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【USAD 2021】Science Resource Guide

SKT - China, CH
An Introduction to Astronomy
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Table of Contents
ASTRONOMY . . . . . . . . . . . . . . . . . . . . .6
Introduction . . . . . . . . . . . . . . . . . . . . . . . . .6
The History of Modern Astronomy . . . . . . 6
Ancient Astronomy . . . . . . . . . . . . . . . . . . . . . .6
Astronomy in Ancient Greece . . . . . . . . . . . . . 6
The Copernican Revolution . . . . . . . . . . . . . . .7
Kepler’s Laws of Planetary Motion . . . . . . . . . 8
Gravitation . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Galileo’s Observations . . . . . . . . . . . . . . . . . . 10
Newton’s Laws . . . . . . . . . . . . . . . . . . . . . . . . .11
SECTION II: THE STARS . . . . . . . . . 21
Introduction . . . . . . . . . . . . . . . . . . . . . . . .21
Distances to Stars . . . . . . . . . . . . . . . . . . . 21
Stellar Spectra . . . . . . . . . . . . . . . . . . . . . . 22
Spectral Lines and Spectroscopy . . . . . . . . . .22
Atomic Structure . . . . . . . . . . . . . . . . . . . . . .
Atomic Energy Levels . . . . . . . . . . . . . . . . . .
Emission Spectra . . . . . . . . . . . . . . . . . . . . . .
Absorption Spectra . . . . . . . . . . . . . . . . . . . . .
The Doppler Effect . . . . . . . . . . . . . . . . . . . . .
Spectral Classes . . . . . . . . . . . . . . . . . . . . . . 25
Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
The Space Race: Yuri Gagarin
Becomes the First Person in Space . . 26
Wave Properties . . . . . . . . . . . . . . . . . . . . . . . . 12
Apparent and Absolute Magnitude . . . . . . . . . 26
The Electromagnetic Spectrum . . . . . . . . . . .13
The Hertzsprung-Russell Diagram . . . . . . . . 28
Electromagnetic Radiation . . . . . . . . . . . . . . 12
Luminosity . . . . . . . . . . . . . . . . . . . . . . . . . . .26
Lower-Frequency Radiation . . . . . . . . . . . . . . 13
Higher-Frequency Radiation . . . . . . . . . . . . . . 14
The Visible Spectrum . . . . . . . . . . . . . . . . . . . 14
The Space Race: President Kennedy
Announces the Moon Shot . . . . . . . . .28
Thermal Radiation . . . . . . . . . . . . . . . . . . 15
The Sun . . . . . . . . . . . . . . . . . . . . . . . . . . .29
The Space Race: Sputnik 1 . . . . . . . . . . 16
Telescopes . . . . . . . . . . . . . . . . . . . . . . . . .16
Structure and Composition . . . . . . . . . . . . . . 29
Sunspots and Activity Cycles . . . . . . . . . . . . .30
Other Solar Activity . . . . . . . . . . . . . . . . . . . . 31
Solar Flares, Prominences, and Coronal Mass
Ejections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Solar Wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Optical Astronomy . . . . . . . . . . . . . . . . . . . . .16
Radio Astronomy . . . . . . . . . . . . . . . . . . . . . .17
Infrared Astronomy . . . . . . . . . . . . . . . . . . . . 18
Ultraviolet, X-Ray, and Gamma Ray
Astronomy . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
The Space Race: John Glenn Orbits
the Earth . . . . . . . . . . . . . . . . . . . . . . . . 32
The Space Race: Explorer 1 . . . . . . . . 19
Stellar Evolution . . . . . . . . . . . . . . . . . . . . 32
Section I Summary . . . . . . . . . . . . . . . . . . 19
Life Cycles of Stars . . . . . . . . . . . . . . . . . . . . 32
Birth of Stars . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Why Stars Shine . . . . . . . . . . . . . . . . . . . . . . . 33
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INTRODUCTION . . . . . . . . . . . . . . . . . .5
Supernovae, Superdense Stars, and Black
Holes . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34
Supernovae . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Neutron Stars and Pulsars . . . . . . . . . . . . . . 35
Black Holes . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Section II Summary . . . . . . . . . . . . . . . . . 36
Neptune . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
Neptune’s Moons . . . . . . . . . . . . . . . . . . . . . . 53
Plutoids and the Kuiper Belt . . . . . . . . . . . . . 53
Asteroids, Comets, and Meteoroids . . . . . . . . 55
Asteroids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
Comets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
Meteoroids . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
Section III Summary . . . . . . . . . . . . . . . . .58
Introduction . . . . . . . . . . . . . . . . . . . . . . . .38
UNIVERSE . . . . . . . . . . . . . . . . . . . . . . 60
Introduction . . . . . . . . . . . . . . . . . . . . . . . .60
The Solar System . . . . . . . . . . . . . . . . . . . 38
The Milky Way Galaxy . . . . . . . . . . . . . . .60
About the Solar System . . . . . . . . . . . . . . . . . 38
The Formation of the Solar System . . . . . . . . 38
The Space Race: The Early Apollo
Missions . . . . . . . . . . . . . . . . . . . . . . . . . 39
The Earth and the Moon . . . . . . . . . . . . . . 39
Earth’s Physical Properties and Structure . . . 39
Earth’s Atmosphere . . . . . . . . . . . . . . . . . . . . 40
The Moon . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
The Surface of the Moon . . . . . . . . . . . . . . . . 42
Tides and Gravity . . . . . . . . . . . . . . . . . . . . . 43
The Space Race: The Apollo 11 Moon
Landing . . . . . . . . . . . . . . . . . . . . . . . . . . 43
The Terrestrial Planets . . . . . . . . . . . . . . . 44
Mercury . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Venus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45
Mars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Mars’ Surface . . . . . . . . . . . . . . . . . . . . . . . . . 46
Mars’ Moons . . . . . . . . . . . . . . . . . . . . . . . . . . 47
Exploration of Mars . . . . . . . . . . . . . . . . . . . . 47
The Jovian Planets and Beyond . . . . . . . . .48
Jupiter and Saturn . . . . . . . . . . . . . . . . . . . . .48
Jupiter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Jupiter’s Moons . . . . . . . . . . . . . . . . . . . . . . . .
Saturn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Saturn’s Moons . . . . . . . . . . . . . . . . . . . . . . . .
Uranus and Neptune . . . . . . . . . . . . . . . . . . . 51
Uranus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
Uranus’s Moons . . . . . . . . . . . . . . . . . . . . . . . . 52
Structure and Properties . . . . . . . . . . . . . . . . 60
Star Clusters . . . . . . . . . . . . . . . . . . . . . . . . . 61
The Interstellar Medium . . . . . . . . . . . . . . . . 61
Mapping Our Galaxy . . . . . . . . . . . . . . . . . . .62
Star Populations . . . . . . . . . . . . . . . . . . . . . . 63
Age and Formation of Our Galaxy . . . . . . . . 63
The Space Race: Skylab . . . . . . . . . . . 64
Other Galaxies . . . . . . . . . . . . . . . . . . . . . 64
Classification of Galaxies . . . . . . . . . . . . . . . 64
Galactic Distances and Distribution . . . . . . .65
Galaxy Clusters . . . . . . . . . . . . . . . . . . . . . . .65
Colliding Galaxies . . . . . . . . . . . . . . . . . . . . .66
Active Galaxies . . . . . . . . . . . . . . . . . . . . . . . 66
Radio Galaxies . . . . . . . . . . . . . . . . . . . . . . . . 67
Quasars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
The Space Race: The Apollo-Soyuz
Test Project . . . . . . . . . . . . . . . . . . . . . . 69
Cosmology . . . . . . . . . . . . . . . . . . . . . . . . 70
The Expanding Universe . . . . . . . . . . . . . . . . 70
Hubble’s Law . . . . . . . . . . . . . . . . . . . . . . . . .
The Big Bang Theory . . . . . . . . . . . . . . . . . . .
Observational Tests . . . . . . . . . . . . . . . . . . . . .
Cosmic Microwave Background Radiation . . .
Twenty-First-Century Cosmology . . . . . . . . .73
Cosmic Acceleration . . . . . . . . . . . . . . . . . . . .
Models of Expansion . . . . . . . . . . . . . . . . . . . .
Big Bang Questions . . . . . . . . . . . . . . . . . . . .
Age and Size of the Universe . . . . . . . . . . . . .
Section IV Summary . . . . . . . . . . . . . . . . 76
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The Space Race: The Gemini
Program . . . . . . . . . . . . . . . . . . . . . . . . . 34
CONCLUSION . . . . . . . . . . . . . . . . . . . .77
NOTES . . . . . . . . . . . . . . . . . . . . . . . . . . 87
TIMELINE . . . . . . . . . . . . . . . . . . . . . . .78
BIBLIOGRAPHY . . . . . . . . . . . . . . . . .89
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GLOSSARY . . . . . . . . . . . . . . . . . . . . . . 81
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In keeping with the overall curricular theme of the Cold
War, we will take opportunities to discuss significant
events in the Space Race between the United States and
the Soviet Union throughout the Resource Guide. The
Space Race marked a period of significant expansion in
humanity’s capabilities for space exploration, motivated
by a competition for superiority between two global
superpowers. The first human Moon landing, an
unparalleled feat of scientific and technological prowess,
took place in July 1969 at the height of the Space Race.
Section I of the guide will cover some of the
foundational topics in astronomy and a basic history
of astronomical discovery. In this section, we will
discuss how models of our Solar System have evolved in
response to new observations. We will also explore how
telescopes work and their role as an astronomical tool.
In Section II, we will explore stars, including our most
familiar star: the Sun. Regardless of their distance from
Earth, stars can be classified according to a specific set
of properties. We will consider what makes stars shine,
how they are formed, and how they change over the
course of millions and sometimes billions of years.
In Section III, we will take a tour of the planets in our
Solar System. Beginning with our home planet of Earth,
we will consider the unique properties of each planet
and the means by which we have learned more about
them. We will also discuss the other bodies that orbit the
Sun—dwarf planets, meteors, asteroids, and comets.
Finally, Section IV will take us beyond our Solar
System to consider galaxies and cosmology. Our
Milky Way galaxy is one of billions of galaxies in our
universe. We will learn how galaxies are classified by
their appearance and properties. We will also consider
what we know about the Big Bang and the age of the
Astronomy is a science that can be appreciated by
anyone who has ever gazed up at the night sky and
wondered about our planet’s place in the vast celestial
array we can observe. By the end of this guide, you will
be able to categorize different types of celestial objects
and describe some of the fundamental properties of the
Solar System, galaxy, and universe to which our planet
belongs. You will also become familiar with the variety
of research methods that astronomers use to make new
discoveries about the universe.
NOTE TO STUDENTS: You will notice as you read through
the Resource Guide that some key terms are boldfaced. These
terms are included in the glossary at the end of the Resource
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Astronomy is perhaps the oldest of all the sciences.
Since ancient humans first looked up in admiration at
the placid beauty of the heavens, the wonders of the
night sky have inspired fantastical myths, fostered
groundbreaking scientific insights, and challenged
us to push the limits of our understanding further
outward. Although the tools of astronomical discovery
have changed since ancient times, humanity’s curiosity
is eternal. In this Science Resource Guide, we will
embark on a journey that will encompass our home
planet of Earth, the Solar System, and the stars beyond,
to the furthest reaches of the universe itself.
Section I
Foundations of Astronomy
A rich history of scientific observation has shaped
our understanding of the Universe over the course of
thousands of years. In this section, we will describe
some of the major events in the history of astronomy.
We will conclude the section by providing an overview
of telescopes and their role in modern astronomy.
Our knowledge of the very first people to study the
behavior of stars and planets is limited because no
written records are known to exist from that era.
Nevertheless, we have evidence that ancient cultures
observed and revered patterns in astronomical
phenomena. A familiar example is Stonehenge, a
massive stone structure in Salisbury Plain, England,
which was constructed in stages between 3100 bce to
about 1600 bce. The fact that particular pathways and
stones that make up Stonehenge align with significant
annual astronomical events suggests that the structure
was built effectively as a massive calendar (Figure 1-1).
Similar properties can be observed in structures
built by other ancient cultures around the world. For
example, the Temple of Isis in Dendera, Egypt, was
constructed to align with the point on the horizon from
which the bright star Sirius appeared to rise. Since
the appearance of this star coincided with the flooding
of the Nile, it played a significant role in Egyptian
agriculture and mythology.2 The Sun Dagger, a rock
sculpture carved by Native Americans in Chaco
Canyon, New Mexico, is another structure that exhibits
alignment with the Sun on the summer solstice.
Observations of astronomical patterns and phenomena
served multiple practical purposes for ancient cultures.
Early astronomers used annual, seasonal, and daily
Alignment of the rising Sun with a “heel stone” of
Stonehenge on the morning of the summer solstice.
patterns in celestial events to establish calendars for
predicting when to plant and harvest crops. Certain
stars in the sky, such as Polaris, became important
navigational aids that fostered exploration. Although
we have no record of how well these humans
understood what they saw in the sky, it is clear that
early astronomy played an important role in their
survival and scientific advancement.
Astronomy in Ancient Greece
Our earliest complete records of observations and
hypotheses regarding astronomical phenomena come
from the ancient Greeks. Early Greek philosophers
sought to explain variations in the brightness of
planets, as well as their apparent “wandering” motion
across the sky. In fact, the word planet comes from the
Greek word planetes, which means “wanderer.”
Around 600 bce, Thales of Miletus was among the first
philosophers to suggest that natural phenomena could
be explained and understood by humans. Pythagoras,
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Ancient Astronomy
The Greek philosopher Aristotle was another student
of Plato who made great contributions across many
scientific disciplines. During the fourth century
bce, Aristotle proposed a geocentric model of the
Universe, which placed the Earth at the center. Not all
the Greek philosophers agreed with Aristotle’s view;
during the third century bce, another astronomer
named Aristarchus proposed that the Earth and other
planets revolve around the Sun. However, Aristotle’s
influence was great enough that the geocentric model
became more widely accepted. Around 150 ce, the
Greek astronomer Claudius Ptolemaeus, now known
as Ptolemy, extended Aristotle’s ideas and made them
fit with the existing data on planetary motion. Ptolemy
described the Sun, Moon, and planets as moving on
small circles called epicycles, which traced larger
circular pathways around the Earth called deferents
(Figure 1-2).
The Ptolemaic model was accepted as the basis for
astronomical work for more than fourteen centuries.
It described with considerable accuracy the observed
positions and motions of the heavenly bodies known
at that time. With minor modifications, Ptolemy’s
geocentric model became central to the accepted
teachings of the Roman Catholic Church during the
Middle Ages.
The Copernican Revolution
Nicholas Copernicus was a Polish astronomer born in
1473. Copernicus spent many years of his life writing
a book, On the Revolutions of the Celestial Spheres,
in which he proposed a heliocentric (Sun-centered)
model of the universe. In the Copernican model, the
Earth is not central to the Universe but instead revolves
about the Sun just like the other known planets.
The Ptolemaic model of the universe.
Copernicus waited several decades to refine his theory
before publishing his work. For one thing, his model
challenged a central teaching of the Catholic Church
and would likely be extremely controversial.
The Copernican model was highly successful at
explaining aspects of planetary behavior that earlier
models could not. For example, as astronomers observed
the paths of certain planets in the night sky, they would
occasionally notice periods in which the planets would
seem to reverse their direction before again resuming
their original motion. This behavior became known
retrograde motion. The Copernican model explained
that Mars never actually moves “backward” in its
orbit—planets always move in a uniform direction.
Instead, the apparent retrograde motion is caused by the
relative motion of Earth and Mars. As the faster-moving
Earth catches up to Mars and moves past it, Mars
appears to us as if it is moving backward (Figure 1-3).
As an analogy, you can think of watching a slower car
as you overtake it in a faster-moving car.
Copernicus published his book in 1543, the year that he
died. Although Copernicus’s theory was not complete,
his heliocentric model was highly influential on later
scientists.4 The term Copernican revolution refers to
Copernicus’ groundbreaking insight that the Earth was
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a renowned Greek mathematician and philosopher,
noticed mathematical patterns in nature and proposed
that these patterns could be extended to the behavior of
stars and planets. Plato, another philosopher, believed
that everything in nature could be reduced to ideal
geometric forms such as tetrahedrons and cubes
(known as the Platonic solids). Plato considered the
most perfect geometric form to be the sphere and thus
proposed that heavenly bodies must be carried around
the Earth upon concentric “nested” spheres. Eudoxus
of Cnidus, a student of Plato, extended this idea to
propose a system of twenty-seven concentric spheres
rotating about the Earth at different rates.
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Retrograde motion of Mars, as explained by the Copernican heliocentric model. The apparent position of Mars in Earth’s
sky changes at five successive moments in time due to relative motion between the planets in orbit.
not the center of the universe.
set of laws describing planetary motion.
Kepler’s Laws of Planetary Motion
Eventually, Kepler succeeded in introducing three laws
of planetary motion:
Tycho Brahe was a Danish aristocrat and astronomer
born in 1546. As a young adult, Brahe developed a
passion for astronomy. Over the course of several
decades, Brahe made daily observations of the
positions of the Sun, Moon, stars, and planets and kept
meticulous and detailed records of his measurements.
The precision with which Brahe collected his data
is particularly noteworthy because the telescope had
yet to be invented! In the year 1600, Brahe joined the
astronomer Johannes Kepler in Prague. Following
Brahe’s death in 1601, Kepler spent several years
attempting to fit Brahe’s extensive numerical data to a
Planetary orbits are elliptical, with the Sun at
one focus.
Although the Greeks envisioned planetary orbits as
perfect circles, Kepler showed that they actually form
ellipses. An ellipse is a curved geometric figure for
which the sum of the distances from any point on the
curve to two fixed points (the foci) is a constant. A
handy way to sketch an ellipse is to attach two tacks
to a piece of paper and loop a string around them. By
stretching the string taut with a pencil and tracing a
complete curve, an ellipse is formed. (Figure 1-4)
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This law describes how the orbital speed of a planet
changes depending on its distance from the Sun. When
a planet is closer to the Sun, it moves faster, thereby
traveling a further distance along its orbit in a given
amount of time (Figure 1-6).
The eccentricity of an elliptical orbit is defined to be
the distance between the foci (F1 and F2 in Figure
1-5) divided by the length of the semimajor axis (a in
Figure 1-5). The eccentricity of a perfect circle is zero,
and as the eccentricity increases from zero, the ellipse
becomes less circular in shape.
An illustration of Kepler’s Second Law. Over the course of
its orbit, the planet will sweep out equal areas A over the
three equal time intervals t.
The square of a planet’s orbital period is
proportional to the cube of its semimajor axis.
This law can be expressed mathematically as
Pyr2 = aAU3, where Pyr is the orbital period in Earth years
and aAU is the length of the semimajor axis, measured
in astronomical units (AU). (One astronomical unit is
defined to be the semimajor axis of Earth’s orbit.) For
example, as seen in Table 1-1, the semimajor axis of
Jupiter’s orbit is a = 5.203 AU. By cubing this value and
taking the square root, we obtain the value of its orbital
period P, which is 11.86 years. Using Table 1-1, you can
verify that the data for the other planets are related in
the same way.
The general form of an ellipse. The two foci are labeled F1
and F2 . The semimajor axis is labeled a and the semiminor
axis is labeled b.
An imaginary line connecting a planet to the
Sun sweeps out equal areas in equal times.
Kepler’s laws of planetary motion are empirical. That
is, they accurately describe a phenomenon (in this
case planetary motion) but do not suggest an explanation for why it occurs. Although Kepler did not propose a theory for why the planets behaved as they did,
that insight would arrive within the century through
the work of Galileo Galilei and Sir Isaac Newton.
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An ellipse can be drawn by stretching a loop of string
between two tacks and the tip of a pencil and tracing the
pencil to create a smooth curve.
TABLE 1-18
FIGURE 1-710
Semimajor Axis
aAU (AU)
Orbital Period
Pyr (Earth
Orbital periods and semimajor axes for planets in the
Solar System.
Galileo’s Observations
Reproduction of Galileo’s nightly observations of Jupiter’s
four largest moons in 1610.
places in their orbits around the Sun. According to the
geocentric model, these planets would only ever appear
in the crescent phase, which is clearly at odds with
observational evidence (Figure 1-8).
FIGURE 1-811,12
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Galileo Galilei was an Italian astronomer and physicist
born in 1564. Galileo developed an interest in the
motion of falling objects, but he discovered that
objects in free-fall moved too quickly for him to
accurately measure. Instead, he recorded how long
it took for metal balls to roll down an incline, using
a water clock of his own invention to measure time.9
Galileo discovered that falling objects do not move
at a constant speed but actually speed up with time.
Furthermore, Galileo found that the rate of acceleration
was the same for every object, regardless of its mass.
In 1609, Galileo became the first person to make
astronomical observations using a telescope. He was
able to view mountains, craters, and extensive dark
areas on the Moon, as well as sunspots and their
movements. Galileo’s discovery of the four largest
moons orbiting Jupiter confirmed that Earth was not
the center of all planetary motion (Figure 1-7). These
observations provided important evidence in support
of the Copernican theory.
Galileo also observed that Venus appears to regularly
change its shape and size in the night sky. Although
the Ptolemaic system could not provide an explanation
for the phases of Venus, the Copernican system had a
simple explanation. Venus and Mercury, the two inferior
planets (i.e., planets closer to the Sun than Earth), exhibit
phases as they reflect sunlight to Earth from different
The expected phases of Venus under the geocentric model
(top) and the heliocentric model (bottom). Venus exhibits all
phases predicted by the heliocentric model. (Note: the above
diagram is not to scale.)
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In 1616, the Roman Catholic Church banned books that
supported the Copernican theory. Galileo was permitted
to continue astronomical research, provided he did not
hold, teach, or defend doctrines contrary to the views
of the Church. Nevertheless, in 1632 Galileo published
“Dialogue Concerning the Two Chief World Systems,”
in support of the Copernican system. The following
year, Galileo was forced by the Inquisition to recant his
astronomical findings and condemned to house arrest.
In 1992, the Catholic Church finally acted to vindicate
Galileo by overturning the centuries-old verdict.13
where m1 and m2 are the masses of the objects, r is
their separation (measured between the centers of the
objects), and G is the gravitational constant (Figure
FIGURE 1-914
Newton’s Laws
English physicist and mathematician Sir Isaac Newton
revolutionized physics and astronomy by formulating
laws that explain why the planets move as they do.
Newton’s book The Mathematical Principles of
Natural Philosophy, also known as the Principia, was
published in 1687. In it, Newton proposed his three
laws of motion and his law of universal gravitation.
An object at rest or in motion will maintain that
motion unless acted upon by an outside force.
An object’s acceleration is directly proportional
to the net force acting on it and inversely
proportional to the object’s mass.
To every action there is an equal and opposite
Newton turned his attention to the particular motion of
falling objects. Just as Galileo recognized that falling
objects changed their speed as they fell, Newton
reasoned that some kind of constant force must be
causing that change in speed. Newton’s great leap
was to recognize that the force that keeps the planets
in orbit around the Sun is the same force that causes
objects to accelerate toward the Earth. By inventing
differential calculus in a geometric formulation,
Newton was also able to calculate the strength of the
force between the Earth and the moon that would be
required to keep the moon in orbit.
Newton’s universal law of gravitation states that the
force between two objects is directly proportional to the
product of their masses and inversely proportional to the
square of the distance between them. Mathematically,
the magnitude of this force can be represented as:
The gravitational force acting between two masses, m1 and
m2 , according to Newton’s law of gravitation.
Newton’s law of gravitation is an inverse-square law.
That is, the magnitude of the attraction between two
objects decreases in proportion to the square of the
distance between them. For instance, if the distance
separating two objects is doubled, the gravitational
force between them is reduced to 1/4 of its original
strength. Furthermore, the gravitational force
between any two objects acts in equal magnitude on
both objects. In other words, the Sun’s gravitational
attraction of the Earth is equal in magnitude to the
Earth’s gravitational attraction of the Sun.
It is important to note that physics, astronomy, and
all sciences are collaborative and interactive in
nature, requiring the effort of many people working
over prolonged periods on similar and slightly more
complicated problems. This concept was famously
stated by Newton himself, who once said, “If I
have seen farther than others, it is because I have
stood on the shoulders of giants.” Indeed, Newton’s
breakthrough came about due to the work of earlier
scientific minds such as Galileo, Copernicus, Brahe,
and Kepler. As we will see later on, Newton’s
model did not provide a complete description of the
role gravity plays in our universe. Other physicists
would stand upon Newton’s shoulders to refine our
understanding of gravitation even further.
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Newton’s laws of motion can be stated as follows:
The information that astronomers gather about distant
stars, planets, and galaxies arrives in the form of light.
Any time you look up at the stars at night, you are
receiving light that has traveled, perhaps, millions of
miles to reach your eyes. But what is light, and how is
it able to traverse the great expanse of the universe?
Electromagnetic Radiation
Light is a form of electromagnetic radiation, meaning
it is made up of electric and magnetic fields that
oscillate back and forth (Figure 1-10). We commonly
use the term light to refer to electromagnetic radiation
that is visible to the human eye, but visible light is only
one form of electromagnetic radiation. Electromagnetic
radiation does not require a physical medium through
which to propagate and can travel through the empty
vacuum of space. This is how light from the Sun and
other stars reaches Earth. All electromagnetic radiation
travels through empty space at a constant speed. This
speed, known as the speed of light, c, is equal to
299,792,458 m/s.
small object, such as a pebble, into a pond causes ripples
of water to spread outward from where the object lands.
The pebble disturbs the water immediately surrounding
it, which disturbs the water further out, which disturbs
the water further out, and so on.
Waves transport energy but not mass. That is, waves
do not carry matter along with them from one point
to another as they travel. Think about a floating buoy
bobbing up and down in the ocean as waves pass.
Although the height of the buoy changes with time,
on average its position along the surface of the water
does not change. Likewise, individual particles of the
medium through which the wave is moving do not
move along with the wave itself.
Although different types of waves have different
physical properties, there are general characteristics
that we can use to describe any wave. The amplitude
of a wave is the maximum displacement from the
equilibrium position. For a water wave or a wave
on a string, the amplitude is the height of the wave
compared to its resting position. For electromagnetic
waves, the amplitude refers to the maximum
magnitude of the electric and magnetic fields. In
general, the amplitude of a wave is related to the
intensity of the wave. The wavelength λ of a wave
is defined as the distance between two consecutive
corresponding points on a wave—for example, the
distance between two successive peaks (Figure 1-11).
Electromagnetic radiation is made up of oscillating
electric and magnetic fields. The wave moves in a direction
perpendicular to both oscillating fields.
Wave Properties
The light emitted by the Sun, the sound generated by
a thunderclap, and the ripples of water in a pond are
all described by the general mathematical concept of
waves. A wave is the motion of a disturbance between
two points. You’ve likely observed how dropping a
The amplitude (y) and wavelength (λ) of a general
A wave can also be described in terms of its frequency
f, which is defined as the number of oscillations it
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FIGURE 1-1215
makes in a given amount of time. (For instance, think
of how many ripples pass by a certain point in a pond
every second.) The standard unit of frequency is the
hertz (Hz), where 1 Hz is equal to one oscillation
per second. The period T of a wave is the amount of
time that elapses between oscillations and is inversely
proportional to the frequency (i.e., T = 1/f ). For example,
if the frequency of a wave is 10 Hz (i.e., 10 oscillations
per second), the corresponding period is 0.1 seconds
because each oscillation is 1/10 seconds apart.
All waves are caused by some kind of initial disturbance
or vibration. Electromagnetic waves are caused by the
vibrations of charged particles, such as electrons and
protons. Charged particles are surrounded by electric
fields that exert forces on other charged particles.
When a charged particle vibrates, the fluctuation in its
electric field causes a self-perpetuating “pulse” that
spreads out like the ripples in a pond, except that the
electromagnetic disturbance (known as a wave packet)
does not need to travel through any medium.
Another important characteristic of a wave is the wave
speed v. The speed of any wave is related to both its
frequency and its wavelength. Let’s determine the
exact relationship by considering the motion of a wave
past a fixed point, such as crests of water past the edge
of a dock. We can use a ruler to measure how far apart
the crests are (i.e., the wavelength) and a stopwatch to
measure how much time passes between the arrival
of one wave crest and the next (the period). Speed is
a distance traveled per unit time, and in the case of a
wave, the distance is the wavelength, and the time is
the period.
The Electromagnetic Spectrum
distance λ
Recalling that period is the reciprocal of frequency, we
can convert this equation to the often more useful form:
v = f λ.
This relationship applies to any kind of wave, from
light to sound to water. Notice that according to this
equation, the higher the frequency of a wave, the
shorter its wavelength.
We are constantly surrounded by an undulating sea of
electromagnetic waves, with each wave characterized
by its own amplitude and frequency. In addition
to visible light, the space around us is filled with
a multitude of waves with different wavelengths,
from radio waves to gamma rays. This continuum of
electromagnetic waves, arranged by either frequency
or wavelength, is called the electromagnetic
spectrum (Figure 1-12).
Lower-Frequency Radiation
Radio waves have the longest wavelengths in the
electromagnetic spectrum, typically 30 cm or longer.
Some AM radio waves even have wavelengths as long
as 1 km. Radio waves are primarily used in radio and
television communication, but also are used in radar
systems, satellites, and walkie-talkies.
Microwaves have wavelengths between 1 mm and
30 cm. Although you are likely most familiar with
their application to cooking food in microwave ovens,
microwaves are also used for aircraft navigation, radar,
and scientific research.
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The electromagnetic spectrum.
Higher-Frequency Radiation
Ultraviolet (UV) light ranges from about 400 nm to
10 nm. The Sun is a significant source of UV light
received here on Earth, although most is absorbed
by the atmosphere. UV light in large doses is
harmful to humans, but it also has practical uses
in the sterilization of medical instruments, silicon
lithography (production of computer chips), scientific
studies, and air and water disinfection from viruses
and bacteria.
X-rays have wavelengths between about 10 –4 nm and
10 nm. The most familiar application of X-rays is in
medicine for the examination of bones, teeth, and
organs. X-rays are energetic enough to be harmful to
living tissue—you may recall wearing thick shielding
to protect your body when having an X-ray performed
at the doctor’s or dentist’s office.
Gamma rays are short-wavelength, highly penetrating
waves that are emitted by nuclear transitions. They
have wavelengths between 10 –14 m and 10 –10 m.
Gamma rays are extremely hazardous to living
tissue and can cause irreversible damage if absorbed.
However, the destructive capability of gamma rays
can also be harnessed to destroy harmful cells such as
cancerous tumors.
It is worth noting that the labels for each category
serve as a historic classification system and do not
indicate fundamental differences between each
type of wave. Each kind of wave in the spectrum is
a manifestation of the same electromagnetic wave
phenomenon, differing from one another in their
frequency of oscillation, which is directly related to
the amount of energy they are capable of depositing
in matter. There is no sharp boundary between one
category of electromagnetic wave and the next.
The Visible Spectrum
Visible light is the set of wavelengths detectable by the
human eye. The colors of the visible spectrum range
from violet (≈400 nm) to red (≈700 nm). Human eyes
are not equally sensitive to all wavelengths; the peak
of sensitivity is around 560 nm (yellow-green light).
Although there are an infinite number of frequencies
within the visible spectrum, conventionally we often
group them into the seven colors: red, orange, yellow,
green, blue, indigo, and violet. You can use the
memory device “ROYGBIV” (often pronounced like
“Roy G. Biv”) to remember the colors of the visible
spectrum in order from lowest to highest frequency.
Light that is made up of a mixture of all colors in
the visible spectrum appears white. For example,
white light from the Sun is composed of all visible
wavelengths of light. In the late 1660s, Isaac Newton
first demonstrated that white light is a composite of
colored light by using a prism to separate sunlight into
a spectrum of colors and then recombining them with a
second prism (Figure 1-13).
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Infrared (IR) waves have wavelengths longer than
visible light, between about 700 nm and 1 mm. IR
waves are readily absorbed and emitted by most
materials. When absorbed, IR waves cause objects to
get hotter by increasing the kinetic energy of atoms
and molecules. Infrared waves have a wide range of
applications in meteorology, night-vision devices,
communication, and astronomy, to name a few.
An illustration of Newton’s double prism experiment, taken
from a 1671 letter to the Royal Society.
The Sun emits a range of electromagnetic wave
frequencies, with the vast majority (over 99 percent)
belonging to the ultraviolet, visible, and infrared
portions of the spectrum. The peak of solar intensity
resides squarely within the visible spectrum for
humans. That’s certainly no coincidence—evolutionary
processes over millions of years have caused human
eyes to be most sensitive to the frequencies of light that
are emitted most strongly by the Sun. Some animals
and insects are sensitive to electromagnetic waves that
are beyond the visible spectrum for humans. Bees, for
example, are sensitive to a portion of the solar spectrum
that extends into the ultraviolet, which enables them to
detect UV patterns that direct them toward pollination
sites on certain species of flowers.
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The temperature of an object is a measure of the
average speed of its constituent particles. All matter
that has a temperature above 0 K (“absolute zero”)
radiates electromagnetic waves, which is known as
thermal radiation. At room temperature, objects radiate
mostly infrared waves, which cannot be detected
by human eyes but can be captured by an infraredsensitive camera. When objects become sufficiently hot
(around 800 K), they begin to glow with a visible color
dependent on their temperature. Familiar examples
include a red-hot stove burner or a yellow-white light
bulb filament (Figure 1-14).
FIGURE 1-1416
evidence has shown that the peak
wavelength is inversely proportional to the temperature
of the object. This relationship is known as Wien’s
displacement law:
Accordingly, if the temperature of a blackbody
is doubled, the peak wavelength of emitted
electromagnetic waves will be halved. This
relationship allows us to easily compare the surface
temperature of distant stars based on their distributions
of emitted light. For instance, since the wavelength
of violet light is just about half that of red light, we
can estimate that a violet-hot star has approximately
twice the surface temperature of a red-hot star. As
an object is heated from room temperature, it first
glows “red-hot” as the peak wavelength shifts into the
visible spectrum from the infrared. Eventually, as the
distribution peak overlaps the entire visible spectrum,
the object will appear “white hot.”
λ peak ∝
A hot stove coil glows red by emitting thermal radiation. A
large fraction of energy is also being radiated as infrared
waves, which we perceive as heat.
The spectrum of thermal radiation emitted by an
opaque object is known as blackbody radiation.
Importantly, the wavelengths of light in a blackbody
radiation spectrum depend on the temperature of
the blackbody. Figure 1-15 shows several intensity
distribution curves for blackbodies at various
temperatures. Notice that the intensity of radiation
is not evenly distributed across wavelengths; for any
given temperature, there is a “peak” wavelength that
is emitted with the greatest intensity. Furthermore,
Intensity distributions of emitted thermal radiation for
blackbodies at 3,000 K, 4,000 K, and 5,000 K. Notice how
the peak wavelength becomes shorter as the temperature of
the object increases.
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Thermal Radiation
On October 4, 1957, the Soviet Union launched Sputnik 1, the
first artificial satellite placed into Earth’s orbit. Sputnik 1 was a
58-cm metal sphere with four external radio antennas (Figure
1-16) that spent three weeks in orbit before losing power. Sputnik
1 burned up upon reentering Earth’s atmosphere about three
months after launch.
A replica of Sputnik 1.
the eyepiece acts as a magnifying lens for the nearby
image formed by the objective lens.
You may be familiar with the process of using a
magnifying lens to make a nearby object easier to see.
A lens is a piece of curved glass that bends the pathway
of light passing through it. This bending of light as it
passes from one transparent medium to another is called
refraction. A single lens can magnify, or enlarge, a
nearby object by creating a larger image of it on your
retina. However, celestial objects such as planets,
comets, and galaxies are too far away to be magnified
using a single lens. A telescope uses a combination
of lenses or mirrors to collect light from a distant
object and concentrate it for analysis. We will begin by
discussing optical telescopes, which produce images
with light in the visible spectrum. There are two main
categories of optical telescopes: refracting and reflecting
(Figure 1-17).
A reflecting telescope uses a curved mirror instead of
a lens to collect and focus light from distant objects.
The large mirror of a reflecting telescope is known
as the primary mirror. Isaac Newton is credited with
assembling the first reflecting telescope in 1688.
Since Newton’s time, reflecting telescopes have been
instrumental in countless astronomical discoveries,
including the discovery of the planet Uranus by
William Herschel in 1781. A Cassegrain telescope
is a form of reflecting telescope that uses a smaller
secondary mirror to reflect light back through a hole
cut in the primary mirror, thereby allowing for a more
compact overall form.
Optical Astronomy
A refracting telescope uses a combination of lenses
to collect light from a distant object and focus it to
produce an image. A refracting telescope has an
objective lens that is fixed at the end of the telescope
tube closer to the object being observed. Light passing
through this lens is refracted so that it forms an image
of the object near the back of the tube. The distance
from the lens to the image is referred to as the focal
length of the lens. A second lens, called an eyepiece,
is used to enlarge the image produced by the objective
lens, which is then viewed by an observer. In this way,
The Hubble Space Telescope, which employs a
2.4-meter concave mirror as the primary and a
0.305-meter secondary mirror, was deployed in
1990 from Space Shuttle Discovery and remains in
operation as of the time of this publication. The Keck
Observatory on Mauna Kea in Hawaii features two
reflecting telescopes with a segmented mirror design
forming a diameter of ten meters each, placing them
among the largest optical telescopes in the world today.
Refracting telescopes are useful for viewing larger,
brighter objects in the night sky such as the moon and
planets. However, some astronomical objects, such as
galaxies, are so distant that a large diameter telescope
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The launch of Sputnik 1 is widely considered the starting event
in the “Space Race” between the Soviet Union and the United
States. News of Sputnik’s launch came as a surprise to the general
American public and called into question the United States’
technological superiority following World War II. As a result,
the federal government directed increased funding toward math,
science, and engineering education at all levels in an effort to
compete more directly with the Soviet Union in space exploration.
Radio waves collected by a radio telescope cannot
be seen, heard, or photographed directly. Rather, a
receiver collects, amplifies, and records their “image”
as an electronic signal. Computers may display radio
images digitized, as a contour map that shows the
strength of the radio source, or as a radiograph, which
is a false color picture that shows how the radio source
in space would “look” to a person with “radio vision.”
Figure 1-18 shows the same celestial object (the
Crab Nebula) imaged with six different wavelengths,
including radio waves.
FIGURE 1-1818
Crab Nebula: Remnant of an Exploded Star
A diagram of a refracting telescope and a Cassegrain
reflecting telescope.
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is necessary to collect enough light for proper
viewing. Manufacturing the large lenses that would
be necessary to construct a refracting telescope at this
scale would be difficult and costly; large-diameter
lenses are also heavy and prone to sagging. For these
reasons, reflecting telescopes tend to be more useful
for the purposes of astronomical research. The 10-m
primary mirrors of the Keck Observatory telescopes
could not rigidly hold their shape to a necessary
precision if they were made of single pieces of glass.
Instead, each mirror is made of an array of thirty-six
hexagonal segments that work together as a single unit.
Radio Astronomy
Some astronomical objects emit most of their radiation
outside the visible spectrum. We would not be able to
make useful observations of such objects using optical
telescopes. Radio astronomy originated in 1931 when
U.S. engineer Karl G. Jansky discovered radio waves
coming from the Milky Way. Since then, radio waves
have been received from a variety of sources, including
the Sun, other planets, cold interstellar gas, pulsars,
distant galaxies, and quasars. Since Earth’s atmosphere
does not block or scatter radio waves, radio telescopes
can be operated in cloudy weather or during the
daytime. Radio telescopes consist of a curved “dish”
antenna that acts like the curved mirror in a telescope.
Because radio waves have wavelengths of many meters,
the antennas must be correspondingly large in size.
Six images of the Crab Nebula, each captured using a
different wavelength of electromagnetic waves.
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The Arecibo Observatory radio telescope.
Infrared Astronomy
Infrared astronomy studies radiation from the infrared
band of the electromagnetic spectrum, which extends
between microwaves and visible red light. First built
in the 1960s, infrared telescopes are essentially optical
reflectors with an IR-sensitive detector at the focus
of the primary mirror. The detectors are shielded and
cooled to about 2 K to ensure that they are registering
infrared rays from space, rather than stray thermal
radiation from people, equipment, and observatory
Since water vapor and carbon dioxide in the air
strongly absorb infrared radiation, large infrared
telescopes must be located on very high mountaintops
where the air overhead is thinnest and driest. Smaller
infrared telescopes can be raised to high altitudes
in airplanes, balloons, rockets, and spacecraft. For
example, the Stratospheric Observatory for Infrared
Astronomy (SOFIA), a collaboration between the
U.S. and Germany, is an airplane modified to fly
a 2.5-m reflecting telescope above an altitude of
twelve km (40,000 feet).22 The NASA Spitzer Space
Telescope, launched in 2003, trails behind the Earth
in a heliocentric orbit, carrying an 85-cm telescope.23
Infrared telescopes provide observations of celestial
objects that are relatively cool or obscured because
infrared rays pass through interstellar clouds of gas
and dust that block shorter visible rays. You can view
false color images of cool stars and galaxies, regions
of star and planet formation in giant molecular clouds,
comets, and galaxy centers at NASA’s Infrared
Processing and Analysis Center (IPAC).24
Ultraviolet, X-Ray, and Gamma Ray
Many of the most energetic objects and violent events
in the universe release high-energy forms of radiation
such as X-rays and gamma rays. Since the 1960s,
ultraviolet, X-ray, and gamma ray telescopes with
suitable detectors have been sent above the Earth’s
atmosphere in orbiting spacecraft. These telescopes
feature solar arrays to generate electricity for
instruments. Insulation protects the instruments from
the extreme heat and cold, low pressure, and energetic
particles and radiation in space. Star tracking systems
and gyroscopes orient space observatories and point
them to sky objects on command.
Like other telescopes, high-energy telescopes collect
and focus incoming radiation. Detectors record the
radiation’s intensity, energy, duration, and the direction
from which it originated. The telescopes receive
commands from mission ground control and transmit
data via radio antennas. After the data has been
processed and analyzed, it can be displayed digitally or
as graphs of intensity over time or an energy range to
reveal how the source is producing its rays, how bright
it is, how long it remains at that brightness, and what
kind of object it is. Data can be manipulated to generate
spectacular false color images, in which colors are used
to show features of objects not emitting in the visible
spectrum (i.e., not colors you would actually see).
Ultraviolet observations have been made of the Sun,
hot stars, stellar atmospheres, interstellar clouds, a
hot gas galactic halo, and extragalactic sources. The
Galaxy Evolution Explorer (GALEX), an orbiting
ultraviolet space telescope launched in 2003, detected
the faintest and most distant sources of ultraviolet
radiation ever observed until its retirement 2012.25
Since X-rays and gamma rays are energetic enough
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One of the largest single radio telescopes ever
constructed is the Arecibo Observatory in Arecibo,
Puerto Rico (Figure 1-19). First operating in 1963, the
305-meter dish features a twenty-acre reflecting area.19
The Robert C. Byrd Green Bank Telescope (GBT) at
the National Radio Astronomy Observatory (NRAO)
in West Virginia is the world’s most powerful, accurate,
and sensitive fully steerable radio telescope.20 The
Green Bank Telescope’s 100 × 110-m dish is specially
shaped to focus radio waves to the side, where a receiver
collects the signals without blocking the dish.
The United States launched its first satellite, Explorer 1, on
January 31, 1958.27 The primary scientific sensor on Explorer
1 was a cosmic ray detector to measure radiation in Earth’s
orbit. Cosmic rays are high-energy radiation in the form of fastmoving particles, such as protons, from distant space. Results
from Explorer 1 revealed the existence of a “belt” of charged
particles trapped in Earth’s magnetic field, which was later
confirmed by subsequent probes. These belts became known
as the Van Allen Belts in honor of Dr. James van Allen, their
discoverer and a member of the Explorer 1 design team.
Explorer 1 team members Dr. William H.
Pickering, Dr. James van Allen, and Dr.
Wernher von Braun holding a model of
Explorer 1.
to pass straight through ordinary mirrors and lenses,
we must use alternate ways to collect and focus these
forms of radiation. The Chandra X-ray Observatory,
launched in 1999, has nested barrel-shaped mirrors.26
Incident X-rays that strike the mirrors at grazing
angles reflect to a focal point and form an image.
Gamma ray detectors such as the Fermi Gamma-ray
Space Telescope detect sudden, intense “bursts” of
radiation that may indicate the presence of black holes,
active galaxies, and distant quasars.
Early astronomers used annual, seasonal, and
daily patterns in celestial events to establish
calendars for timekeeping and agricultural
purposes. Astronomy was also important for
navigation and exploration.
Many early models of the universe, such as
the one proposed by Ptolemy, were geocentric,
meaning Earth-centered.
Copernicus proposed a heliocentric model,
in which the Earth orbits the Sun along with
the other planets. Astronomical observations
by Galileo Galilei and Tycho Brahe provided
evidence in support of the Copernican model.
Kepler’s laws of planetary motion state that: (1)
planetary orbits are ellipses, (2) planets move
faster when closer to the Sun, and (3) there
is a relationship between the dimensions of a
planet’s orbit and its orbital period.
Newton discovered that all objects, including
planets, attract one another by a gravitational
force that is proportional to the product of their
masses and inversely proportional to the square
of their separation.
Light is a form of electromagnetic radiation,
made up of oscillating electric and magnetic
fields. All electromagnetic waves travel
at the same speed in a vacuum (empty
space), the speed of light (𝑐). The speed
of light is equal to 299,792,458 m/s. The
electromagnetic spectrum is a classification of
all electromagnetic waves by frequency.
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The United States continued to launch successors to the Explorer
1 satellite throughout 1958. Some, such as Explorer 2 and
Explorer 5, suffered launch complications and did not make it
to orbit. Explorer 3 and Explorer 4 launched on March 26 and
July 26 of 1958, respectively, and each operated for about four
The visible spectrum represents a small fraction
of the entire electromagnetic spectrum. White
light from the Sun is composed of all visible
wavelengths of light and can be separated into a
full spectrum using a prism.
Many types of telescopes operate at
wavelengths outside the visible spectrum.
Radio, infrared, ultraviolet, X-ray, and gamma
ray astronomy all offer the ability to “see”
astronomical phenomena that cannot be
otherwise observed with our eyes.
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Refracting telescopes use a combination of two
lenses to collect and focus light from a distant
object to form an image. Reflecting telescopes
collect and focus light with one or more curved
mirrors and then use a lens to enlarge an image.
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Section II
The Stars
The pinpoints of light we see in the night sky are
actually distant, massive power generators that
recombine nuclei to release energy. The light we
receive from the Sun is the product of constant nuclear
reactions in the Sun’s core. Stars are born, evolve, and
die over the course of millions (and sometimes billions)
of years. In this section, we will explore the properties
of stars, how astronomers classify them based on these
properties, and the changes they undergo over the
duration of their lifetimes.
FIGURE 2-129
Because the distances to stars are so vast, stellar
parallaxes tend to be very small. The standard unit of
measure for stellar parallax is arc seconds (”), where
1” = 1/3600°. To give you a sense of scale, an M&M
candy would appear to have a parallax angle of 1” if it
were viewed from a distance of about 2 km away. The
parallaxes of even the nearest stars are less than 1”.
The distance to an imaginary star whose parallax is
exactly 1 arc second (1”) is defined to be one parsec
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The observable universe contains on the order of
several tens of sextillions of stars (1 sextillion = 1021).28
One way of measuring the distances to nearby stars is
called the method of parallax. The method of parallax
is based on the fact that objects shift their position
relative to a background as the observer’s position
changes. First, the position of a star is carefully
determined relative to other stars. As Earth revolves
about the Sun, nearby stars appear to shift back and
forth relative to more distant stars. An apparent change
in the position of a star is observed when the star
is sighted from opposite sides of Earth’s orbit. This
apparent change is known as stellar parallax. The
distance to the star is determined from its parallax
angle, which is equal to one half of the apparent
change in the angular position of the star (Figure 2-1).
An illustration of stellar parallax. A nearby star that is
sighted from opposite sides of Earth’s orbit appears to shift
its position. (The amount of shift is exaggerated here for
illustrative purposes.)
(pc), for “parallax in arc seconds.” One parsec equals
about 31 trillion kilometers (19 trillion miles), or 3.26
light-years. To calculate the distance to any star from
its measured parallax, we can use the formula:
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Astrometry is the measurement of the positions,
parallaxes, and motion of stars. The first space
experiment for precision astrometry, known as
Hipparcos (HIgh Precision PARallax COllecting
Satellite), was launched in 1989 by the European
Space Agency and operated until 1993. Its name
also honors Hipparchus, a Greek astronomer who
used the method of parallax in 120 bce to calculate
the distance between the Earth and the Moon. The
Hipparcos satellite collected precise measurements
of the positions, parallaxes, and motions of more than
118,200 stars. A follow-up mission to Hipparcos,
known as Gaia, was launched in 2013.
Spectral Lines and Spectroscopy
Astronomers use the light emitted by stars to gather
information about their composition and other
properties. Light produced by stars is made up of many
different wavelengths. In order to understand how light
is emitted by stars, we must first consider the structure
and properties of the atom.
Atomic Structure
Atoms are the building blocks of the world around us.
Atoms contain a nucleus, which is a dense core that
comprises most of the atom’s mass, and an outer region
inhabited by bound electrons. The nucleus is made
up of positively charged particles, called protons, and
neutrons, which have no electric charge. The nucleus
is surrounded by negatively charged electrons. Atoms
are electrically neutral because they contain the same
number of electrons as protons (Figure 2-2). (However,
atoms can gain or lose electrons to become ions, which
have an overall negative or positive charge.)
FIGURE 2-230
All atoms of the same element contain equal numbers
of protons and electrons. This carbon atom contains six
protons and six electrons.
A substance composed of only one kind of atom is
called an element. Each element in the periodic table
has a characteristic atomic number, which is equal
to the number of protons in the nucleus of an atom of
that element. Hydrogen, which has an atomic number
of 1, contains a single proton in its nucleus, orbited by
a single electron. Likewise, all carbon atoms (atomic
number 6) contain six protons and six electrons. Atoms
with the same number of protons but different numbers
of neutrons are known as isotopes of an element.
Atomic Energy Levels
In 1913, Niels Bohr proposed a model of the hydrogen
atom, now known as the Bohr atomic model. At the
time, experimental evidence supported a model of
the atom in which electrons revolved around a central
nucleus. Bohr proposed that the electrons could only
occupy orbits of certain radii, corresponding to fixed
energy levels (Figure 2-3). Bohr indexed these levels
according to a quantum number that could take the
whole number values n = 1, 2, 3, and so on. A staircase
serves as a useful analogy for the energy levels in
the Bohr model. Just as it is not possible for an object
to reside in between two steps, an electron cannot
occupy a state between two energy levels. (Today, this
is a fundamental concept in a field of physics called
quantum mechanics.)
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parallax (in arc seconds)
Thus, stellar parallax decreases as the distance to a star
increases. As an example, the closest star to the Sun is
Proxima Centauri, which has the largest known stellar
parallax, 0.77”. Proxima Centauri is therefore 1.3 pc
(1/0.77”), or 4.3 light-years from the Sun, which equals
about 40 trillion km. With instruments we have today,
stellar parallaxes can be measured down to about
0.01”, corresponding to a distance of 100 pc. Only a
small fraction of visible-to-the-eye stars are within
this distance or have accurately measured parallaxes.
Other indirect methods must be used to determine the
distances to the great majority of stars beyond 100 pc.
distance (in parsecs) =
The Bohr model of the hydrogen atom includes fixed orbitals
(left) with corresponding energy levels (right). Only the first
three levels are shown.
The lowest allowable energy level of an atom is known
as the ground state. In this state, the electron is also
closest to the nucleus. Energy levels that are higher
than the ground state are called excited states. Each
element has a characteristic set of energy levels that
are common to every atom of that element. You can
think of this characteristic set of energy levels as
arising from the unique configuration of electrons and
protons for each atom.
Electrons must accept energy in order to move from one
energy level to a higher one. The process of an electron
moving from a lower to a higher energy level is called
excitation, which occurs when the atom absorbs energy
corresponding exactly to the difference between two
particular energy levels. This energy input can result
from absorbing light but can also come from the energy
due to collisions with other particles or from thermal
energy from being heated. Once they are in an excited
state, electrons can drop to lower energy levels by
emitting a packet of radiation called a photon that has
An atom emits a packet of electromagnetic radiation called
a photon when an excited electron drops to a lower energy
level. The energy of the photon is equal to the difference in
energy between the two levels.
both particle- and wave-like properties. The energy of
the photon is equal to the difference in energy between
the two levels.
Emission Spectra
Emission lines are produced when excited electrons
transition from higher energy levels back down to
lower energy levels. The frequency of the light emitted
is directly proportional to the energy difference
between the energy levels. Since every element has a
unique set of energy levels determined by its particular
configuration of electrons, each element produces its
own unique set of brightly colored emission lines.
The characteristic set of emission lines emitted by
a collection of excited atoms is called an emission
spectrum. (The plural of spectrum is spectra.)
Emission spectra can be viewed with a measurement
device called a spectroscope, which uses a prism or
grating to separate light emitted by a collection of
excited atoms into component wavelengths. Figure
2-5 shows a possible configuration for a spectroscope,
in which the emitted light is passed through a thin slit
and projected onto a viewing screen. Each component
color is refracted to a definite position on the screen,
forming a distinct image of the slit as a narrow line.
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The modern view of the atom is more complex than
the one Bohr proposed. Although each electron orbital
does have a precise energy level, the electron is now
envisioned as being smeared out in an “electron cloud”
that surrounds the nucleus. It is common to speak of
the average distance from the cloud to the nucleus as
the “radius” of the electron’s orbital. For illustrative
purposes, we will continue to use solid lines to represent
electron orbitals in this section, but bear in mind that
this represents a simplified model of the atom.
FIGURE 2-531
The emission spectra for an incandescent source, hydrogen,
and iron.
one another and with the nucleus, creating many more
energy levels and thus many more possible transitions.
Iron, which has an atomic number of 26, contains
26 electrons. Notice that iron has many more visible
spectral lines than hydrogen due to its more complex
electronic configuration.
A basic spectroscope apparatus using a prism and a
narrow slit in cardboard (top). Notice the difference in
emission lines from an incandescent light bulb (middle) and
fluorescent bulb (bottom).
Figure 2-6 shows emission spectra for an incandescent
light source, a hydrogen lamp, and a collection of
excited iron atoms. The emission spectrum for hydrogen
contains four distinct lines, which occur as a result of
energy level transitions that emit photons in the visible
The more electrons an atom contains, the more
complicated the energy level structure becomes.
The hydrogen spectral lines are relatively simple,
containing only a few visible lines; this is because
hydrogen contains only a single electron. For atoms
with larger atomic numbers, the electrons interact with
Electrons can move to higher energy levels by
absorbing electromagnetic radiation. An absorption
spectrum is created by passing a continuous spectrum
of electromagnetic waves through a collection of
atoms. What emerges from the atoms is a continuous
spectrum except for black lines, called absorption
lines, where specific frequencies of light have been
absorbed. Figure 2-7 shows a segment of the emission
and absorption spectra for hydrogen. Notice that the
positions of absorption lines exactly correspond with
the emission lines for the same element.
Only certain frequencies of light can be absorbed by
an individual collection of atoms, and therefore each
energy level diagram is unique for each type of atom.
We can therefore think of these spectral lines as a sort
of “fingerprint” that indicates the presence of a specific
element. The process of analyzing spectral lines to
identify the chemical makeup of an excited sample
is called atomic spectroscopy. Atomic spectroscopy
became a widely used chemical analysis technique in
the late 1800s.
The emission spectrum of the Sun is not perfectly
continuous—instead, there are thin lines where certain
frequencies of light are missing. These absorption
lines are known as Fraunhofer lines after Joseph von
Fraunhofer who first discovered them and determined
their wavelengths. Fraunhofer lines occur because the
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Absorption Spectra
Spectral Classes
Emission spectrum (top) and absorption spectrum (bottom)
for hydrogen.
light emitted from the body of a star is absorbed by the
atmosphere of cooler gases that surround it. In 1868,
scientists deduced that a set of then-unknown spectral
lines from the Sun belonged to a yet undiscovered
element, which they called helium. Thus, solar atomic
spectroscopy revealed helium to be a new element
almost three decades before it was first found on Earth.
Since that time, astronomers have catalogued thousands
of dark lines in the Sun’s spectrum. By comparing
these lines with the spectral lines produced by different
chemical elements on Earth, they have detected more
than seventy different chemical elements in the Sun.
Absorption spectra are used to classify stars into
nine principal types, known as spectral classes.
In comparing the stellar spectra of various stars
(Figure 2-8), we notice some similarities but also key
differences. For example, hydrogen absorption lines
appear much stronger in the spectra of some stars than
in others. Astronomers once mistakenly concluded that
these stars had more hydrogen than other stars and
classified them by the strength of the hydrogen lines in
their spectra, in alphabetical order, from the strongest
(called Class A) to the weakest (called Class Q).
Between 1911 and 1915, American astronomer Annie
J. Cannon developed a modified classification system
by examining the spectra of 225,300 stars. Cannon’s
system of spectral classes is still in use today: O B A
F G K M L T. (Cannon’s original classification system
included only O through M, but more recently the L
and T classes were added in order to classify dwarf
stars cooler than class M stars.)
FIGURE 2-832
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The Doppler Effect
If you have ever stood near a train as it passes, you
may have experienced how the sound of the whistle
seems to become lower in pitch at the moment the train
moves past your position. Although the frequency of
the whistle has not changed, the sound waves that reach
your ears are shifted lower in frequency. This frequency
shift due to relative motion between the source and
observer, known as the Doppler effect, also applies to
light. Because we know the exact frequencies of spectral
lines due to measurements here on Earth, we can
compare them with spectral lines from distant objects to
determine the relative shift due to the Doppler effect. If
the spectral lines are shifted toward the red side of the
spectrum (i.e., lower in frequency), we know the star or
galaxy is moving away from Earth.
Stellar spectra for each spectral class.
All visible stars are roughly uniform in composition,
composed mainly of hydrogen and helium. American
astronomer Cecilia Payne-Gaposhkin showed that the
differences in the dark line patterns of stars are due
primarily to their vastly different surface temperatures.
The sequence of spectral classes corresponds to a
sequence of surface temperatures. The O stars are
hottest, with the temperature continuously decreasing
down to the coolest T stars. Each spectral class is
arranged in ten subclasses numbered 0 to 9, also in
order of decreasing temperature.
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Gagarin’s mission was a historic milestone in space exploration.
President Kennedy joined other world leaders in congratulating
the Soviet Union for their “outstanding technical achievement.”
At the same time, however, the mission presented further
evidence of significant Soviet advantages in the Space Race and
increased pressure on the United States to increase its launch
The front page of the Huntsville, AL
newspaper from April 12, 1961, announcing
Gagarin’s successful mission.
Astronomers distinguish a star’s apparent brightness—
the way the star appears in the sky from Earth—from its
luminosity, which is the actual amount of energy a star
emits into space each second. Luminosity is an intrinsic
property of a star and does not depend on an observer’s
location. Thus, we cannot tell simply by looking at stars
in the sky which ones have the greatest luminosity. The
luminosity of other stars is often reported in terms of
the Sun’s luminosity (L), which is equal to 3.86 × 1026
watts. In other words, the Sun’s luminosity is equivalent
to 3,860 billion trillion 100-watt light bulbs all shining
together. The most luminous stars are over a million
times as luminous as the Sun, and the dimmest known
stars are less than 0.0001 times the luminosity of the
Sun. The star Deneb in the constellation Cygnus is
about 60,000 times more luminous than the Sun, yet it
is so distant that it appears less bright than other stars in
the night sky.
A star’s apparent brightness B depends on both its
luminosity L and the distance d between the star and the
observer. The equation relating these quantities is
B = L/4πd2. Light spreads out uniformly in all directions
from a source so that the amount of starlight shining
on a unit area falls off as one over the square of the
distance away from the star. This relationship is called
the inverse square law (Figure 2-10). Our Sun has an
exceptionally high apparent brightness because of its
proximity to us relative to other stars. If the Sun were
located 100,000 times more distant from us in space, it
would appear (100,000)2 = 10,000,000,000 (10 billion)
times fainter.
Apparent and Absolute Magnitude
Apparent magnitude is a measure of how bright a
star appears to an observer on Earth. The magnitude
scale was invented by the ancient Greek astronomer
Hipparchus around 150 bce. Hipparchus ranked the
stars he could see in terms of their brightness, with
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Prior to beginning manned space missions, the Soviet Union
began testing spacecraft carrying life-size human models and
animal passengers. Yuri Gagarin, a twenty-seven-year-old senior
lieutenant in the Russian Air Force, was selected to pilot the first
manned mission to space. Because the effects of zero gravity
on human pilots were not understood at the time, Gagarin’s
spacecraft had limited onboard controls and was piloted from
the Soviet ground control. Gagarin spent 108 minutes in space
and completed one full orbit of the Earth. The Vostok 1 was not
designed to land, so Gagarin ejected from the spacecraft upon
reentry and landed safely with parachutes.
stars. What we see as a linear increase in brightness (a
difference of one magnitude) is precisely measured as
a geometrical increase in brightness (the fifth root of
100, or 2.512 times brighter). Magnitude differences
between stars measure the relative brightness of the
stars. The most negative magnitude numbers identify
the brightest objects, while the largest positive
magnitude numbers identify the faintest objects.
An illustration of the inverse-square law as it relates to
apparent brightness. As the distance from a source of light
increases, the power radiated from that source is spread
over an increasingly large area.
1 representing the brightest and 6 representing the
faintest. Modern astronomy has extended this system
to stars brighter than Hipparchus’ first-magnitude stars
and ones much, much fainter than 6.
Absolute magnitude is a measure of luminosity, or
how much light a star is actually radiating into space.
If you could line up all stars at the same distance from
Earth, you could see how they differ in their intrinsic, or
“true,” brightness. Astronomers define a star’s absolute
magnitude as the apparent magnitude the star would
have if it were observed from a standard distance of 10
parsecs. With the effects of distance canceled out, we
can use absolute magnitude comparisons to determine
differences in the actual light output of stars.
If a star is farther than 10 parsecs from us, its apparent
FIGURE 2-1134
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The modern magnitude scale defines a firstmagnitude star to be exactly a hundred times brighter
than a sixth-magnitude star. This ratio agrees with the
way our eyes respond to increases in the brightness of
Full moon
Alpha Centauri
Andromeda Galaxy
Apparent and absolute magnitudes for various celestial
A reproduction of the original Hertzsprung-Russell diagram
from their 1914 paper. Each point represents the data for
a single star. Absolute magnitude is plotted on the vertical
axis versus spectral class on the horizontal axis.
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In the speech, Kennedy acknowledged the monumental costs that would be involved in achieving this goal.
“No single space project in this period will be more impressive to mankind, or more important for the longrange exploration of space; and none will be so difficult or expensive to accomplish.” Kennedy requested
billions of dollars in new government funding over the next decade to support the necessary operational and
technological advances this mission would require.
President Kennedy’s speech was ultimately persuasive to the nation and Congress, who approved the
unprecedented federal investment toward scientific achievement. The speech is remembered as a turning
point in marshaling national support for the American space program, placing the United States on a stronger
footing in the Space Race.
magnitude is numerically bigger than its absolute
magnitude. (Large positive magnitude numbers
indicate faint objects.) For example, Polaris is 130
pc away. Its apparent magnitude is +2.0, whereas its
absolute magnitude is –3.6. On the other hand, if a star
is closer than 10 parsecs, its apparent magnitude is
numerically smaller than its absolute magnitude. Thus,
Sirius (the brightest star in the Northern Hemisphere’s
night sky) is 2.6 pc away. Its apparent magnitude is
–1.4, whereas its absolute magnitude is only +1.5.
The Hertzsprung-Russell Diagram
A basic relationship between the luminosities and
temperatures of stars was discovered early in the
twentieth century by two independent astronomers,
Henry N. Russell of the U.S. and Ejnar Hertzsprung of
Denmark. The Hertzsprung-Russell (H-R) diagram
is a plot of luminosity versus temperature for a group
of stars. Every dot on an H-R diagram represents a
star whose temperature (spectral class) is read on
the horizontal axis and whose luminosity (absolute
magnitude) is read on the vertical axis (Figure 2-11).
As Hertzsprung and Russell plotted more and more
stellar temperatures and luminosities in this manner,
they found that stars are not uniformly scattered
across the H-R diagram. Instead, most are confined to
a fairly well-defined band stretching diagonally from
top left (high-temperature, high-luminosity) to bottom
right (low-temperature, low-luminosity). This pattern
indicates that a meaningful connection exists between
a star’s luminosity and its temperature. To put this into
words, cool stars tend to be faint (less luminous), and
hot stars tend to be bright (more luminous). This band
of stars spanning the H-R diagram is known as the
main sequence, which includes roughly 90 percent of
all stars.35
Red dwarfs are relatively small, cool main sequence
stars that can be found in the lower right corner of the
H-R diagram. Giant stars can be found off the main
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On May 25, 1961, in a speech to a joint session of Congress,
President John F. Kennedy set forth an ambitious objective for
the country: “I believe that this nation should commit itself to
achieving the goal, before this decade is out, of landing a man
on the moon and returning him safely to the Earth.” Kennedy’s
speech came at a time when the Soviet Union had a perceived
advantage in the Space Race, having achieved the first human
spaceflight only a month earlier. In preparing the announcement,
Kennedy had consulted with NASA Administrator James
President John F. Kennedy announces a goal
E. Webb and Secretary of Defense Robert McNamara, who
of reaching the moon in his May 25, 1961,
recommended that a lunar landing was an objective that the
speech to Congress.
United States could conceivably achieve before the Soviet Union.
Structure and Composition
Our understanding of the structure of the Sun (and
other stars) comes from direct observations of its outer
layers and theoretical calculations of the behavior
of gases deep inside that we cannot see. The three
outermost layers of the Sun are the photosphere, the
chromosphere, and the corona. The photosphere, from
the Greek photos for “light,” is the visible surface of
the Sun. The photosphere is a hot, opaque gas layer
with a temperature around 5,800 K (10,000° F) from
which energy is radiated into space. The photosphere
is about 500 km thick, less than 0.1 percent of the
Sun’s radius, which is why the Sun appears to have a
well-defined edge. The limb is the apparent edge of the
FIGURE 2-1336
Sun’s disk. It appears darker than the center, an effect
called limb darkening, because light from the limb
comes from higher, cooler regions of the photosphere.
The chromosphere, from the Greek chroma for “color,”
is a thin, transparent layer that extends about 10,000
km (6,000 miles) above the photosphere. Normally
the chromosphere is only visible from Earth during a
total eclipse of the Sun, when it glows red due to the
hydrogen gas it contains. The temperature rises sharply
as one moves outward through the chromosphere,
reaching temperatures of around 20,000 K.
The corona, from the Latin for “crown,” is the
outermost atmosphere of the Sun just above the
chromosphere. It is a rarified, hot gas that extends
many millions of kilometers into space. Because of its
high temperature—up to three million K in the outer
part—the corona emits high-frequency radiation in the
form of X-rays. During a total eclipse of the Sun, the
corona is prominently visible as a jagged white halo
around the briefly hidden photosphere (Figure 2-14).
The Sun’s interior resides within the photosphere.
Without any direct measurements of the interior of the
Sun, astronomers construct mathematical models that
fit the data from indirect observations. According to the
standard solar model, the temperature and density of
the Sun increase inward from the surface. Deep inside,
FIGURE 2-1437
The structure of the Sun.
A total solar eclipse, photographed in 2017 above Madras,
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sequence in the upper right of the H-R diagram. These
stars are cool but relatively luminous, which means
they must be larger in size than main sequence stars
of the same temperature. A giant star has a radius
between 10 and 100 times that of the Sun. Even larger
stars, with radii 100 to 1000 times that of the Sun,
are called supergiants. In the lower left corner of the
H-R diagram are white dwarfs, which are stars with
high temperatures but low luminosities due to their
relatively small size.
The core is the most interior region of the Sun. The
core is the “power plant” where nuclear fusion reactions
generate the Sun’s energy. The intense energy released
in the core provides heat inside the Sun and enough
pressure to balance the inward pull of gravity. Energy
from the core propagates outward through the other
interior layers of the Sun. In the radiation zone, light is
repeatedly absorbed and re-emitted at lower energies.
From there, circulating currents of gas in the convection
zone transfer most of the energy as heat to the outer
layers. Light that reaches the photosphere is able to
escape into space. It can take hundreds of thousands
of years for energy produced in the core to reach the
surface and become sunlight.
The Sun rotates on its axis from west to east, as the
Earth does. However, the Sun does not rotate as a
solid body—instead, the rate of rotation varies by
latitude. The period of rotation, or the length of time
for one complete turn, is shortest at the Sun’s equator
(about twenty-five days), longer at middle latitudes,
and slowest at the poles (about thirty-five days). This
strange rotation pattern probably contributes to the
violent activity that takes place on the Sun, described
in the sections that follow.
The nebular theory, first proposed by German
philosopher Immanuel Kant, says that our Sun and
its planets formed together from a rotating cloud
of interstellar gas and dust called the solar nebula.
The solar nebula condensed into the newly forming
Sun, encircled by a rotating disk of gas and dust out
of which the planets, moons, and other solar system
objects formed. The Sun has more than 99 percent
of the mass of the solar system and provides the
gravitational force that keeps the planets in orbit
around it. Its surface gravity is approximately twentyeight times that of the Earth.
More than seventy chemical elements have been
identified in the Sun’s spectrum. The Sun’s outer layers
likely have the same chemical composition as the Sun
FIGURE 2-1538
This large sunspot, observed on Oct. 18, 2014, was about
129,000 km (80,000 miles) across.
had at birth: about 71 percent hydrogen, 27 percent
helium, and 2 percent other elements by weight. The
Sun’s core has likely subsequently changed to about 38
percent helium due to nuclear fusion reactions.
Sunspots and Activity Cycles
Sunspots are temporary, dark, relatively cool blotches
that exist on the Sun’s bright photosphere (Figure 2-15).
Sunspots usually appear in groups of two or more,
and often occur in pairs of opposite magnetic polarity.
Individual sunspots can last anywhere from a few hours
to a few months. The largest sunspots are visible at
sunrise or sunset or through a haze. Observations of
sunspots were first recorded in China before 800 bce. A
typical sunspot is roughly twice the size of Earth. The
largest sunspots may be larger than ten Earth diameters.
Although sunspots appear dark, in actuality they
shine brighter than many cooler stars. They look dark
only in comparison to the much brighter surrounding
photosphere. The temperature of a sunspot is about
4,200 K in the umbra, or center, of the sunspot. The
penumbra, or outer gray part of a large spot, is a few
hundred degrees cooler than the photosphere. Sunspots
almost always appear in pairs, corresponding to
opposite magnetic polarities connected by magnetic
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the temperature rises to 15 million K, the pressure to
200 billion Earth atmospheres, and the density to over
a hundred times that of water. No known element can
exist in a solid or liquid state at these extremely high
solar temperatures. Instead, the Sun is composed of
very hot gases throughout. The Sun’s temperatures are
so great that electrons can actually separate away from
atoms, forming a gas of ions called a plasma.
field lines. The first telescopic observations of sunspots
and their motions, reported by Galileo in 1610, had
a historic impact. Galileo correctly concluded that
the rotation of the Sun transports sunspots across its
FIGURE 2-1640
At any one time, more than three hundred sunspots—
or none at all—may appear on the Sun’s disk. The
number of sunspots regularly rises to a maximum and
falls to a minimum in an approximately eleven-year
cycle, known as the sunspot cycle. The sunspot cycle
is watched carefully from Earth because it coincides
with the solar activity cycle. The Sun is most active,
with the greatest outbursts of energy and radiation, for
about 4.8 years during which sunspots are increasingly
numerous. After sunspot maximum, the number of
sunspots decreases for about 6.2 years to a sunspot
minimum as solar activity decreases. The most recent
solar activity cycle, Solar Cycle 24, began in 2008.39
Other Solar Activity
Other solar activity can be much more violent than
sunspots. Occasionally, eruptions of energetic particles
occur on the photosphere, most often during the most
active periods of the solar cycle. Locations on the Sun’s
surface where these eruptions take place are known
as active regions. Solar prominences, one form of
solar activity, are fiery arches of ionized gases that
move upward through the corona under the influence
of the Sun’s magnetic field. A typical solar prominence
can span 100,000 km—almost ten times Earth’s
diameter—and last for days or weeks.
A solar flare is a sudden, explosive outburst of
radiation from all parts of the spectrum and highvelocity particles from the Sun. A single large solar
flare may release as much energy as the whole world
uses in 100,000 years. Solar flares tend to be shortlived, in many cases lasting only a few minutes.
However, the longest solar flares last a few hours. Solar
flares seem to be energized by strong local magnetic
fields. A flare usually follows the most energetic of all
solar eruptions, a coronal mass ejection, which blasts
plasma out from the corona (Figure 2-16).
Electromagnetic radiation such as gamma rays, X-rays,
visible radiation, and ultraviolet rays reach Earth from
the Sun in just 8.3 minutes. Particles released during
solar flares arrive a few hours or even days later.
A coronal mass ejection that occurred on May 1, 2013.
Earth’s magnetic field and atmosphere shield us from
these particles. However, exposure to solar radiation
can be hazardous for people and instruments at high
altitudes, such as airplane passengers, astronauts, and
spacecraft electronics.
When electrically charged particles from the Sun collide
with particles in Earth’s atmosphere, they can excite the
atmospheric atoms and ions to higher energy levels. The
resulting de-excitation of these atoms and ions causes
the phenomena known as aurora borealis (Northern
lights) and aurora australis (Southern lights). Oxygen
atoms emit a pale green color, nitrogen molecules
produce red-violet light, and nitrogen ions emit blueviolet light. In addition to visible light, auroras also emit
infrared, ultraviolet, and X-ray radiation. Auroras are
typically visible only in the Arctic and Antarctic regions
but occasionally also down to middle latitudes about
two days after a solar flare. Auroras reach a peak about
two years after sunspot maximum.
Strong blasts of electrically charged solar particles
interact with Earth’s magnetic field and disturb it,
causing geomagnetic storms, which can interfere
with the operation of compasses and other magnetic
instruments. Particularly strong storms have resulted
in satellite damage, surges in electric power and
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Solar Flares, Prominences, and Coronal Mass
The United States’ first human spaceflight program was known as
Project Mercury. The first manned Mercury mission was piloted
by Alan Shepard, who achieved spaceflight on May 20, 1961, but
did not reach orbit. A similar mission was flown by Virgil “Gus”
Grissom on July 21. By the end of 1961, the United States was
nearing the capability of a manned orbital mission. The Mercury
5 mission in November 1961 successfully launched and landed
a chimpanzee passenger, and a manned orbital mission was
scheduled for early the following year. John Glenn was selected as
the pilot for the Mercury 6 mission.
FIGURE 2-1743
John Glenn entering his spacecraft, Friendship
7, in advance of his successful orbital mission.
Project Mercury continued for several years following Glenn’s flight. Astronaut Scott Carpenter replicated
Glenn’s three-orbit flight in May 1962, and Wally Schirra undertook six orbits in October 1962. The final
Mercury mission was a twenty-two-orbit flight completed by Gordon Cooper in May 1963.
telephone lines, and blackouts.41 By increasing
ionization, solar outbursts can also disrupt radio
transmission and navigation signals. Because of the
potential impact of solar flares, researchers with the
U.S. National Oceanic and Atmospheric Association
(NOAA), along with partners worldwide, monitor the
Sun’s magnetic field and activity daily. The NOAA
Space Weather Prediction Center issues space weather
alerts, warnings, and forecasts.42
High-energy radiation from solar flares can also heat
the upper atmosphere, causing it to expand. This
expansion increases the effects of friction and drag on
spacecraft in low-Earth orbits. As a result, increased
drag during times of maximum solar activity can
cause satellites to plunge from orbit and be destroyed
on reentry. Skylab, the first U.S. space station, was a
casualty of a solar maximum. Skylab’s orbit decayed
faster than expected due to increased drag due to solar
activity and plummeted to Earth in 1979.
Solar Wind
The solar wind is a stream of energetic, electrically
charged particles that flows outward from the Sun
at all times. It is much faster, thinner, and hotter
than any wind on Earth. The solar wind is detected
by instruments carried on spacecraft above Earth’s
atmosphere. Near Earth, the average solar wind speed
is about 450 km/second (1 million miles/hour). Earth’s
atmosphere and magnetic field provide protection from
the potentially harmful effects of the solar wind.
The solar wind is most intense during periods when
many sunspots are visible and solar activity is at a
maximum. Particularly large emissions of solar wind
occur during coronal mass ejections. Strong blasts of
solar wind produce especially brilliant auroras. The
solar wind comes mainly from coronal holes, which
are regions in the Sun’s corona where gases have a
much lower density than neighboring areas. Magnetic
fields are relatively weak at coronal holes, allowing
high-speed solar wind streams to escape.
Life Cycles of Stars
Stellar evolution refers to the changes that stars undergo
as they age—in other words, the life cycle of stars.
These changes cannot be observed directly because
they take place over millions or even billions of years.
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On February 20, 1962, Glenn’s spacecraft, Friendship 7, launched
from Cape Canaveral, Florida. Glenn completed three full orbits before splashing down in the north Atlantic
Ocean, for a total flight time of just over 4 hours and 55 minutes. Glenn and the spacecraft returned safely,
marking a key milestone in the Unites States space program and providing encouragement that the U.S. could
regain ground in the Space Race.
Astronomers construct a theory of stellar evolution that
is consistent with their observations and other physical
laws. Then they check their theory by comparing the
predictions of their theory with observations of stars at
various points in their lifetimes.
the protostar becomes a newborn star. Our Sun was
mostly likely formed in this manner about 4.6 billion
years ago. Infrared images of protostars within dense
cores of gaseous clouds allow us to see the jets of gas
that are often seen streaming away from young stars.
An important tool in tracking stellar evolution is
the Hertzsprung-Russell (H-R) diagram, which was
described earlier in this section. Astronomers make
predictions regarding a sequence of changes in
luminosity and temperature that stars will undergo
as they age from birth to death. These changes are
then plotted on an H-R diagram, forming theoretical
“tracks” of evolution. Theoretical H-R diagrams can
then be compared with H-R diagrams constructed
from measurements of groups of real stars.
Why Stars Shine
A protostar is a star in its earliest observable phase of
evolution. You can think of a protostar as a star that is
being born. Protostars are formed from high-density
clumps inside huge turbulent clouds of dust and gas
(mostly hydrogen). A shock wave from an exploding
star (supernova) may trigger the formation process.
Protostars are held together by the force of gravity.
Initially, the force of gravity pulls matter toward the
center of a dense clump, causing it to contract and
become even denser. Matter continues to accrete onto
the protostar as it contracts. Gravitational contraction
of the cloud and protostar causes the temperature and
pressure inside to rise.
Heat flows outward from the protostar’s hot center
to its cooler surface, and the protostar radiates this
energy into space at infrared wavelengths. In a rotating
cloud, a disk of dust and gas may surround a protostar.
This disk also re-radiates the energy as infrared
radiation. Possibly particles in the disk accrete to form
planets. When the temperature in the protostar’s center
reaches 10 million K, nuclear fusion reactions start.
These nuclear reactions release tremendous amounts
of energy. Energy is generated in the center as fast as it
is being radiated out into space. The very high internal
temperatures and pressures are thus maintained.
Eventually, the outward pressure due to the expansion
of hot gases within the protostar balances the inward
pull of gravity. This balance is called hydrostatic
equilibrium. Once it has reached hydrostatic
equilibrium, the protostar stops contracting and
radiates its own light steadily into space. At this point,
FIGURE 2-1844
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Birth of Stars
Main sequence stars can be thought of as “adult” stars.
In comparison to the rate of change that protostars
undergo, evolution of main sequence stars is very
slow. A star spends the majority of its lifetime shining
steadily, with luminosity and temperature values found
along the main sequence of H-R diagrams. A main
sequence star gets its energy from a sequence of nuclear
fusion reactions in which hydrogen at the center of the
star is converted into helium (Figure 2-18). During this
Gamma Ray
An illustration of the sequence of nuclear fusion processes
that produces energy in stars. Individual protons combine
with hydrogen nuclei to form more massive nuclei, releasing
energy in the form of gamma rays.
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FIGURE 2-1946
Project Gemini was NASA’s second human spaceflight program
and the successor to the Mercury program. The objective of
the Gemini program was to develop and improve space travel
capabilities that would be necessary for the Apollo mission
to land on the Moon. The Mercury program had established
that a single astronaut could undertake a spaceflight of several
hours. The Gemini missions would demonstrate that teams of
astronauts could perform on missions lasting several weeks.
In November 1966, the final Gemini mission, Gemini 12, demonstrated that astronauts could work effectively
outside the spacecraft for extended periods. By the end of the Gemini program, NASA had developed many
of the necessary techniques and technologies for the Apollo lunar missions. Many of the Gemini astronauts,
including Neil Armstrong, Edwin “Buzz” Aldrin, Michael Collins, and Jim Lovell, would go on to take part
in Apollo program missions.
sequence, known as the proton-proton chain reaction,
four hydrogen nuclei are fused into one lighter, helium
nucleus. The mass “lost” from the initial protons to
the final nucleus is converted into energy and released.
There are other more exotic nuclear fusion reactions that
allow for the creation of elements as massive as iron.
The energy from the nuclear fusion reactions
eventually reaches the star’s surface, where it is
released in the form of electromagnetic radiation. The
amount of energy released in a single nuclear fusion
reaction can be calculated from Albert Einstein’s
famous equation E = mc2, where E represents the
energy released, m represents the difference in mass
before and after the reaction, and c equals the speed
of light. According to Einstein’s equation, when many
nuclear fusion reactions occur together, enormous
amounts of energy are released.
Nuclear fusion reactions occur at a tremendously high
rate in order to continually supply a star’s energy
output. Every second, around 1038 reactions occur
within the Sun, converting about 4 million tons of
matter into energy at the same rate. Nevertheless, the
Sun is so massive that this extremely rapid output of
energy results in effectively no appreciable change in
its size or temperature. Over the course of its estimated
lifetime of 12 billion years, the Sun will convert only
around 0.1 percent of its mass into energy.45
A supernova is a large, violent explosion that takes
place at the end of a star’s life cycle. Supernovae
occur due to changes in the core of a star. There are
two primary ways these changes can happen, each
corresponding to a different type of supernova. Type
I supernovae occur in binary-star systems, which
are two stars that orbit the same point. One of the
stars, a carbon-oxygen white dwarf, pulls matter onto
itself from its companion star. Eventually, the white
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The Gemini 4 mission, which took place in June 1965, involved
the first spacewalk by an American astronaut, Edward “Ed”
Gemini 4 astronaut Ed White undertaking a
White. The following mission, Gemini 5, set a new space
spacewalk in June 1965.
endurance record at eight days thanks to new fuel cells capable
of powering longer missions. This was notable because a lunar
mission would be expected to take about eight days. The later Gemini missions were dedicated to perfecting
docking capabilities between spacecraft. Gemini 8 accomplished the first docking of two spacecraft in orbit
but suffered a system failure that necessitated an early return to Earth.
dwarf accumulates too much matter. As the white
dwarf becomes more massive, the internal pressure
required to support its weight increases. If the dwarf’s
mass exceeds the limit beyond which it can no longer
maintain equilibrium, it will collapse inward and
rapidly undergo carbon fusion before exploding.
FIGURE 2-2047
Type I and Type II supernovae affect different types of
stars. All high-mass stars eventually become Type II
supernovae, but only a small fraction of low-mass stars
become white dwarfs that eventually undergo a Type I
supernova explosion. Although our Sun is a single star,
it is not massive enough to become a Type I supernova.
Neutron Stars and Pulsars
After a very massive star explodes, it can collapse
inward into an extremely dense star composed almost
entirely of neutrons, which are uncharged atomic
particles. This is known as a neutron star. A neutron
star with the Sun’s mass could be packed into a sphere
that is only about 16 km across. Neutron stars tend to
have high rotational speeds, as a result of conservation
of angular momentum, and also have very strong
magnetic fields.
A pulsar is a rapidly rotating, highly magnetic neutron
star. Pulsars were first observed in 1967 by Jocelyn
Bell, a graduate student at Cambridge University—
work for which her advisor Anthony Hewish won the
1974 Nobel Prize. Pulsars emit sharp, strong bursts
of radio waves to Earth with clocklike regularity, at
intervals between milliseconds and four seconds.
The characteristic short, regular pulses come from
radiation beams, emitted by very energetic accelerated
charged particles, sweeping past Earth as the neutron
star rotates. The rotation and pulse rates of pulsars
gradually slow down as energy is radiated away.
Astronomers predicted that a neutron star should exist
at the center of the Crab Nebula, and in 1968 a pulsar
was detected there (Figure 2-20). The Crab Pulsar
An X-ray image of the Crab Nebula captured by the
Chandra X-ray Observatory. The white dot near the center
is a pulsar.
has since been observed across all electromagnetic
wavelengths, from low-frequency radio waves to highfrequency gamma rays.
Black Holes
A very massive star may continue to collapse after
the pulsar stage to become an even more tightly
compressed object called a black hole. A star will form
a black hole if it is sufficiently massive that the inward
force of gravity overcomes the resistance of neutrons
to become tightly packed together. According to stellar
evolution theory, any star whose main-sequence mass
is greater than about twenty-five times that of the Sun
will eventually become a black hole.48 Black holes are
in fact not “holes” at all—they are massive objects
with extremely small sizes and enormous densities.
The gravitational attraction of a black hole is so great
that, according to Einstein’s theory of relativity, even
light could not escape from within a given radius to the
center of the black hole. The “surface” of a black hole,
or in other words the boundary through which no light
can escape, is called the event horizon.
The Schwarzschild radius (RS) is the critical radius at
which a spherically symmetric body becomes a black
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The second type of supernova (Type II) occurs at
the end of a single star’s lifetime. As a massive star
begins to expend the supply of hydrogen in its core to
support nuclear fusion, the core becomes composed of
heavier and heavier elements and rapidly increases in
temperature. Eventually, the core becomes so heavy that
it can no longer withstand its own gravitational force.
The core collapses so rapidly that it actually “rebounds”
outward, resulting in the giant explosion of a supernova.
where G is the gravitational constant, M is the
mass of the body, and c is the speed of light. The
Schwarzschild radius for the Sun is about 3 km
whereas for Earth it is about 1 cm. Theory predicts that
a star of over three solar masses at its final collapse
must cross its event horizon and disappear from view.
No known force could stop further collapse, so the star
may continue to shrink to a point at the center called a
RS =
Cygnus X-1 is an intense X-ray source over 2,500 pc
(8,000 light-years) distant in the constellation Cygnus.
Cygnus X-1 was discovered in 1964 and was found to
be an eclipsing binary star system (period 5.6 days)
whose unseen component was the first black hole
reported. The visible primary star in Cygnus X-1 is
a blue supergiant that shows variations in spectral
features from one night to the next. The black hole
pulls in material from the star and heats it to millions
of degrees, causing the intense emission of X-rays we
observe from Earth.
The method of parallax can be used to
determine the distance to stars. The more
distant the star, the smaller the angle of parallax.
The measurement of the position, parallax, and
motion of stars is called astrometry.
Atoms are the building blocks of the world
around us. Each element has a characteristic
atomic number, which is equal to the number
of protons in the nucleus of an atom of that
element. Neutral atoms contain equal numbers
of protons and electrons.
Electrons occupy fixed energy levels within
atoms. The lowest energy level is known as the
ground state; higher energy levels are called
excited states.
When atoms become excited, electrons move
to higher energy levels. An electron can
transition to a lower energy level by emitting a
photon with energy equal to the difference in
energy between the two levels.
The characteristic set of wavelengths emitted
by a collection of excited atoms is called an
emission spectrum. Since every element has a
unique electronic configuration, the collection
of wavelengths emitted by an element is also
Stellar spectra, or the spectrums of stars, are
absorption spectrums. Absorption spectra are
used to classify stars into nine principal types,
known as spectral classes. The spectral classes,
from highest to lowest surface temperature,
are: O B A F G K M L T.
The Hertzsprung-Russell (H-R) diagram
is a plot of luminosity versus temperature.
H-R diagrams are a basic tool in astronomy,
allowing astronomers to classify stars and
refine theories. About 90 percent of all stars
lie along a diagonal band on the H-R diagram
known as the main sequence.
The three outermost layers of the Sun are the
photosphere, the chromosphere, and the corona.
The Sun’s interior resides within the
photosphere. No known element can survive
as a solid or liquid at the extremely high solar
temperatures within the sun. Elements within
the sun exist as superheated gases or plasma.
Sunspots are temporary, dark, relatively
cool blotches that exist on the Sun’s bright
photosphere. The number of sunspots regularly
rises to a maximum and falls to a minimum in
an approximately eleven-year cycle, known as
the sunspot cycle.
Other forms of solar activity include solar flares,
solar prominences, and coronal mass ejections.
The solar wind is a stream of energetic,
electrically charged particles that flows outward
from the Sun at all times. The solar wind is
strongest during periods when many sunspots
are visible and solar activity is at a maximum.
Stellar evolution refers to the changes that stars
undergo as they age over the course of millions
or billions of years. A protostar is a star in
its earliest observable phase of evolution.
Protostars become young stars once they reach
hydrostatic equilibrium.
A main sequence star gets its energy from a
sequence of nuclear fusion reactions where
hydrogen at the center of the star is converted
into helium. The energy from the nuclear
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hole. The equation for the Schwarzschild radius is:
fusion reactions eventually reaches the star’s
surface, where it is released in the form of
electromagnetic radiation.
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After a very massive star explodes, it can
collapse inward to form a neutron star, an
extremely dense star composed almost entirely
of neutrons. A pulsar is a rapidly rotating,
highly magnetic neutron star. A very massive
star may continue to collapse after the pulsar
stage to become a black hole.
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Section III
The Planets
Our Solar System is made up of eight planets, each
with different properties and appearances. Some
planets can be observed from Earth with our eyes
alone, while others have only been photographed by
space probes traversing billions of miles across the
Solar System. In this section, we will explore the
eight planets of our Solar System and describe their
individual features. We will also describe other smaller
bodies that make up our Solar System, such as dwarf
planets, comets, and asteroids.
FIGURE 3-149
About the Solar System
The Solar System is made up of the Sun and all
objects gravitationally bound to it. These objects
include planets, dwarf planets, moons, as well as
small solar system bodies such as asteroids (also
called minor planets), comets, Kuiper Belt objects,
and interplanetary dust and gas. According to the
International Astronomical Union (IAU), a planet is
defined as a celestial body that satisfies the following
three conditions:
The body orbits the Sun.
The body has sufficient mass to be nearly round.
The body clears the region around its own orbit.
(In other words, the body does not share its
orbital region with other bodies of similar size,
except its own satellites.)
According to this definition, there are eight planets
in the Solar System: Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, and Neptune. The planets
range in mass and size from lightest, smallest Mercury
to heaviest, largest Jupiter. All the planets together
represent only 0.001 of the mass and 0.3 of the volume
of the Sun (Figure 3-1).
The Solar System. The sizes of the Sun and planets are
correctly scaled, but distances between them are not.
Bodies that satisfy the first two planetary conditions
but not the third are known as dwarf planets. In other
words, a dwarf planet orbits the Sun and is massive
enough to be nearly round but does not clear the region
around its orbit. In 2006, Pluto was reclassified from
a planet to a dwarf planet. Although some planets are
visible in the night sky from Earth, planets do not
generate their own light as stars do. Rather, planets
shine by reflecting light from the Sun.
The Formation of the Solar System
According to the solar nebular model, the solar system
formed out of a rotating interstellar cloud about 4.6
billion years ago. The nebula contracted into the protoSun surrounded by a spinning disk where the planets
formed as dust and gas accreted. The newly formed
Sun blew away most residual gas and dust by emitting
a very intense solar wind. The solar nebular model is
supported by observations of the current solar system.
All the planets revolve, or orbit around, the Sun in
the same direction as this original interstellar cloud.
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FIGURE 3-250
The Apollo 1 crew: Gus Grissom, Ed White,
The next crewed spaceflight mission, Apollo 7, launched on
and Roger Chaffee.
October 11, 1968 and successfully tested the command and
service module (CSM), the cabin module that would eventually
carry astronauts to the Moon and back. In December 1968, Apollo 8 became the first manned mission to orbit
the Moon and return. The Apollo 8 crew conducted unprecedented observations of the lunar surface, and the
success of the mission was a powerful demonstration that a Moon landing was within reach. The Apollo 9
mission in early 1969 tested the docking capabilities of the CSM and demonstrated the life support system
that would sustain astronauts on the Moon.
The Apollo 10 mission served as a “dress rehearsal” for the Moon landing. During this mission, the crew tested
the equipment and procedures for landing on the Moon and practiced entering the 15.6 km approach orbit from
which a landing mission would begin descent. Apollo 10 orbited the Moon thirty-one times before returning
successfully to Earth, thereby setting the stage for the Apollo 11 mission and the first Moon landing.
This movement is called direct motion. The planets
also rotate as they revolve about the Sun. The rotation
of most planets (except for Venus and Uranus) is also
direct. The mean plane of Earth’s orbit around the
Sun is called the ecliptic. The orbits of all the planets
are in nearly this same plane, similar to car lanes on a
circular racetrack. The planets whose orbits are closer
to the Sun than Earth’s are called inferior, while those
whose orbits are outside Earth’s are called superior.
Earth’s Physical Properties and
Earth, the third planet from the Sun, is humanity’s
home and the planet with which we are most familiar.
The total surface area of our planet is about 5.10 × 108
km2 (199 million square miles). Over 70 percent of
Earth’s surface is covered by water, which is unique
among the other planets. The highest point on the
Earth’s surface is Mt. Everest, on the border between
Nepal and China in Asia, which rises 8,848 m (29,029
ft) above sea level. The deepest measured underwater
spot is the Marianas Trench, more than 11 km (36,000
feet) below the Pacific Ocean’s surface. Earth’s mass is
about 6 × 1024 kg.
Earth was formed about 4.6 billion years ago, from the
same contracting cloud of gas and dust that formed
the Sun and other planets. The oldest rocks discovered
so far on Earth, amid the remote lakes and tundra
of northwest Canada, have been dated to around 4
billion years old.51 It’s not possible to go inside the
Earth to directly observe its interior structure. Instead,
geologists indirectly determine the composition of
our planet by observing how seismic waves, which
result from earthquakes and other disturbances, are
transmitted through the Earth and along its surface.
Earth’s interior is divided into three main layers:
the crust, the mantle, and the core. The crust is the
thin, outermost, solid layer. The crust has an average
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The Apollo program was the third American spaceflight program,
with the specific goal of landing the first men on the Moon. The
first crewed mission of the Apollo program, Apollo 1, suffered a
tragic accident on February 21, 1967, in which a cabin fire took
the lives of crewmembers Virgil “Gus” Grissom, Ed White, and
Roger Chaffee during a prelaunch test. Manned spaceflights
were subsequently suspended for twenty-one months while
NASA investigated the causes of the accident and revised safety
procedures. During this time, unmanned test flights of the Saturn
V rocket and Lunar Module continued.
FIGURE 3-352
km (3,000 miles) in 200 million years. Movement of
the plates is also responsible for earthquakes, volcanic
activity, and the formation of mountain ranges. These
events occur at boundaries between the moving plates
where tremendous forces are exerted.
Earth has a magnetic field, or region of magnetic
forces, that affects compass needles. The Earth’s
magnetic poles are located at about 83°N latitude,
115°W longitude in northeast Canada—1300 km (800
miles) from the geographic North Pole—and at about
65°S, 138°E in Antarctica. The magnetic poles drift
some 40 km (25 miles) a year. Earth’s magnetic field
is generated by its liquid iron-nickel core, which acts
as a giant dynamo as the planet spins. The complex
motion of this metallic core likely causes the long-term
migration of the magnetic poles.
Earth’s magnetosphere, the region around the planet
where its magnetic field is influential, extends outward
into space in an asymmetric manner. On the side closer
to the Sun, the magnetosphere extends four times
Earth’s radius. On the side further from the Sun, the
magnetosphere extends like a tail to a distance of about
10 to 1,000 Earth radii. Earth’s magnetic field traps
many energetic, charged particles from the solar wind.
These particles move rapidly within two doughnutshaped regions in the magnetosphere known as the
Van Allen belts.
Earth’s Atmosphere
A cross-section of Earth’s interior layers.
Earth’s surface is constantly changing due to erosion
and geological activity. About 200 million years ago,
all the world’s continents were joined in one huge
supercontinent called Pangaea, which later broke apart
to form the continents we are familiar with today.
According to the theory of plate tectonics, also called
the continental drift theory, the continents and ocean
floor are embedded in plates, or rock slabs, several
thousand miles across. The plates move slowly on
the slightly yielding mantle beneath. Earth’s crust is
reshaped at plate boundaries. Where the plates move
apart, the continents separate slowly, at a rate of about
2.5 cm (1 inch) per year. That adds up to over 5,000
Earth is surrounded by an atmosphere that extends
several hundred kilometers out into space. The
composition of Earth’s atmosphere has evolved over
the course of millions of years due to processes such
as photosynthesis, in which green plants absorb carbon
dioxide from the air and release molecular oxygen
(O2). Today, Earth’s atmosphere contains about 78
percent nitrogen, 21 percent oxygen, and 1 percent
argon, carbon dioxide, and other gases. It also contains
trace amounts of water vapor, dust, carbon monoxide,
chemical products of industry, and microorganisms.
Over half of this air is packed within the first six km
(four miles) above Earth’s surface.
The layer of Earth’s atmosphere below about 12 km is
called the troposphere. Everything on Earth’s surface
lies within the troposphere, including Mt. Everest.
Above the troposphere, extending up to an altitude of 40
to 50 km, lies the stratosphere. Within the stratosphere
lies the ozone layer, where atmospheric oxygen, ozone,
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thickness of thirty-five km (twenty-two miles) and is
thicker at the location of continents and thinner under
the oceans. The crust is composed mainly of lightweight
rocks such as granite and basalt. The mantle is the layer
below the crust. The mantle extends about 2,900 km
(1,800) miles below the crust. Laboratory analysis of
samples from volcanoes indicates that the thick mantle
consists mostly of dense silicate rock that behaves
plastically, somewhat like taffy—yielding under steady
pressure but fracturing under impact. The central layer,
which is about 3,500 km (2,170 miles) thick, is called
the core. The core consists of an outer, molten, metallic
layer about 2,100 km (1,300 miles) thick surrounding a
solid center. The core is most likely made of dense iron
and nickel at a temperature of about 6,400 K.
The layers of Earth’s atmosphere.
and nitrogen absorb incoming ultraviolet radiation
from the Sun. Ozone is a molecule made up of three
oxygen atoms, which is formed in the atmosphere when
ultraviolet radiation interacts with oxygen molecules.
Its concentration is greatest at an altitude of around 25
km. The ozone layer serves as a shield from ultraviolet
radiation, which can have harmful effects on the animals
(including humans) and plants on Earth’s surface.
Between 50 and 80 km from Earth’s surface lies the
mesosphere. Above about 80 km, in the thermosphere,
the atmosphere is kept partly ionized by solar ultraviolet
radiation. The various atmospheric regions are
distinguished from one another by the behavior of the
temperature (decreasing or increasing with altitude) in
each. Atmospheric pressure decreases exponentially
with increasing altitude.
Researchers use sophisticated computer simulations
and instruments on the ground and aboard airplanes
and spacecraft to study potentially dangerous changes
in the atmosphere and climate caused by human
activity. For example, elevated levels of ozone-
destroying chlorofluorocarbons (CFCs) released by
refrigerators, air conditioners, and some aerosol
sprays have been shown to create holes in the ozone
layer over the polar regions. Furthermore, increasing
concentrations of carbon dioxide and other compounds
released by burning coal and oil contributes to a
greenhouse effect in which thermal energy from
the Sun passes through our atmosphere but then
becomes trapped. Most scientists view the additional
greenhouse effect due to human sources as a leading
cause of global temperature rises over the course of
the last several decades and as a significant threat to
Earth’s climate and the well-being of life on Earth.
The total mass of Earth’s atmosphere is about 5,000
trillion tons, or about one-millionth the mass of the
planet itself.54 Gravity keeps the atmosphere bound to
Earth, although atoms occasionally escape at the top. At
sea level the atmosphere presses down with a pressure
of 101.3 kPa (14.7 pounds per square inch), which is
known as 1 atmosphere of pressure. The millibar is
another common unit of atmospheric pressure. At sea
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FIGURE 3-453
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FIGURE 3-555
An image of the near side of the Moon, with major maria and craters labeled.
level, the air pressure on Earth is about 1,013 millibars.
The Moon
The Moon is unusually large for a satellite, in
comparison to its parent planet. The Moon’s size can be
determined from measurements of its angular diameter
and its distance from Earth. The distance to the Moon
has been measured to an accuracy of one part in 10
billion (a few centimeters) by precisely timing how long
it takes a beam of laser light to reach reflectors placed
on the lunar surface and return. The Moon’s equatorial
diameter is 3,476 km (2,160 miles), which is about onefourth the equatorial diameter of Earth.
The Moon’s mass, which can be determined from
the forces the Moon exerts on spacecraft, is 7.35 ×
1022 kg, or 1/81 that of Earth. Orbital spacecraft have
collected data to more precisely determine the Moon’s
mass distribution and gravitational characteristics.
The Moon’s average density is 3.34 g/cm3, or roughly
3/5 that of Earth. The Moon’s surface gravity is about
1/6 that of Earth due to its smaller mass and size. That
means an 84-kg (180-pound) astronaut only weighs 14
kg (30 pounds) on the Moon’s surface. Additionally,
objects accelerate downward at 1/6 the rate they would
on Earth.
The Surface of the Moon
The Moon’s surface is pitted with bowl-shaped
indentations called craters. Craters on the Moon
are customarily named after famous scientists and
philosophers, such as Copernicus and Plato. The
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FIGURE 3-650
On July 16, 1969, Neil Armstrong, Michael Collins, and Edwin
“Buzz” Aldrin launched from Kennedy Space Center on the
Apollo 11 mission that resulted in the first Moon landing. Three
days after launch, Apollo 11 achieved orbit around the Moon,
at which point Armstrong and Aldrin transferred to the Lunar
Module (named Eagle) and descended to the surface while Collins
remained in orbit. The Eagle successfully landed in a mare basin
named the Sea of Tranquility.
Astronaut Buzz Aldrin salutes the U.S. flag
during the Apollo 11 moon landing.
The Apollo 11 mission successfully fulfilled President Kennedy’s goal of achieving the first Moon landing by
the end of the 1960s. It was an unparalleled milestone in human scientific and technological achievement and
reclaimed American dominance in spaceflight capability. The Soviet Union pursued several manned lunar
missions before abandoning them in favor of Earth orbital space stations.
largest, flat-floored craters, such as Clavius, are nearly
240 km (150 miles) across. The smallest are known
as craterlets. Typical craters are circular, ranging in
size from tiny pits to huge circular basins hundreds of
meters across with walls ranging up to 3,000 m (10,000
feet) in height. Most craters were formed by highspeed meteoroids striking the surface of the Moon.
The energy of these impacts vaporizes the meteoroid
and sends shockwaves through the lunar surface,
forming a circular crater with a high rim and often a
central peak. Some prominent craters, such as Tycho,
are surrounded by spoke-like bright streaks called
rays, which are splash patterns caused by material
ejected from the impact. Smaller secondary craters can
also be formed when this material lands.
When Galileo made observations of the Moon’s surface
through his telescope in 1609, he mistakenly thought the
large, relatively smooth dark areas he saw were oceans.
He called these regions maria (singular mare), meaning
“seas.” The maria are actually dry lava beds made of
basalt, a dark igneous rock that formed over three billion
years ago when molten lava from the Moon’s hot interior
flooded huge impact basins. The largest mare of this
type, Mare Imbrium, the Sea of Showers, is about 1,100
km (700 miles) across. The brighter areas of the Moon
are called the lunar highlands. They are higher, more
rugged, older regions than the maria.
Highlands contain light-colored igneous rocks and
cover about 80 percent of the Moon’s surface. When
the Moon is full, the maria stand out prominently, but
the lack of surface shadows makes the surface relief
hard to discern. Spacecraft photographs show that the
far side of the Moon has craters and highlands, but it
does not have large maria, which are so conspicuous
on the near side. This suggests that the thicker outer
layer on the Moon’s far side prevented upwelling lava
from pouring into the basins.
Tides and Gravity
Gravitational effects between the Earth and the Moon
result in interesting phenomena, such as tides. Tides
are daily fluctuations in the ocean level on Earth,
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The lunar landing was broadcast live on television. Armstrong’s
first step on the Moon was witnessed by about 723 million people,
about one-fifth the population of Earth. The first words spoken
on the Moon were Armstrong’s now-famous phrase, “That’s one
small step for [a] man, one giant leap for mankind.” While on the
surface, Armstrong and Aldrin planted an American flag, took
photographs, and collected samples of lunar soil. After about 21
½ hours on the Moon, they ascended from the surface to rejoin
Collins in the orbiter and prepare for the return to Earth.
The Moon has a near side that always faces Earth,
and a far side (sometimes called the “dark” side) that
always faces away. This is due to the fact that Earth’s
gravity has locked the Moon into a synchronous orbit,
meaning that the Moon’s rotational period about its
axis equals its orbital period around Earth. The Moon
rotates on its axis every 27.3 days, the same amount
of time it takes to travel around Earth. As a result,
the same side of the Moon faces Earth at all times.
The visible disk of the Moon does appear to shift,
due to slight variations in the Moon’s motions, called
libration. It’s possible to see a total of 59 percent of the
Moon’s surface over time.
Mercury is the smallest planet in the Solar System and
the one closest to the Sun. Mercury was named for
the speedy messenger god in Roman mythology—an
appropriate name since Mercury has the greatest orbital
speed of all the planets (around 172,000 km/hour). Our
first close-up views of Mercury came from the U.S.
robotic probe Mariner 10, which photographed half
of the planet on three flybys in 1974–75 (Figure 3-7).
The surface of Mercury has many craters, making it
similar in appearance to our Moon. The largest crater
on Mercury, Caloris Basin, is 1,300 km across. Large,
smooth areas resembling the Moon’s maria suggest that
extensive lava flooding occurred in Mercury’s past.
Mercury’s surface is crisscrossed by scarps, or cliffs,
up to 2 km high and 1,500 km (930 miles) long. They
apparently formed when the planet’s interior cooled and
shrank, thereby compressing the crust.
FIGURE 3-757
The cratered surface of Mercury, photographed by the
Mariner 10 probe. The distinctive Dürer ring basin,
appearing in the top left as two concentric craters, has a
diameter of about 190 km (120 mi).
Unlike Earth, Mercury has a vertical axis of rotation,
so the Sun is always directly overhead on its equator.
As a result, the planet has no seasons, and some
sunlight always shines on its poles.
Mercury is known for its wide temperature variations,
from intensely hot (430°C, or 800°F) in direct sunlight,
to extremely cold (–180°C, or –300°F) on the dark side.
The extreme temperatures cause volatile substances to
be released from the surface, which creates a very thin,
unstable atmosphere. Helium, sodium, hydrogen, and
oxygen have been detected in Mercury’s atmosphere,
and the surface air pressure is barely 2 trillionths
of Earth’s at sea level. A magnetic field about one
percent as strong as Earth’s affects the moving charged
particles in the solar wind.
The topography of Mercury has been examined by
studying how radar signals reflect from the surface of
the planet. High radar reflectivity could indicate the
presence of ice deposits within craters near Mercury’s
poles, where temperatures average about –148°C
(–235°F). The NASA spacecraft Messenger, which
stands for MErcury Surface Space ENvironment
GEochemistry and Ranging Mission, was launched in
2004 to investigate key scientific questions and fully
map the planet from orbit in 2011, after three flybys in
2008 and 2009.58
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which can range from a few centimeters to many
meters, depending on the location and time of year. At
most coastal locations on Earth, there are two low tides
and two high tides each day. Tides are caused by the
fact that the Moon’s gravitational attraction is slightly
greater on the side of Earth that faces the Moon than
on the opposite side, some 12,800 km farther away.
This difference in the gravitational force is small,
only about 3 percent, but it produces a noticeable
deformation—a stretching along the line joining Earth
to the Moon called a tidal bulge. The effect is greatest
in Earth’s oceans because liquid can most easily move
around on our planet’s surface. The daily tides we see
result as Earth rotates beneath this deformation.
The planet Venus, named for the Roman goddess of love
and beauty, is the next-furthest planet from the Sun.
Venus is visible as a bright star-like object in the night
sky, sometimes referred to as the “morning star” and
“evening star” for its appearance just before sunrise and
sunset. Venus shines brilliantly because it is surrounded
by a highly reflective layer of clouds, about 20 km thick.
The presence of sulfuric acid in these clouds gives
Venus a yellow appearance. Venus’ atmosphere consists
of about 97 percent carbon dioxide and 1 to 3 percent
nitrogen with traces of water vapor, helium, neon, argon,
sulfur compounds, and oxygen. The temperature of the
cloud tops is about –23°C (–9° F).
Venus’ surface is obscured from our direct view by
its cloud layer. In order to map the surface, scientists
bounce radar signals off Venus and analyze the
reflections. These signals may originate from radar
telescopes on Earth, such as the Arecibo Observatory,
or from orbiting spacecraft. Our best radar imaging
data of Venus was gathered by the NASA Magellan
orbiter, which mapped 99 percent of the planet and
also studied its atmosphere and interior (Figure 3-8).
Radar images show that the terrain of Venus is dry and
rocky. About 80 percent is relatively flat plains with
FIGURE 3-859
fractures, impact craters, and volcanoes that are within
1 km of the planet’s mean surface. The difference
between the lowest and highest elevations on Venus is
15 km (9 miles).
Surface temperatures on Venus reach 482°C (900°F)
due to the greenhouse effect caused by the high density
of carbon dioxide in its atmosphere. Atmospheric
pressure is a crushing ninety times greater than that on
Earth, and lightning and thunderstorms are frequent.
The Soviet Union’s Venera 9 lander first captured
pictures of Venus’ surface in 1975, and other Venera
landers followed within the next few years. The landers
found a very inhospitable world, and each one stopped
working within two hours due to the severe conditions.
Beneath Venus’ thick clouds, rocks and soil have an
orange appearance, whereas in direct sunlight on Earth
they would appear gray.
Many areas of Venus have volcanic features. The most
common volcanoes on the planet are of the type known
as shield volcanoes, which have a broad, dome-like
appearance resembling a shield. Shield volcanoes
are built up over long periods of time by successive
eruptions and lava flows and often contain a caldera,
or hollow indentation, at the summit where the surface
has collapsed due to the withdrawal of lava. The
largest volcanic structures on Venus are huge, roughly
circular regions known as coronae. Coronae are unique
to Venus and appear to be the result of upwelling
FIGURE 3-960
A cloudless view of Venus’s surface, based on radar data
from the Magellan orbiter.
A computer image of Venus’ Sapas Mons volcano,
constructed from Magellan spacecraft data. The coloration
is based on images collected by the Venera probes.
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motions in the mantle that caused the surface to bulge
outward. Volcanoes can generally be found both in and
around coronae, and their rims usually show evidence
of extensive lava flows into the plains below.
Venus is the planet with the closest approach to Earth.
It was the first planet beyond Earth to be visited by a
spacecraft, the Mariner 2 in 1962, and the first planet
to be successfully landed on, by Venera 7 in 1970.
More than twenty American and Russian robotic
spacecraft have successfully conducted flybys, orbits,
or landings of Venus and transmitted data back to
Earth for analysis and image processing.
in the entire Solar System. Olympus Mons has an
elevation of almost 25 km, three times the elevation
of Mt. Everest. The great height of Mars’ volcanoes is
a consequence of its lower surface gravity (about 40
percent that of Earth).
FIGURE 3-1162
Mars is the fourth planet from the Sun, named for the
Roman god of war. High-resolution images and 3D
maps from measurements by orbital spacecraft show
Mars to be a harsh, rugged, and dry planet. Although
Mars’ southern hemisphere is mostly cratered and
ancient, its northern hemisphere is basically smooth
plains and typically several kilometers lower. The
northern lowlands may be an ancient ocean basin filled
in by sedimentation. Mars has huge shield volcanoes,
including Olympus Mons, which is the largest volcano
FIGURE 3-1061
The rugged landscape of Mars, captured by the Mars
Pathfinder mission in July 1997.
Martian soil is about 45 percent silicon oxide and 19
percent hydrated iron oxide (rust), which provides its
distinctive red color. The Martian sky is colored pink
in daytime by red dust that hangs in the atmosphere
like smog. Air pressure on Mars is a fraction of that
on Earth, at only about 7 or 8 millibars. Consequently,
Mars’ atmosphere is too thin to effectively shield the
surface from the Sun’s harmful ultraviolet rays. Mars’
atmosphere is made up of about 95 percent carbon
dioxide, with 2 to 3 percent nitrogen, 1 to 2 percent
argon, and 0.1 to 0.4 percent oxygen, with traces of
water vapor and other gases.
A true-color image of Mars, captured by the European
Space Agency’s Rosetta spacecraft in 2007.
Wild dust storms swirl out of Mars’ southern
hemisphere in the summer and often rage across the
entire planet. Winds up to 120 km (75 miles) per hour
blow light-colored dust about, sculpting and exposing
dark rock. Thin layers of ice and dust hundreds of
kilometers long have been laid down at the north
and south poles by global dust storms in alternating
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Mars’ Surface
The Martian surface is lined with canyons and
pockmarked by craters. Mars’ largest canyon, Valles
Mariner, is a complex network of rocky valleys
extending 5,000 km (3,000 miles) around the equator
with an average depth of 6 km (4 miles). The lowest
point on Mars, 7 km (4 miles) below the mean surface,
is the bottom of the circular Hellas Planitia basin.
Hellas, the largest impact crater in the Solar System,
is about 4 billion years old. Some younger craters,
such as the 18-km (11-mile) wide crater Yuty, are
surrounded by multiple layers of ejected material that
form an apparent splash-like pattern. Multiple theories
exist to explain the formation of these “splosh” craters.
Mars does not have liquid surface water today. However,
there is ample evidence of ancient catastrophic flooding.
The deep, winding channels and gullies on Mars
resemble riverbeds with tributaries, possibly carved
long ago by floodwaters and streams. Rocks found on
Mars show evidence of weathering and erosion, and the
abundance of rounded pebbles, sand, and dust particles
similarly argue for a previously water-rich planet.
Additionally, some rock types form only underwater.
Water that once flowed freely on Mars may now be
locked in the polar ice caps and under the surface in
the form of permafrost. Global-scale climate changes
may have transformed Mars from a running water
environment into the cold, dry world it is today.
Exploration of Mars
More American, Russian, and European probes have
explored Mars than any other planet. The first images
of the surface of Mars were provided by the U.S. robot
Viking Lander 1, which touched down on Mars on July
20, 1976. Scattered rocks, powdery dirt, sand dunes,
and distant low hills came into view on Chryse Planitia
(“Plains of Gold”). Two months later, Viking 2 landed
7,500 km (4,600 miles) northwest, near rocks pitted
by gaseous volcanoes or meteorite impacts. Viking 1
and Viking 2 sent back over 4,500 pictures, 3 million
weather reports, and data from chemical and biological
In July 1997, the U.S. Pathfinder lander and Sojourner
rover explored the Martian surface. Although expected
to last no more than a few weeks, both remained in
operation until September 1997. The following decade,
NASA explored Mars with two exploration rovers,
Spirit and Opportunity, which landed on opposite sides
of Mars’ equatorial region in January 2004. These
rovers discovered evidence of past flowing water in
rocks, minerals, and geologic landforms. The Spirit
rover ceased functioning in March 2010, whereas
Opportunity fell silent in June 2018, greatly exceeding
its original operating plan (of ninety days) by more
than fourteen years.
In 2008, NASA’s Phoenix Mars lander discovered
subterranean ice and chemicals that are important
for life, and minerals that are created in liquid water.
FIGURE 3-1264
Mars’ Moons
Mars has two small moons, named Phobos and Deimos.
They were discovered in 1877 by American astronomer
Asaph Hall. Phobos and Deimos are small, irregular
rock chunks that are only about 28 km (17 miles) and
16 km (10 miles) long, respectively. Phobos orbits Mars
every 7.7 hours, while Deimos completes a circuit in 1.3
days. Both moons appear fairly old, with many impact
craters of varying ages. Phobos has striations and chains
of small craters. Stickney, its largest crater, measures
practically 10 km (6 miles) across.
A self-portrait taken by the Curiosity rover on June 15, 2018.
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Gravitational data, derived from changes in Mars
Global Surveyor’s orbits, as well as the presence of
extremely tall, massive volcanoes indicate that Mars’
crust is about 50 km (30 miles) thick and does not
drift as Earth’s continents do. Mars likely has a mantle
that is cooler and thicker than Earth’s. Linear bands
of highly magnetized material appear in some of the
oldest crust. In the past, Mars must have had a molten
iron core and magnetic and geologic activity, although
there is no evidence of any such activity today.
The NASA Curiosity rover arrived at Mars’ Gale
Crater in August 2012 to search for signs of ancient
microbial life and assess the conditions for future
human exploration (Figure 3-12). More recently, the
Mars InSight lander launched in May 2018 and touched
down in November of that year, with a mission of
studying Mars’ interior and seismic activity. Mars is in
many ways the next frontier in space exploration.63 The
next Mars mission, currently known as Mars 2020, is a
rover based on Curiosity’s design.
FIGURE 3-1366
Jupiter and Saturn
Jupiter, the largest planet in the Solar System, was
named for the king of the gods in Roman mythology.
In the night sky, Jupiter appears brighter than all
the stars and all the planets except Venus. Jupiter’s
colorful, parallel, dark and light cloud bands, Great
Red Spot, and four largest moons can be seen from
Earth through a small telescope (Figure 3-13). The
Jovian system, consisting of Jupiter and its moons,
was explored by the U.S. robotic probes Voyager 1 and
Voyager 2 in 1979. Voyager 1 flew within 206,700 km
and Voyager 2 within 570,000 km of Jupiter’s cloud
tops. The two spacecrafts sent back more than 33,000
pictures. In 1995, the U.S. spacecraft Galileo reached
Jupiter and split into two parts. An atmospheric probe
plunged through Jupiter’s clouds and transmitted
data for an hour before it succumbed to Jupiter’s
tremendous heat and pressure. The orbital probe
collected and transmitted data and images of Jupiter
and its moons for eight years.65
Jupiter is more massive than all the other planets and
their moons combined. In fact, if Jupiter were about
eighty times more massive, nuclear fusion reactions
could have started, and it would have become a star!
Jupiter has a thick atmosphere composed primarily
of hydrogen and helium. As one descends from
the outermost edge of the atmosphere, increasing
temperature and pressure compress the gas into a liquid
and an Earth-size solid core. Jupiter is encircled by a
faint system of thin rings, made up of dust grains that
have been blasted off the inner moons by meteoroids.
The outermost part of Jupiter’s rings extends about
210,000 km (130,000 miles) from the planet’s center.
An image of Jupiter, captured by the Hubble Wide Field
Camera on April 21, 2014. The Great Red Spot is clearly
Colorful changing cloud features and convoluted
weather patterns circulate in Jupiter’s observable
atmosphere, and massive bolts of lightning flash.
Although the hydrogen, methane, and water vapor in
Jupiter’s atmosphere are all colorless, the sulfur and
phosphorous compounds and ammonia at various
depths give Jupiter its distinctive red, orange, yellow,
and brown colors. Jupiter’s famous Great Red Spot
is a colossal atmospheric storm, about 1.3 times the
diameter of Earth. It has been observed for over three
hundred years at varying sizes, brightness, and color.
The Great Red Spot rotates counterclockwise and
also moves around the planet. It is also cooler than
surrounding clouds, partly because it towers up to 24
km (15 miles) above them. Smaller storms and eddies
appear throughout the banded clouds of Jupiter.
Temperatures reach –112°C (–170°F) at the cloud tops.
The atmosphere extends down about 21,000 km (13,000
miles). The density of hydrogen increases steadily from
the top inward as the pressure increases, until it changes
to liquid hydrogen. The pressure below must be high
enough to compress hydrogen to an extraordinarily
dense form known as liquid metallic hydrogen. At the
core, temperatures may be 30,000 K (53,000°F), which
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FIGURE 3-1467
would explain the observation that Jupiter radiates about
twice as much heat as it receives from the Sun.
Jupiter has a powerful magnetic field that traps ions
and electrons in a complex system of large, intense
radiation belts. The magnetic field is essentially dipolar
but opposite Earth’s in direction. Electrical currents
in the liquid hydrogen layer could be its source. At
the cloud tops, Jupiter’s magnetic field is 1.5 to seven
times more powerful than Earth’s. Jupiter’s enormous
magnetosphere varies in size, possibly due to changes
in the solar wind pressure. It may stretch sunward 7
million km (4 million miles) and outward nearly 650
million km (400 million miles) to Saturn’s orbit.
Jupiter’s Moons
There are at least sixty-three confirmed moons orbiting
Jupiter. The four largest moons are collectively called
the Galilean moons after their discoverer Galileo
Galilei (Figure 3-14). Colorful Io has active volcanoes
that spew sulfur-rich materials that color the surface
bright orange, red, brown, black, and white. Io’s bright
white spots are sulfur dioxide frost, and its tenuous
atmosphere is primarily sulfur dioxide gas. The
volcanoes may be due to the heating that occurs as
Europa and Ganymede, two of Jupiter’s other moons,
tug gravitationally on Io, and Jupiter alternately pulls
Io back to its regular orbit. This pumping creates tidal
bulges on Io’s surface that are up to a hundred times
greater than the typical 1 m (3.3 feet) tidal bulges on
A gigantic cloud of charged particles, mostly ions
of sulfur and oxygen, wobbles around Jupiter at Io’s
distance. The particles are likely stripped off Io by
magnetic forces as Jupiter’s magnetosphere rotates
with the planet. Cloud particles may also travel along
Jupiter’s magnetic field lines into its north and south
polar atmospheres, causing brilliant Jovian auroras.
There is evidence of ice on the surfaces of Europa,
Ganymede, and Callisto. Europa, about the same size
and density as Earth’s Moon, is the brightest Galilean
moon. Its smooth, icy crust, crisscrossed by long lines,
may hide a global ocean of water warmed by tidal heat.
Ganymede and Callisto most likely have a rocky core
with a water/ice mantle and a crust of rock and ice.
Ganymede is the largest moon in the solar system
at 5,262 km (3,262 miles) in diameter. It has dark,
probably ancient, areas with many craters and lighter,
younger terrain that is grooved, suggesting global
tectonic activity. Callisto’s surface appears oldest,
with numerous impact craters. The largest craters on
Callisto may have been erased by the flow of icy crust.
Features that look like the remains of very large basins
may be the result of collisions with large chunks of
rock and metal.
Saturn, the most distant planet that is visible to
the unaided eye, was named for the Roman god of
agriculture. Saturn is known for the impressive rings
that encircle it (Figure 3-15). Since Saturn has an
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A composite image of the four Galilean moons. From left to right, in order of increasing distance from Jupiter: Io, Europa,
Ganymede, and Callisto.
FIGURE 3-1568
Saturn, as photographed by the Cassini spacecraft in July 2008.
Voyager 1 in 1981 and Voyager 2 in 1982 sent back
33,000 images of the Saturnian system as they flew
by. The main rings contain thousands of tiny ringlets,
which are partly intertwined and kinked by the
gravitational forces of small moons that “shepherd” the
ring material. In 2004, the U.S./European spacecraft
Cassini, using gravity assists from Venus, Earth, and
Jupiter, neared Saturn and ejected a probe, called
Huygens. After twenty days, Huygens descended
through the clouds of Saturn’s moon Titan via
parachute. Huygens collected information about the
chemical composition of the atmosphere and clouds
and snapped pictures for two and a half hours during
its descent. Upon landing on the frozen ground,
Huygens sent a surface report for a few minutes. The
Cassini orbiter continued to observe Saturn and its
moons for thirteen years before being steered into
Saturn’s surface to avoid hitting any of the moons.69
Like Jupiter, Saturn is a huge multilayered ball of
hydrogen and helium gas. However, the fraction of
helium in Saturn’s atmosphere (about 7 percent) is
less than the fraction in Jupiter’s atmosphere (about
14 percent). Inside, a central iron-silicate core is
surrounded by a metallic hydrogen layer under high
pressure. A dynamic atmosphere is flattened at its
poles by rapid rotation. Colors and features such as
belts and zones and long-lived ovals are much less
distinct because of a hazy layer above the visible
clouds. Saturn also radiates more energy than it
absorbs from the Sun, likely due to its internal heat.
Saturn’s mass is ninety-five times that of Earth, while
its volume is 844 times that of Earth. Saturn has the
lowest average density of all the planets in the Solar
System. Its density is even lower than that of water,
meaning that it could float in water if a large enough
sea existed. Saturn’s magnetosphere is about one-third
the size of Jupiter’s and may extend nearly 2 million
kilometers toward the Sun. The magnetosphere varies
in size during shifts in the intensity of the solar wind.
Saturn’s magnetic field drags along charged particles,
which circle the planet as it rotates.
Saturn’s Moons
Saturn has at least sixty confirmed and several
suspected moons. Others may be discovered as
scientists continue to analyze the massive amount
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orbital period of 29.5 Earth years, we can see it in
essentially the same orientation toward Earth in the
same region of our sky for months. Although Saturn’s
brightest rings are 282,000 km (175,000 miles) wide,
they are only a kilometer (less than a mile) thick.
Saturn’s rings consist of billions of ice particles that
resemble icy snowballs or ice-frosted rocks, orbiting
Saturn. These particles can range in size from
micrometers to meters. The larger particles are likely
the remnants of moons, asteroids, and comets that
broke apart due to collisions or were torn apart by
gravitational interactions. The rest may be material
that never collected into a single moon.
FIGURE 3-1670
Cassini Division
Division (Pan)
Cassini Saturn Orbit Insertion
Ring Plane Crossing
F ring
G ring
E ring
(to Titan)
of data transmitted by the Voyager and Cassini
spacecraft. Saturn’s largest moon, Titan, has a thick,
hazy, orange-colored atmosphere that is mostly
nitrogen, with hydrocarbons such as methane. Titan is
made of rock and ice, although data from Cassini in
2012 suggests that liquid water may exist underneath
the icy exterior. The presence of liquid water on Titan
could indicate the possibility of biological processes
despite the moon’s otherwise harsh conditions. Cassini
also observed mountains and an ethane lake, which
suggests that methane and ethane form clouds, rain out
into pools and rivers, and evaporate into clouds again.
Saturn’s other large moons appear to be mainly ice.
Except for Enceladus, all these moons are heavily
cratered. Geyser-like jets near the south pole of
Enceladus spray ice crystals far out, perhaps providing
evidence of liquid water beneath its surface. Hyperion
appears to be the oldest, with evidence of meteoritic
bombardment. Iapetus has icy and dark material on
opposite sides. The irregular shapes of Saturn’s small
moons indicate they are fragments of shattered larger
bodies. The two moons Prometheus and Pandora
orbit on the inside and outside edge of one of Saturn’s
thinnest rings. Their gravitational effects at varying
distances may cause the ring’s kinks (Figure 3-16).
Uranus and Neptune
Uranus was the first planet identified by means of
a telescope. It was discovered in 1781 by British
astronomer William Herschel, using a 150-mm (sixinch) telescope he made himself. Almost named
for King George III, Uranus was finally named
traditionally for the Greek god of the heavens. Uranus,
with a maximum magnitude of +5.7, appears as a
small disk (sometimes tinted blue) through a telescope.
Uranus remained largely a mystery until 1986, when
the Voyager 2 spacecraft flew within 81,500 km
(50,600 miles) of its cloud tops. Voyager 2 sent back
7,000 images of the Uranian system. Uranus is tipped
on its side and surrounded by a system of narrow
rings, making it resemble a giant bull’s eye. The angle
between its axis and the pole of its orbit is a unique
98°. The north and south polar regions are alternately
exposed to sunlight and darkness as Uranus orbits
the Sun. Astronomers hypothesize that Uranus may
have suffered a collision with a planet-sized body that
knocked it over early in its history.
Uranus’ atmosphere is composed of mostly hydrogen
and about 15 percent helium, with smaller amounts
of methane and other hydrocarbons. Uranus’ blue
appearance is caused by the methane in its atmosphere,
which preferentially absorbs red light from sunlight.
The atmosphere has clouds running east to west like
those of Jupiter and Saturn. Winds blow in the same
direction as the planet rotates, at speeds of 40 to 160
meters/sec (90 to 360 miles per hour). Surprisingly,
sunlit and dark cloud tops show the same average
temperature, about –212°C (–350°F). Voyager 2
detected haze around the sunlit south pole and large
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An artist’s concept of Saturn’s rings and major icy moons.
FIGURE 3-1771
that the ring system may have formed after Uranus.
Ring particles may be remnants of a moon that was
broken by a high-velocity impact or torn apart by
gravitational effects.
Uranus’s Moons
An image of Uranus, captured by Voyager 2 in 1986.
amounts of ultraviolet light, called dayglow, radiated
from the sunlight hemisphere.
Uranus has a magnetosphere with intense radiation
belts and radio emissions. Its magnetic field axis
is tilted 60° to the rotational axis. This magnetic
field is comparable to Earth’s in intensity, but it is
more irregular because it is offset from the planet’s
center. Uranus’ magnetic field may be generated by
an electrically conductive, super-pressurized ocean
of water and ammonia located between the planet’s
atmosphere and rocky core. A rotating cylindrical
magnetotail extends at least 10 million km (6 million
miles) behind the planet. It is twisted into a long
corkscrew shape by the planet’s extraordinary rotation.
Uranus has narrow rings that are distinctly different
from Jupiter’s and Saturn’s. These rings are very dark
and are composed mainly of large icy chunks several
feet across. Intense irradiation may have darkened
any methane trapped in their icy surfaces. Collisions
between the ice chunks may create the fine dust that
appears to be spread throughout the ring system.
Atmospheric drag due to a hydrogen corona that
Voyager 2 observed around Uranus may cause dust
particles to spiral into the planet. Incomplete rings and
varying opacity in several of the main rings suggest
Uranus’ two largest moons, Titania and Oberon, are
about half the size of Earth’s moon. Ariel has the
brightest and possibly youngest surface, with many
fault valleys and what appear to be extensive flows
of icy material. Titania has huge fault systems and
canyons that provide evidence of past geologic activity.
Umbriel and Oberon have the darkest appearance of
Uranus’ largest moons. Their surfaces are heavily
cratered and old, indicating little past geologic activity.
Ten smaller moons of Uranus were discovered by the
Voyager 2 spacecraft. Puck, the largest of these, is
155 km (96 miles) in diameter. The smaller moons
are composed of more than half rock and ice. Uranus’
other small moons were discovered by the Hubble
Space Telescope and Earth-based telescopes.
Like Uranus, the planet Neptune has a thick hydrogen,
helium, and methane cloud cover that gives it a bright
blue appearance. Long bands and several large spots are
also visible in Neptune’s atmosphere. When Voyager 2
flew within 5,000 km (3,000 miles) of Neptune in 1989,
the planet was more distant from the Sun than Pluto.
The eight thousand images Voyager sent back gave us
our first good look at the Neptunian system (Figure
3-19). Neptune’s discovery was a triumph for theoretical
astronomy. Uranus did not follow the path Newton’s law
of gravity predicted it should. Astronomers John Adams
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Uranus has five large moons and at least twenty-seven
small moons. The largest moons appear as tiny bright
dots through our large telescopes. Titania was the first
moon discovered, in 1787, and Miranda the last, in 1948.
Voyager 2 found that the moons are dark gray ice-rock
conglomerates, composed of about 50 percent ice, 20
percent carbon and nitrogen-based materials, and 30
percent rock. Miranda, the smallest of the five larger
moons, has the strangest appearance. It has huge fault
canyons as deep as 20 km (12 miles), terraced layers,
a chevron feature, large relief mountains, ridges, and
rolling plains. This mixture of different terrain types on
older and younger surfaces suggests that Miranda has
been affected by tectonic activity, violent impacts, and
tidal heating caused by Uranus’s gravitational pull.
FIGURE 3-1872
FIGURE 3-1973
in England and Urbain Leverrier in France calculated
that its motion was being disturbed by another planet’s
gravity. They predicted where that unknown planet
should be in the sky. In 1846, astronomer Johann Galle
at the Berlin Observatory in Germany pointed to the
predicted spot and found Neptune. The planet was
named for the Roman god of the sea.
Although Neptune, the smallest planet of the gas
giants, receives only 3 percent as much sunlight as
Jupiter, it has a dynamic atmosphere. The strongest
winds on any planet can be found on Neptune, blowing
opposite its direction of rotation. Like Uranus, Neptune
has a magnetic field that is highly tilted, at 47° relative
to its rotational axis. This magnetic field causes radio
emission and weak auroras. Voyager 2 found four rings
circling Neptune. They are so diffuse and the material
in them is so fine that they could not be fully resolved
from Earth.
Neptune’s Moons
Neptune has at least thirteen confirmed moons. Of
these, Triton is the largest and the most interesting.
Voyager data showed that Triton’s surface contains
methane ice, which is thought to contribute to its
reddish color. Recent infrared measurements of
Triton’s crust indicated the presence of nitrogen,
carbon monoxide, and carbon dioxide, all in ice form.
Active geyser-like eruptions spray invisible nitrogen
gas and dark dust particles up several kilometers. The
An image of Neptune, captured by Voyager 2 in August
surface temperature of Triton is the coldest observed
in the Solar System, about –235°C (–391°F). Triton’s
large south polar cap is slightly pink and is composed
of frozen nitrogen and methane. From the ragged edge
of this polar cap northward, Triton appears darker and
redder, possibly as a result of methane ice turning to
carbon upon exposure to ultraviolet radiation.
A very thin atmosphere extends some 800 km (500
miles) above Triton’s surface. The surface pressure is
about 14 microbars, or 1/70,000 that of Earth. Nitrogen
ice particles may form thin clouds a few kilometers
above the surface. Six small, dark moons discovered
by Voyager 2 and five discovered by enhanced groundbased telescopes remain close to Neptune’s equatorial
plane. These moons are named for mythology’s water
gods and nymphs. Proteus, the largest, is 420 km (250
miles) in diameter. Neptune’s small moons and rings
are probably fragments of larger moons shattered in
Plutoids and the Kuiper Belt
The Kuiper Belt is a vast region of the Solar System
beyond Neptune containing icy bodies and larger
masses known as Kuiper Belt objects (KBOs) (Figure
3-20). It was named for Dutch-American astronomer
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A composite image of Uranus’ large moons and one
smaller moon. From left to right: Puck, Miranda, Ariel,
Umbriel, and Oberon. Original images were captured by
Voyager 2. Size proportions are correct.
FIGURE 3-2074
Gerard Kuiper, who predicted the existence of KBOs
in 1951 from the theory of planet formation. KBOs
are likely leftover materials from the formation of the
Solar System that never coalesced into planets.
Pluto, named for the Greek god of the underworld, was
the first KBO to be discovered. Following its discovery
in 1930 by American astronomer Clyde Tombaugh,
Pluto was designated as the ninth planet in the Solar
System. However, the discovery of similarly sized
objects in the late twentieth century called Pluto’s
planetary status into question. Eris, in 2003, and its
moon Dysnomia, in 2005, were discovered by U.S.
astronomer Michael Brown and his team. They were
named amidst intense controversy over how to classify
newly discovered or understood objects: Eris, for the
Greek goddess of discord and strife, and Dysnomia,
for Eris’s daughter, spirit of lawlessness. In 2006,
following extensive discussion, the International
Astronomical Union (IAU) formally introduced the
designation of dwarf planet to classify bodies such as
Pluto and Eris that did not fully meet these criteria.
Trans-Neptunian (beyond Neptune) dwarf planets are
also known as plutoids.
Both Pluto and Eris have very eccentric and inclined
orbits. Pluto moved inside Neptune’s orbit in 1980,
reached perihelion (closest approach to the Sun) in
1989, and crossed outbound toward aphelion (furthest
distance from the Sun) in 1999. Pluto, about 2,300 km
(1,430 miles) in diameter, is two-thirds as big and onesixth as massive as Earth’s Moon. Eris is about 2,400
km (1,490 miles) in diameter. Little is known about the
exact interior of Pluto and Eris, but their low densities
indicate that both must be made of ice and rock. A
thick layer of water ice and frozen methane, nitrogen,
and carbon monoxide, polar caps, and large, dark spots
near the equator cover Pluto’s surface, and a layer of
frozen methane appears to cover the surface of Eris.
Exterior ices thaw and form thin atmospheres when
the bodies are closer to the Sun and refreeze during the
coldest, most distant parts of their orbits.
Pluto’s moon Charon is about half its size. Charon
orbits Pluto in 6.387 days, exactly the time both take
to rotate once. Consequently, an astronaut on Pluto
would always see Charon in the same spot in the sky
and only from one hemisphere. Pluto also has several
smaller moons, including Nix, named for the goddess
of darkness and night, and Hydra, named for a multiheaded monster that guarded the underworld. More
objects are added to the dwarf planets category as
they are discovered (Figure 3-21). In 2006, the U.S.
space probe New Horizons launched with the mission
of studying the Pluto system. Nine years later, on July
14, 2015, New Horizons became the first spacecraft to
explore Pluto during a historic flyby 12,500 km from
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An illustration of the Kuiper Belt surrounding the Solar System. Pluto’s orbit is highlighted in yellow.
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FIGURE 3-2176
The largest known trans-Neptunian objects. The top four objects are categorized as dwarf planets by the International
Astronomical Union (IAU), whereas the bottom six are dwarf-planet candidates.
the surface of the dwarf planet.75
estimated to be less than 0.01 percent that of Earth.
Asteroids, Comets, and Meteoroids
The U.S. robotic probe Galileo sent the first closeup
image of an asteroid, 951 Gaspra, in 1991 and of
a small satellite, Dactyl, orbiting 243 Ida, in 1993.
The U.S. robotic probe NEAR (Near Earth Asteroid
Rendezvous) Shoemaker, launched in 1996, orbited
the asteroid Eros for over a year before successfully
landing on its surface on February 12, 2001. The
U.S. spacecraft Dawn entered a fourteen-month orbit
around Vesta in July 2011 and began orbiting Ceres
in March 2015. In 2016, Dawn discovered evidence
of water molecules on the surface of Ceres. Dawn
has since exhausted its fuel supply and remains in an
uncontrolled orbit around Ceres.77
Asteroids, or minor planets, are irregularly shaped
bodies of rock and/or metal. Most asteroids orbit the
Sun in the asteroid belt, a region between the orbits
of Mars and Jupiter. Through a telescope, asteroids
appear as stars—in fact, the term asteroid actually
comes from the Greek for “star-like.” The largest
known asteroid, named Ceres, was discovered in
1801 by Sicilian astronomer Giuseppi Piazzi. Ceres
is 950 km (590 miles) across and was classified as a
dwarf planet along with Pluto and Eris in 2006. Some
400,000 asteroids have been observed, and more are
added each year. The total mass of these asteroids is
The amount of sunlight that asteroids reflect to Earth
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varies and repeats after several hours, which indicates
that they have irregular shapes and are rotating.
Asteroids are classified by how they reflect sunlight
using spectrophotometry, the accurate determination of
magnitudes within specified wavelength regions. Very
dark C-type asteroids, so called because they contain
a large amount of carbon, are very common in the
outer asteroid belt. Moderately bright S-type asteroids
contain silicates mixed with metals. S-type asteroids
are common in the inner belt. Very bright M-type
asteroids are metallic. The bright asteroids are likely
clumps of matter that condensed from the original
solar nebula but never became massive enough to form
a large planet. The fainter ones are likely fragments
resulting from collisions.
FIGURE 3-2379
The comet Hale-Bopp, photographed in 1997.
A size comparison of the asteroids Ceres, Vesta, and Eros.
Comets are icy, small celestial objects that produce
a distinctive tail as they pass near the Sun (Figure
3-23). Comets have served as a source of fascination
for centuries, and records of comets extend as far back
as the fourth century bce. Throughout history people
have dreaded comets as omens of human disasters
such as wars or famines. Comets were named for their
appearance. Both the Greek word kometes and the
Latin word cometa mean “long-haired.” When it shines
in the sky, a bright comet has a head with a star-like
core called the nucleus surrounded by a glowing halo
called the coma and long semi-transparent tails. The
nucleus is several kilometers in size. The coma may
extend 100,000 km (60,000 miles) or more outside the
nucleus. Comet tails can stream millions of kilometers
into space. Although billions of comets likely orbit the
Sun far from Earth, they only shine in the sky when
they travel near the Sun.
The most widely accepted description of a typical
comet is the “dirty snowball” model, proposed by
American astronomer Fred Whipple in 1950. When a
comet is far out in the Solar System it consists of only
an irregularly shaped nucleus composed of mostly ice
and other frozen gases (the “snow”) loosely mixed with
stony or metallic solids (the “dirt”). As a comet nucleus
comes in from the edge of the solar system to within
a few hundred million kilometers of the Sun, it heats
up. Gases sublimate and escape to space, pushing dust
from the surface of the nucleus. The comet’s gravity
is too weak to hold back the gases and dust, which
expand outward around the nucleus for thousands
of kilometers, forming the coma. The comet shines
because the gases absorb and emit light and the dust
reflects light from the Sun.
As a comet approaches the Sun, it may develop tails of
gases and dust released from the nucleus. Ultraviolet
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FIGURE 3-2278
FIGURE 3-2480
Comets have been explored several times by robotic
spacecraft. In 1986, the European Space Agency’s
Giotto spacecraft approached Halley’s Comet at a
distance of 596 km, becoming the first spacecraft to
make closeup observations of a comet. In 2001, the
U.S. Deep Space 1 probe collected high-resolution
photographs of comet 19P/Borrelly. In 2005, the U.S.
Deep Impact probe excavated debris from the interior
of the nucleus of comet Tempel 1, which when analyzed
was found to contain more dust and less ice than
scientists had expected. More recently, the European
space probe Rosetta successfully landed on the comet
67P/Churyumov-Gerasimenko in August 2014.
Countless bits and pieces of rock and dust, called
meteoroids, populate the inner Solar System.
Meteoroids enter Earth’s atmosphere continually.
Astronomers collect them at high altitudes, from arctic
ice sheets, and from the ocean floor for analysis in
laboratories. These meteoroids are similar to dust grains
ejected from the nucleus of Comet Halley. Meteors,
or “shooting stars,” are streaks of light created by
meteoroids that plunge through Earth’s atmosphere at
speeds up to 72 km (45 miles) per second. Air friction
burns tiny particles when they are 60 to 110 km (40 to
70 miles) above Earth. On any clear, dark night you
can see an average of six meteors an hour flashing
unpredictably in the sky. Meteors occur but are not
visible during the daytime because the sky is too bright.
On several predictable dates every year you may see
meteors pour down from one part of the sky. This
type of display is called a meteor shower (Figure
3-25). Meteor showers occur when Earth, moving
along its orbit around the Sun, crosses a swarm of
meteoroids left behind by an active comet. During a
meteor shower, all the meteors appear to come from
one common point in the sky, called the radiant of the
shower. Meteor showers are usually named for the
constellation where they seem to originate, such as the
Perseids from Perseus and the Orionids from Orion.
Meteor showers are best seen with your unaided eye on
nights when the Moon is not bright, as a full Moon can
blot out a meteor shower.
An artist’s conceptual rendering showing the gas and dust
tails trailing behind a comet.
When a piece of stone or metal from outer space lands
on Earth, it is called a meteorite. The largest meteorite
ever found, the Hoba West, weighs an estimated sixtysix tons and still lies in Namibia in Africa where it
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radiation tears the gases apart into molecular
fragments and ions, which interact with the charged
particles blowing out from the Sun in the solar wind.
These ions are ultimately swept millions of kilometers
straight back into a gas, or ion, tail. Radiation pressure,
or intense sunlight striking, pushes the dust particles
outward. The comet keeps moving, and a dust tail
curves behind it. Comet tails are thin enough that
stars can be viewed through them. Neutral molecules
and atoms continue to expand outward until they are
ionized. The most common atom, hydrogen, forms the
huge hydrogen cloud. The hydrogen cloud surrounding
the nucleus of Comet Halley at its 1986 apparition
grew to several hundred thousand kilometers in
diameter. Effects of hydrogen ions released by Comet
Halley on the solar wind were detected as far as 35
million km (21 million miles) from the nucleus.
FIGURE 3-2581
FIGURE 3-2682
A meteor streaks across the night sky during the annual
Perseid meteor shower in August 2015.
landed (Figure 3-26). Meteoroids smaller than about
a meter in diameter generally burn up in Earth’s
atmosphere. Larger meteorites can reach the surface
and cause significant damage upon impact. Nearly a
hundred craters larger than 0.1 km in diameter can be
found on Earth’s surface although most of these have
been significantly worn away by erosion. The Barringer
Meteor Crater, near Winslow, Arizona, is 1.2 km in
diameter and 0.2 km deep, and resulted from a meteorite
impact about 25,000 years ago. The huge Chicxulub
crater buried under the Yucatan Peninsula in Mexico is
the postulated impact site for a meteoric event that could
have caused the catastrophic disruption of the biosphere
and mass extinction of dinosaurs, many plants, and
other animal species 65 million years ago.
One of the most recent documented meteoric events
occurred in central Siberia on June 30, 1908, leaving a
blasted depression at ground level but no well-formed
crater. Recent calculations suggest that the object in
question was a rocky meteoroid about thirty meters
across. The explosion, estimated to have been equal
in energy to a 10-megaton nuclear detonation, was
heard hundreds of kilometers away and produced
measurable increases in atmospheric dust levels across
the Northern Hemisphere.
Is our planet at risk for another massively destructive
meteoric event? Fortunately for life on Earth, major
destructive collisions between meteorites and the Earth
are believed to be rare events, occurring only once every
few hundred thousand years. The Center for Near Earth
Object Studies, a division of NASA’s Jet Propulsion
Laboratories, computes the trajectories of near-Earth
objects and tracks their closest approach to our planet.83
The Solar System is made up of the Sun and all
objects gravitationally bound to it, including
planets, dwarf planets, moons, comets, and
Kuiper Belt objects. There are eight planets in
the Solar System.
Earth is the third planet from the Sun. Earth’s
interior is composed of three distinct layers:
the crust, mantle, and core. Over 70 percent
of Earth’s surface is covered by water, and its
relatively oxygen-rich atmosphere supports a
diversity of lifeforms.
The Moon has a diameter about one-fourth that
of Earth. The Moon’s surface has many craters
from meteorite impacts, as well as large, dry
plains known as maria. Tides on Earth are
caused by gravitational interactions with the
Mercury is the smallest planet in the Solar
System and the one closest to the Sun. Mercury
is known for the extreme temperature variations
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The Hoba West meteorite in Namibia.
Venus is a rugged, inhospitable planet
surrounded by a thick layer of clouds. Many
areas of Venus have volcanic features. Surface
temperatures are relatively high on Venus
due to the greenhouse effect from its carbon
dioxide-rich atmosphere.
Mars’s soil is rich in iron oxide, giving it a
distinct red color. Dust storms are frequent,
and the atmosphere is thin compared to Earth.
Mars’ surface is lined with canyons and
craters, providing possible evidence of past
flooding. More U.S. and Russian probes have
explored Mars than any other planet.
Jupiter is the largest planet in the Solar System.
Jupiter has a thick atmosphere composed
primarily of hydrogen and helium. Temperatures
and pressures increase rapidly from the surface
of its atmosphere to its core. Jupiter is orbited by
at least sixty-three moons, the largest of which
are known as the Galilean moons.
Saturn is a gaseous planet encircled by a set
of distinctive rings made up of ice particles.
Saturn has at least sixty confirmed moons.
Saturn’s largest moon, Titan, is composed
of rock and ice, while its other moons are
primarily composed of ice.
Uranus is a blue-colored planet that is tipped
on its side, causing it to “roll” along its orbit.
Uranus has narrow rings made up of icy
chunks, and it is orbited by many moons of
varying sizes and features.
Neptune has a similar blue appearance to
Uranus. Neptune’s presence was predicted
before it was formally observed due to its
gravitational influence on Uranus. Neptune has
at least thirteen moons, the largest and most
interesting of which is Triton.
The Kuiper Belt is a vast region of the Solar
System beyond Neptune containing icy bodies
and larger masses known as Kuiper Belt
objects (KBOs). Pluto and Eris are the largest
known KBOs, and both were classified as
dwarf planets in 2006.
Asteroids are irregularly shaped bodies of rock
and/or metal, most of which can be found in
the asteroid belt between Mars and Jupiter.
Comets are icy, small celestial objects that
produce a distinctive tail as they pass near the
Sun. Meteoroids are pieces of rock and dust
that continually enter Earth’s atmosphere.
Meteoroids that reach Earth’s surface are
known as meteorites.
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between its sunlit and dark sides, which also
contributes to its thin, unstable atmosphere.
Section IV
Galaxies and the Universe
Already in this Guide we have pondered truly vast
distances—planets that occupy the furthest reaches of
our Solar System, stars that are located many lightyears from Earth, and comets that traverse the great
expanses in between. As it turns out, these distances
are dwarfed by the size and scope of our galaxy—a
system over 100,000 light-years across. In this section,
we will discuss the properties and structure of our own
Milky Way galaxy as well as the billions of others that
serve as “building blocks” of the universe. In doing so,
we will uncover what the behavior of these galaxies
can tell us about the universe’s earliest origins.
FIGURE 4-185
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A galaxy is an enormously large collection of matter—
stars, gas, dust, and black holes all held together by
gravitational attraction. The galaxy to which our Sun
and all the visible stars in our sky belong is named the
Milky Way galaxy. Our Galaxy is visible as a cloudy
band of light across the sky on a very clear, dark night
(Figure 4-1). That band is the combined glow of billions
of stars in our huge galaxy, and the name “Milky Way”
originates from its likeness to a trail of milk spilled in
the sky. The Milky Way can be viewed from Earth in
the direction of the constellation Sagittarius between the
months of March and October. Using a telescope, you
can see that the cloudy band is actually made of many
individual bright stars. Astronomers estimate that the
entire Milky Way galaxy contains between 100 billion
and 400 billion stars.84
Structure and Properties
Since we are bound to our own Solar System, which is
located inside the huge Milky Way galaxy, we cannot
photograph our own galaxy from the outside. Instead,
astronomers use observations of distant galaxies to
help us draw conclusions about our own galaxy’s
properties and appearance. If it were possible to go
The Milky Way, visible in the night sky.
far into space and look down on our galaxy, it would
appear as a brilliant spiral pinwheel about 100,000
light-years (30 kpc) in diameter. From the side, the
Milky Way galaxy would look like a thin, shiny
disk that bulges out at its center (Figure 4-2). The
galactic disk is a flattened circular region that contains
most of a galaxy’s stars and interstellar matter. The
thick distribution of gas and stars near the center of
the galactic disk is called the galactic bulge. The
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thickness of the Milky Way’s galactic disk is about
3,000 light-years (1 kpc), and the thickness of the bulge
is about 10,000 light-years (3 kpc). The disk and bulge
are surrounded by the galactic halo, a spherical region
containing fainter and older stars.
FIGURE 4-387
FIGURE 4-286
A diagram of the parts of the Milky Way galaxy.
The Milky Way is a spiral galaxy. All galaxies of this
type are shaped like a flattened disk containing spiral
arms that wind outward from in a central galactic bulge
(Figure 4-3). Our Solar System is located out in the
Orion spiral arm of the galaxy, about 25,000 light-years
(8 kpc) away from the center. The entire Milky Way
galaxy is rotating in space, which can be deduced from
the observed Doppler shift of radiation from the spiral
arms. Our Solar System is racing around the center
of our galaxy at about 250 km/sec (563,000 miles per
hour). Even at that incredible speed, our solar system
requires about 220 million years to complete just one
revolution. Our Galaxy appears to be hurtling through
space in the direction of the constellation Hydra at a
speed of over 600 km/sec (1 million miles per hour).
Star Clusters
While some stars travel through the galaxy alone,
many move in star clusters, which are groups of stars
that were formed from the same parent cloud and stay
close together due to mutual gravitational attraction.
Star clusters are important to astronomers because all
the different-mass stars within a cluster are about the
same age, making them ideal for testing models of star
formation and evolution. Loose, irregular star clusters
are known as open clusters and contain between 100
to 10,000 stars across a distance of about a parsec.
Open clusters are strongly concentrated in the spiral
arms. Stars that belong to open clusters are relatively
young and typically hot and highly luminous.
A small fraction of stars in our galaxy belong to
globular clusters located in the galactic halo (Figure
4-4). About 150 globular clusters, containing some
100,000 to 1 million tightly packed stars each, have
been detected. Globular clusters contain the oldest
known stars—astronomers estimate that all known
globular clusters are at least 10 billion years old.88
Some globular clusters also contain a small number of
“blue stragglers,” which are atypically hot, blue, highly
luminous stars. Blue stragglers are believed to be able
to fuse hydrogen longer than most cluster stars due to
the transfer of mass from a companion star.
The Interstellar Medium
The matter between the stars is referred to as the
interstellar medium. Interstellar matter is particularly
important because it serves as the raw material for
new stars and planets. It is about 99 percent gas (about
75 percent of the mass of the gas is hydrogen and 23
percent is helium) and 1 percent interstellar dust,
very tiny solid particles. In our galaxy, most of the
interstellar gas and dust is concentrated in the spiral
arms, which is also where the newest stars are located.
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M101, also known as the Pinwheel galaxy, is a spiral
galaxy with a similar structure to our own.
FIGURE 4-590
Tucanae is one of the brightest globular clusters.
The Orion Nebula
A concentration of gas and dust in space is called a
nebula. Historically, the term nebula, from the Latin for
“cloud,” was used to describe all kinds of hazy patches
in the sky, including many now known to be star
clusters or galaxies. An emission nebula is a cloud of
ionized gas that glows by absorbing and then re-emitting
starlight from very hot, young stars nearby. The Orion
Nebula is a well-known example of an emission nebula
(Figure 4-5). A dark absorption nebula, or molecular
cloud, is a relatively dense concentration of interstellar
matter whose dust absorbs or scatters starlight and hides
stars that are behind it from our view.
radiation is emitted by neutral hydrogen atoms, which
make up much of the gas in interstellar space. This
radiation is emitted most strongly from regions with the
greatest concentration of neutral hydrogen atoms—the
spiral arms.
FIGURE 4-691
Mapping Our Galaxy
Using optical wavelengths, even with the largest optical
telescopes, we are limited in our ability to see more than
about a thousand light-years in most directions into the
Milky Way galaxy because dust clouds in the interstellar
medium absorb light in the visible spectrum. Instead,
astronomers use radio, infrared, and high-energy waves,
which can pass through these clouds, to image the
space beyond (Figure 4-6). The spiral structure of our
Galaxy can be mapped by detecting radio waves with a
wavelength equal to 21-centimeters. This 21 centimeter
An infrared image of our Galaxy’s core, collected by
NASA’s Spitzer Space Telescope.
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FIGURE 4-489
A*) and typically pronounced “sadge A-star,” was
conclusively determined to be a black hole in research
published in October 2018.92 As matter falls in toward
the central black hole, it is compressed and heated to
millions of degrees, producing the observed X-rays.
In other regions of the galaxy, hydrogen atoms join
together to form hydrogen molecules (H2). These
regions contain exceptionally dense concentrations of
hydrogen in dark, cool molecular clouds. Molecular
hydrogen does not emit or absorb radio waves, so it
cannot easily be used to probe cloud structure in these
regions. Instead, radio astronomers map the densest
gas concentrations by looking at the strong radio
emission lines of “tracer” molecules such as carbon
monoxide (CO), water (H2O), and formaldehyde
(H2CO). Although these molecules are rare (about one
per billion hydrogen molecules), their detection is a
strong indication of the presence of a molecular cloud.
Star Populations
Observations of radiation across the electromagnetic
spectrum provide ongoing insight about the structure
and properties of our galaxy. Stellar coronas and very
hot interstellar gas can be observed at ultraviolet
wavelengths. Powerful signals in radio, infrared, and
X-ray wavelengths can be detected from the galactic
nucleus. These detections have led to the discovery of
a very massive, compact object surrounded by very
hot, chaotic gas clouds and dust in the galaxy’s nucleus
(Figure 4-7). This object, called Sagittarius A* (Sgr
Population I stars include the hottest and most
luminous stars. These relatively young stars are located
in the disk, especially in the spiral arms, embedded in
the dust and gases from which they formed. They are
relatively high in heavy elements (similar to the Sun,
about 2 percent by mass) in addition to their hydrogen
and helium. Population II stars, like those in globular
clusters, are found toward the galactic nucleus and in
the halo. These stars are older and are made almost
entirely of hydrogen and helium.
Age and Formation of Our Galaxy
FIGURE 4-793
Astronomers can use the estimated age of star clusters
to determine the approximate age of the galaxy itself.
The oldest open clusters in the Milky Way are 9 billion
to 10 billion years old, indicating that the galactic disk
has an age of at least 10 billion years. The galactic halo
is believed to have formed over 13 billion years ago,
perhaps a few hundred million years after the universe
American astronomer Vera Rubin demonstrated that
stars make up only a fraction of the mass of a normal
galaxy such as ours. However, gravitational attraction
between luminous matter cannot explain the observed
velocities of stars and gas clouds in galaxies or
galaxies in clusters. Most of the matter in the universe
is dark matter, inferred from its gravitational effects
but otherwise undetectable. Our visible galaxy must
contain a lot of dark matter and be surrounded by
a huge, massive, dark matter galactic halo at least
300,000 light years in diameter (Figure 4-8).
An image of Sgr A*, a supermassive black hole at the
center of the Milky Way galaxy, collected by NASA’s
Chandra X-ray Observatory. The inset shows the detection
of an unusually large X-ray flare from Sgr A*.
Although the exact process that led to the formation
of our Galaxy is still a matter of debate, the current
hypothesis starts with an extended cloud of cosmic gas
spread out over an irregular region of space. This gas
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In 1944, American astronomer Walter Baade divided
stars into two classes: Population I and Population
II. Although now known to be oversimplified, this
classification system provided early insight into
the relationship between age, dynamics, and the
production of elements in stars and galaxies.
FIGURE 4-996
The Skylab missions succeeded in obtaining vast amounts of
scientific data and demonstrated to the American public that astronauts could live and work productively
in space for months at a time. After Skylab’s final crew left in February 1974, the station remained in orbit.
Although return missions were originally planned, financial constraints and the development of the Space
Shuttle program prevented Skylab from being used again. On July 11, 1979, Skylab disintegrated upon
re-entering Earth’s atmosphere, scattering debris across the Indian Ocean and part of Australia. Although
the mission itself lasted less than a year, Skylab represented a crucial step toward later orbital research
laboratories such as the International Space Station.
flattened distribution of the galactic disk. In this way,
the galaxy’s thin profile took shape.
FIGURE 4-895
Classification of Galaxies
An artist’s impression of the expected dark matter halo
surrounding the Milky Way galaxy.
cloud may have resulted from the merging of several
other systems. Over time, the rotation of the cloud
caused gas and dust to contract and form a spinning
galactic disk. As the disk flattened, stars that had
already formed were left behind in the surrounding
halo while new generations of stars adopted the
Galaxies do not all look the same—they come in a
variety of shapes and sizes. Galaxies were first classified
into groups according to their structure by astronomer
Edwin Hubble in 1924. Hubble conducted a systematic
study of distant galaxies using the 2.5-m Mount Wilson
telescope, the world’s largest at the time. The Hubble
classification scheme, still in use today, consists of
four distinct types: elliptical, spiral, barred-spiral, and
irregular (Figure 4-10). Elliptical galaxies (class E)
have no disk, no spiral arms, and very little visible gas
and dust. They range from nearly perfect spheres, E0, to
the flattest, E7. Like the halo of the Milky Way, elliptical
galaxies are composed mostly of old, reddish, low-mass
Spiral galaxies (class S) are divided into two major
subcategories. As we have seen from the example of
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Following the success of the Apollo program, NASA sought to
establish a lasting presence in space that could serve as the next
step in wider space exploration. Hardware from the canceled
final Apollo missions was repurposed to develop Skylab, the first
American space station. Skylab was launched on May 14, 1973.
Its first crew was Pete Conrad, Paul Weitz, and Joe Kerwin,
who spent twenty-eight days in orbit. A second crew orbited
for sixty days, and Skylab’s third and final crew spent eightyfour days in orbit, a record that would not be broken until the
Shuttle-Mir program over twenty years later. Astronauts aboard
Skylab conducted about 270 experiments in solar astronomy,
biology, and materials science. In addition, the mission served as
a landmark investigation of the physiological effects of extended Skylab, NASA’s first space station,
photographed in 1974.
space flight on humans.
The Hubble galactic classification scheme.
our Milky Way, spiral galaxies are disk-shaped with
spiral arms winding out from a central bulge. They are
subdivided into subclasses Sa, Sb, and Sc, according to
the size of the central bulge and how tightly wound the
spiral arms are. Barred-spiral galaxies (class SB) look
like normal spiral galaxies except that the spiral arms
unwind from the ends of a bar-shaped concentration of
material. Spiral galaxies have large amounts of gas and
dust in the disk and contain young, middle-aged, and
old stars. Disk-shaped galaxies with no spiral arms or
recent star formation are known as lenticular (lensshaped) galaxies (class S0).
Irregular galaxies (class Irr) have no regular
geometric shape or structure. They usually contain
gas and dust, mostly bright young stars and clouds of
ionized gas, and some old stars. The Large and Small
Magellanic Clouds are a pair of irregular galaxies that
orbit the Milky Way galaxy.
Galactic Distances and Distribution
The distance to a galaxy is a key to determining the
galaxy’s basic properties. These distances are so
large that the units of light-years, parsecs, or even
kiloparsecs are not convenient to measure them. The
typical astronomical unit for galactic distances is the
megaparsec (Mpc), which equals 1 million parsecs or
3.26 million light-years. In order to find the distance
to a galaxy, astronomers must search among its stars,
nebulae, or star clusters for astronomical objects with
known luminosities. An object of known brightness
that serves as a reference for astronomical distance
is called a standard candle. Useful standard candles
include Cepheid variable stars out to 100 million lightyears (30 Mpc) and standard types of galaxies out to
500 million light-years (150 Mpc). Type Ia supernovae,
a kind of Type I supernova, are also important
standard candles because they have very consistent,
well-known luminosities.
When the absolute magnitude of a standard candle
is known, the distance to that standard candle can be
determined by comparing the absolute magnitude with
the apparent magnitude. In this way, the distance to
the standard candle can give estimates to the distance
to the object the standard candle is near or in which it
is embedded. When the distance to a galaxy has been
determined, the star’s luminosity and diameter can be
determined from its apparent magnitude and apparent
diameter. A galaxy’s mass is calculated from observed
gravitational effects on its stars or gas clouds or on
neighboring galaxies. Observational data indicate that
most of the mass is unobserved dark matter.
Galaxy Clusters
A group of galaxies held together by gravitational
attraction is known as a galaxy cluster. Most galaxies
are members of galaxy clusters, and a single cluster
can contain from several to thousands of individual
galaxies (Figure 4-11). The galaxies are held together
by the force of gravity as they orbit one another at
velocities of about 1,000 km per second. The Milky
Way galaxy belongs to a typical small cluster, the
Local Group, with about forty members. “Local”
refers to the fact that the galaxies are within a region
3 million light-years across. Three of these galaxies—
our Milky Way, Andromeda (M31), and M33 in
Triangulum—are spirals. The others are ellipticals
(including M31’s bright companions NGC 205 and
M32) or irregulars (including the Magellanic Clouds).
Most are dwarf galaxies, or small galaxies a few
thousand light-years in diameter.
Clusters can be divided into two classes by shape.
Regular clusters are relatively compact, with highest
density near the center. Members are mostly elliptical
and S0 galaxies. Many regular clusters emit radio
radiation from active galaxies. About a third emit
X-rays from intracluster gas at about 100 million K. In
contrast, irregular clusters, including our Local Group,
have a looser structure with little central concentration
and less very hot gas. They contain many spiral and
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example, the two spiral galaxies NGC 2207 and IC
2163 (Figure 4-12).
The Virgo Cluster.
irregular galaxies. Fewer emit radio waves or X-rays.
Clusters of galaxies can themselves form clusters. A
supercluster is the term for a cluster of clusters of
galaxies. Superclusters are the largest gravitationally
bound systems observed so far. They are about 100
million to 1 billion light-years across. Both the Local
Group with our Milky Way galaxy and the Virgo
Cluster are members of the Local Supercluster.
Superclusters are located in thin sheets that border
voids, or regions where few galaxies are observed.
These voids can be thought of as gigantic “bubbles”
with clusters of galaxies along their surfaces. The
observable universe consists mostly of vast voids in
between superclusters.
Colliding Galaxies
When two galaxies collide, they pass through each
other. As they do, their clouds of gas and dust
become unusually dense and trigger a galaxy-wide
increase in star formation, resulting in what’s known
as a starburst galaxy—a galaxy in which stars are
forming at a rate hundreds of times greater than in our
galaxy. Collisions between galaxies take place over
the course of several million years but can be modeled
by astronomers using computers. Galaxies presently
undergoing collisions can also be observed—for
Other forms of interaction between galaxies are also
possible, particularly when the galaxies have very
different masses. A more massive galaxy can gradually
strip away and take in gas, dust, and stars from a smaller
one. If the masses are extremely different, it is possible
for a very large galaxy to consume a much smaller one
in a process nicknamed “galactic cannibalism.” The
nucleus of the smaller galaxy, falling to the center of
the more massive galaxy, could fuel the larger galaxy’s
energy output for millions of years.
Active Galaxies
An active galaxy is a galaxy that emits exceptionally
large amounts of energy in the form of radiation. The
center of an active galaxy is called an active galactic
nucleus, which acts as the source of the galaxy’s
tremendous output of energy (Figure 4-13). The
energy output of an active galaxy far exceeds the total
output from normal nuclear fusion reactions in stars
in normal galaxies. Active galactic nuclei often eject
great jets of hydrogen gas outward at very high speeds.
Astronomers have concluded that the source of an
active galaxy’s colossal energy production is a central,
powerful source of gravity that strongly attracts
nearby matter. An extremely massive object, such as
a black hole with a mass millions of times that of the
Sun, is one possible source of gravitational attraction.
According to this model, as dust, gas, and even stars
spiral in toward the black hole, they accelerate and
heat up. This hot infalling matter emits the radiation.
Astronomers are able to determine the mass of the
black hole based on the speed of infalling matter.
One type of active galaxy is the Seyfert galaxy, first
described by American astronomer Carl K. Seyfert in
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While a collision between galaxies can lead to dramatic
changes for the overall structure of the galaxies
involved, it has essentially no effect on the individual
stars that make up the galaxies. This is because the stars
are so small compared to the distance between them
that they simply glide past one another. The likelihood
of any two stars colliding or interacting is vanishingly
small. Even when the density and star population of the
galaxy temporarily increases while the galaxies overlap,
the separation between the stars is still great enough that
no interstellar interactions occur.
1943 (Figure 4-14). A Seyfert nucleus, typically less
than a single light-year across, emits a much larger
brightness than a normal spiral galaxy. Its spectrum
has broad emission lines that indicate turbulent
motions of very hot gas at velocities of thousands
of kilometers per second. Heated dust enveloping
the nucleus probably absorbs high-energy radiation
emitted from the energized core and re-emits it at
longer infrared wavelengths. About two percent of all
spiral galaxies are Seyfert galaxies.101
The spiral galaxies NGC 2207 and IC 2163 undergoing a
galactic collision.
FIGURE 4-13100
NGC 1448, a galaxy with an active galactic nucleus.
A radio galaxy is an active galaxy that emits large
amounts of radiation in the form of radio waves. The
radiograph of a typical radio galaxy shows two large
regions of energy at radio wavelengths, known as radio
lobes, on opposite sides of a visible galaxy (Figure
4-15). This pattern of radio energy is characteristic
of synchrotron radiation, or radiation produced by
electrons spiraling around at nearly the speed of light
in a strong magnetic field. If the hypothetical central
black hole exists, the jets of high-speed electrons are
ejected by matter as it disappears into the black hole.
The electrons emit the colossal radio energy while
accelerating within a strong magnetic field.
FIGURE 4-14102
The Circinus galaxy is a Seyfert galaxy.
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Radio Galaxies
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FIGURE 4-15103
A composite image of radio galaxy Centaurus A. The radio lobes can be seen in purple.
During the 1950s, astronomers made observations
of distant sources of radio waves that bore a
striking resemblance to stars in visible-wavelength
photographs. It was apparent from their spectra that
these objects were not stars, so they were termed
quasi-stellar radio sources, or quasars (Figure 4-16).
Most of the hundreds of thousands of observed quasars
emit extraordinary power across a broad range of
wavelengths, from radio to gamma rays. Although
most quasars are not strong radio sources, the original
name has stuck. Quasars are extremely compact,
typically about one light-day across (not much bigger
than our Solar System), but they shine brighter than
a thousand normal galaxies. Most quasars vary
irregularly in their light output.
Quasars exhibit the highest redshifts observed. Light
emitted by quasars exhibits extremely strong shifts
toward lower frequencies when received on Earth (i.e.,
a Doppler redshift). This high redshift indicates that
quasars are racing away from us at speeds of over 90
percent the speed of light. Ultraviolet light emitted
by quasars with large redshift is received as red light
on Earth. This redshift implies that quasars are very
distant objects. They were shining when the universe
was young. Observations of their environments
confirm that quasars are extremely far away. Radiation
detectors show identical spectra for quasars, the
faint haze around the quasars, and remote galaxies.
Brilliant, compact quasars must be at the centers of
very distant galaxies.
Twin or multiple images of the same quasar, formed
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FIGURE 4-17107
Tensions between the United States and the Soviet Union remained
high as the Cold War dragged into the 1970s. As the Vietnam
War drew to a close, relations between the two nations began to
gradually improve, and the prospect of a joint spaceflight mission
seemed achievable. Discussions between the U.S. and U.S.S.R.
led to both nations signing a joint agreement in 1972 to launch the
first two-nation cooperative space mission, the Apollo-Soyuz Test
Project (ASTP), in 1975.
The ASTP was considered a success both as a technological mission and a political milestone. Apollo
crewmember Vance Brand was quoted as saying, “I really believe that we were sort of an example…to the
countries. We were a little of a spark or a foot in the door that started better communications.”106 Not only was
the ASTP the symbolic end of the Space Race, but it also marked a significant step toward the end of the Cold
War and served as a model for international cooperation in space exploration. In the 1990s, the two nations
would again work together on docking missions between the Space Shuttle and space station Mir.
FIGURE 4-16104
A Hubble Space Telescope image of Quasar 3C 273.
by a gravitational lens, provide further evidence that
quasars are located at large cosmological distances
(Figure 4-18). According to Einstein’s theory of
general relativity, the modern theory of gravitation,
starlight passing near a massive body will be deflected.
A gravitational lens is a concentration of mass that
bends the path of light emitted by distant objects.
Galaxies, intracluster gas, or dark matter can act as a
gravitational lens and let us obtain images and spectra
of outlying objects too faint to examine otherwise.
Different hypotheses were proposed and abandoned
to explain the stupendous energy output of these
quasars. Einstein’s theory of general relativity was
used to attribute the quasars’ extraordinary redshift to
an enormous gravitational force (i.e., a gravitational
redshift), which would have implied that the quasars
were closer and not so powerful after all. Alternative
theories involved collisions of material particles
with antimatter, a hypothetical type of exotic matter.
The evidence shows that a quasar is powered by a
supermassive black hole. Since quasar activity was
much more common in the early universe than it is
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The Soyuz and Apollo spacecrafts launched within seven and a
half hours of each other on July 15, 1975. After the spacecraft had
Three American astronauts and two Soviet
successfully docked on July 17, Thomas Stafford and Aleksey
cosmonauts composed the crew of the ApolloLeonov, the two mission commanders, exchanged the historic
Soyuz Test Project. From left to right: Donald
first international handshake in space. The crew members then
Slayton, Thomas Stafford, Vance Brand,
proceeded to conduct scientific experiments, visit each other’s
Aleksey Leonov, and Valeriy Kubasov.
spacecraft, eat together, and exchange gifts. The Apollo and
Soyuz crews separated after forty-four hours together, and after
undocking each spacecraft remained in orbit for several days before returning to Earth.
FIGURE 4-18105
FIGURE 4-19108
The “Einstein cross,” four images of the same distant
quasar caused by a galaxy acting as a gravitational lens.
today, a quasar could be a development phase in young
The Expanding Universe
The question of how the universe began has tantalized
humankind for thousands of years. Previously only
discussed in the realm of philosophy, this question
has become central to the branch of astronomy
known as cosmology. Cosmology is the study of the
origin, present structure, evolution, and destiny of
the universe. Cosmologists construct mathematical
descriptions that try to explain how the universe
began, how it is changing as time goes by, and what
will happen to it in the future. These models must be
consistent with the observational data that has been
collected from stars and galaxies. Two basic types of
cosmological models, evolutionary and steady state,
have been tested in the last fifty years.
The basic observation that must be accounted for
by any cosmological model is that light from distant
galaxies is shifted in wavelength toward the red
end (longer wavelengths) of the spectrum. This
phenomenon is called the cosmological redshift.
According to modern theory, cosmological redshift
results from an expansion of space. As distances
between galaxy clusters increase, traveling light
waves also become “stretched,” and their observed
wavelengths increase (Figure 4-19). Therefore, greater
redshifts must correspond to more distant galaxies
and earlier eras. The most distant, youngest galaxies
we observe have the greatest observed redshifts, and
in accordance with Hubble’s law, these galaxies are
receding from us at the fastest rate.
The basic assumption we make in attempting to
understand the universe is called the cosmological
principle. The cosmological principle states that on a
sufficiently large scale the universe is homogeneous
and isotropic. In other words, at any given time, the
distribution of matter is the same everywhere in space,
and the universe looks the same in all directions.
According to the cosmological principle, there is
nothing special about our region in space—at any
given time, an observer anywhere in the universe
would see just about the same things we do on a large
scale. The cosmological principle is important because
it lets us assume that the small portion of space that
we can see is truly representative of all the rest of the
universe that we cannot see. It allows us to formulate
a theory that explains the entire universe, including
those parts we cannot observe.
There are two notable implications from the
cosmological principle. First, the universe cannot have
an edge because a universe with an edge would not
be homogeneous. Second, the universe has no center,
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An illustration of cosmological redshift. As the surface
of the balloon expands, the wavelength of a wave on its
surface increases.
Hubble’s Law
American astronomer Edwin Hubble, who spent most
of his life studying galaxies, examined the relationship
between the velocity of recession and the distance
away for many galaxies. In doing so, he discovered a
linear relationship between the velocity and distance:
The farther away a galaxy is, the faster it is receding
(Figure 4-20). Hubble’s law says that a galaxy’s
velocity of recession, v, is directly proportional to its
distance from us, d. Hubble’s law is often expressed as:
v = H0 d, where H0 is known as the Hubble constant.
It is difficult to determine an accurate value for the
Hubble constant due to uncertainties in distance scales
outside our own galaxy. There are multiple methods of
determining the value of H0, but most methods agree
on a value approximately equal to 70 km/s/Mpc. In
recently published research using different methods,
researchers obtained values of H0 equal to 74.03 ± 1.42
km/s/Mpc109 and 69.8 ± 0.8 km/s/Mpc110.
observed redshift of a galaxy corresponds to a velocity
of recession v = 700 km/s, and we approximate H0 as 70
km/s/Mpc, then the galaxy’s distance d from Earth in
Mpc must be about 700 divided by 70, or 10 Mpc.
As an analogy for the expanding universe model,
consider a loaf of raisin bread expanding as it bakes
(Figure 4-21). As the dough expands, the raisins
are pushed away from one other at speeds that are
proportional to the distance between them. Two raisins
that were originally close to each other are pushed apart
slowly, but two raisins that were far apart, having more
dough between them, are pushed apart faster. There are
limitations to this analogy—the universe does not have
an edge in the way that the loaf of bread has a crust, for
instance—but it is useful for the purposes of illustration.
FIGURE 4-21112
FIGURE 4-20111
Raisins in a baking loaf of bread accelerate away from one
another in a similar fashion to galaxies in the expanding
The Big Bang Theory
A plot of observed galactic redshift (i.e., rate of recession)
versus distance from Earth. The slope of the line of best fit
provides an estimate of Hubble’s constant.
The Hubble constant is an important and fundamental
quantity in cosmology—it gives the rate at which
the galaxies are receding, or equivalently, the rate at
which the universe is expanding. It also can be used
in Hubble’s law to determine the distance to galaxies
based upon their observed redshifts. For example, if the
If we consider the expansion of the universe as a
movie, we can imagine running the movie backward
to the time at which all the matter and radiation of our
present universe were packed together at a singular
point. This was the state of the universe at the time of
the Big Bang—a violent expansion event that marked
the beginning of time and space as we know it. The
Big Bang theory states that about 13.77 billion years
ago our universe expanded rapidly from an infinitely
hot, dense state, and it has been evolving ever since.
At 10 –43 seconds after the Big Bang, the temperature
was 1,032 K. The early universe was opaque, made of
a nearly featureless hot, charged gas that emitted and
trapped high energy photons of light. Expansion cooled
the matter and photons of that early inferno. Within a
few seconds, protons (hydrogen nuclei), neutrons, and
electrons formed. Within minutes, deuterium (heavy
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because if it did, the universe would not appear the
same in all directions from any noncentral point.
An illustration of the expansion of the universe over its 13.77-billion-year lifetime.
hydrogen), helium, and a few lithium nuclei formed.
After about 380,000 years, the expanding universe
cooled sufficiently for electrons and nuclei to combine
to form neutral atoms (Figure 4-22). As expansion
continued, electromagnetic radiation interacted less
frequently with matter, allowing photons to spread
freely throughout the universe. Several million years
later, stars and galaxies began to form. The universe
has continued to expand in space-time, the galaxies
have continued moving apart, and the radiation has
continued to cool ever since. Today we continue to
observe evidence of the universe’s expansion. Stars
are still forming inside galaxies, using the original
hydrogen from the Big Bang. The observed material of
the universe is approximately 74 percent hydrogen and
24 percent helium, with traces of other light elements,
such as deuterium and lithium, as predicted.
Observational Tests
Astronomers test a cosmological model by seeing
whether it agrees with all the observational data we
have gathered about the universe. The most direct way
to check how the universe is evolving is to compare
the way it appears today with the way it appeared
billions of years ago. Since we cannot actually make
observations over billions of years as the universe
ages, astronomers instead look at galaxies that are at
different distances away from us. Although the idea—
to look back in time, you study distant galaxies—is
simple, it is very difficult to carry out in practice.
We can measure redshifts readily, but technology
is not sufficiently developed for precise distance
measurements. Consequently, all data that might be
used to check cosmological models have uncertainties.
Distant galaxies differ from nearby galaxies, which
confirms that our universe evolves.
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FIGURE 4-22113
Although the Big Bang theory is the prevailing model
of universal expansion, other theories have been
proposed. The Steady State theory, first proposed
by British astronomer Sir Fred Hoyle in 1948, was
previously considered an alternative to the Big Bang
theory. According to the Steady State theory, the
universe does not evolve or change in time. There was
no beginning in the past, and there will be no end in
the future—the universe remains the same forever.
In order to explain the observation that the universe
is expanding, the Steady State model says that new
hydrogen is created continuously in empty space at
a rate just sufficient to replace matter carried away
by receding galaxies. However, the theory does not
explain where the new hydrogen comes from. The
Steady State theory has been rejected by the scientific
community because the Big Bang theory is more
strongly consistent with observed evidence.
In 1964, American physicists Arno Penzias and
Robert Wilson detected a constant microwave signal
radiating equally from all directions in the sky
and, after accounting for all other possible sources,
correctly concluded that the source of the radiation was
outside our own galaxy. Penzias and Wilson had in
fact made the first detection of the cosmic microwave
background, for which they were later awarded the
1978 Nobel Prize in Physics.114 Data from the U.S.
Cosmic Background Explorer (COBE) satellite in
1989 matched this nearly uniform radiation to that of
a blackbody at a temperature of 2.7 K (Figure 4-23).
Astronomers had conclusively detected the predicted
remnant of the Big Bang radiation, which was a key
piece of evidence in support of the Big Bang theory.
Cosmic Microwave Background Radiation
Cosmic Acceleration
FIGURE 4-23115
Temperature of the cosmic microwave background
radiation, as collected by the COBE satellite. Different
colors represent extremely slight variations compared to
the 2.7 K average temperature.
Twenty-First-Century Cosmology
Has the universe’s rate of expansion remained constant
since the Big Bang? In order to answer this question,
cosmologists compare the current value of the Hubble
constant—that is, the current rate of expansion—
with the value billions of years ago. Surprisingly,
researchers discovered that the Hubble constant is not
constant over time, and in fact it has increased over
time. Observations of Type Ia supernovae in distant
galaxies show that they are fainter than expected from
redshift data, which indicates that the expansion of the
universe began accelerating a few billion years ago
(Figure 4-24). Astrophysicists Saul Perlmutter, Brian
P. Schmidt, and Adam G. Riess were jointly awarded
the 2011 Nobel Prize in Physics for demonstrating the
accelerating expansion of the universe.116
This accelerating rate of expansion poses an apparent
contradiction. If gravity were the only force acting
between components of the universe, we would
expect the rate of expansion to decelerate due to the
continued gravitational attraction. The supernova
redshift data indicate that there must be some strong
negative pressure that opposes and overcomes gravity.
Cosmologists have proposed that our universe is full of
dark energy, an as-yet unknown source of gravitational
repulsion, to explain the accelerating expansion. Dark
energy was originally introduced by Einstein in his
formulation of the theory of general relativity, in what
he referred to as a “cosmological constant.”
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The Big Bang theory predicts that the universe should
still be filled with cosmic microwave background
radiation, a vestige of the tremendous energy released
by the Big Bang. According to the theory, the Big Bang
sent intense short-wavelength radiation (corresponding
to a blackbody temperature of trillions of degrees)
in all directions like an enormous explosion. Over
time, that radiation would spread out and fill the
expanding universe uniformly as it cooled. By the
time this radiation reached Earth, the theory predicts
that it would be observed as microwave radiation,
corresponding to a blackbody temperature of only 2.7
K above absolute zero.
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FIGURE 4-24117
An illustration (not to scale) of how the universe’s rate of expansion has accelerated rather than remained constant.
Models of Expansion
With the conclusion that the universe is presently
expanding, a natural question emerges: will it continue
to expand forever? In order to answer this question,
astronomers must consider the average density of the
universe. If the average density is low, there is less
mass and less gravity, and the universe’s expansion
will not be slowed very much by gravitational
attraction. It can therefore expand forever. Higher
average density, on the other hand, means there is
more mass and more gravity and that the stretching of
space might slow down enough that the expansion will
eventually stop. An extremely high density might even
cause the universe to collapse again.
open universe model says that the universe will
continue to expand indefinitely. The flat universe
model predicts a continuing but slowing expansion
that approaches zero as time approaches infinity. In
contrast, a closed universe model predicts that the
universe will not expand forever. Instead, gravity will
cause the expansion to gradually slow until it collapses.
If the universe is closed, we happen today to be in the
observed expanding phase. In the future, our expanding
universe will slow down, come to a complete stop,
and then begin to contract. As the universe contracts,
galaxies will fall back in toward one another until all
matter is once again compressed into an extremely hot,
dense state known as the “Big Crunch.”
Each of these possibilities corresponds to a different
hypothetical model of the universal expansion. The
The critical density is the minimum average density
of matter and energy required to make the universe
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The preferred Einstein-de Sitter model of inflationary
cosmology calls for a homogeneous, flat universe.
However, observations indicate that ordinary matter
contributes only 5 percent of the mass necessary to
reach the critical density, and dark matter contributes
an additional 27 percent. If we only account for
ordinary and dark matter, the resulting gravitational
attraction is less than would be necessary to eventually
halt the observed expansion. If the universe is flat as
theorized, the remaining 68 percent must be supplied
by dark energy. In other words, according to modern
cosmological theory, 95 percent of the universe is made
up of either dark matter or dark energy! Other forms of
matter have been proposed as possible alternatives to
dark matter, such as MACHOs (massive compact halo
objects), as well as WIMPs (weakly interacting massive
particles). Future observations will test how much and
what kinds of mass and energy actually exist.
Big Bang Questions
For all its successes, the standard Big Bang model fails
to explain how an explosive beginning resulted in both
the homogeneity of the cosmic background radiation
and the large-scale structure of the observable
universe. If the initial distribution of energy and
mass was smooth, gravity alone could not clump
ordinary matter into the observed large clusters and
superclusters of galaxies in the calculated lifetime of
the universe. Instead, there were most likely some
initial anisotropies and inhomogeneities. Between
2001 and 2010, the Wilkinson Microwave Anisotropy
Probe (WMAP), a NASA observational satellite,
measured slight temperature variations in the cosmic
FIGURE 4-25119
The Cosmic Microwave Background temperature
fluctuations from the seven-year Wilkinson Microwave
Anisotropy Probe (WMAP) data seen over the full
sky. Different colors correspond to extremely slight
temperature variations.
microwave background radiation (Figure 4-25). These
variations provided evidence of tiny fluctuations in the
nearly uniform density of the early universe—ripples
of wispy matter whose gravitational pull could have
grown the galaxies, clusters of galaxies, and the great
voids in space today.
A critical question with the Big Bang theory involves
what’s known as the flatness problem. WMAP’s
measurements of fluctuations in the cosmic microwave
background indicate that the shape of the observable
universe is relatively flat. The earliest density of the
universe must likewise have been extraordinarily close
to the critical density—but why? In other words, why
did the universe essentially start off as being flat? In
1981, American physicist Alan Guth proposed inflation,
a brief phase of incredibly rapid expansion shortly after
the Big Bang, to account for the present vast extent of
the universe and its uniformity. The addition of inflation
theory to the Big Bang theory resolves the flatness
problem—essentially, a rapid inflation would force the
universe to become flat, much as a spot on the surface of
a balloon becomes flat as the balloon inflates.
Age and Size of the Universe
Astronomers have two primary methods of determining
the age of the universe: (1) measure the rate of expansion
and extrapolate back to the Big Bang; and (2) determine
the age of the oldest stars, which places a lower limit
on the age of the universe. The Hubble time, equal
to 1/H0, is an upper limit on the age of the universe in
accordance with Hubble’s law. This upper limit depends
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flat—in other words, the average density that will be
just enough to slow the expansion of the universe to
zero at some future time. The calculated value of the
critical density, which depends on the value of the
Hubble constant, is approximately 9 × 10 –27 kg/m3,
which is equivalent to a few hydrogen atoms per cubic
meter.118 The abundance of the lightest elements in
space today places a limit on the maximum possible
amount of ordinary matter in our universe. (By
“ordinary” matter, we mean matter that interacts
visibly or electromagnetically with its surroundings,
i.e., matter composed of protons, neutrons, and
electrons.) Essentially all existing hydrogen, helium,
and lithium is presumed to have been created at the
time of the Big Bang, so it is tightly linked to the initial
density of matter.
In 2008, WMAP measurements put the time of the Big
Bang at 13.77 ± 0.059 billion years ago. Hubble Space
Telescope observations of white dwarf stars in the oldest
globular clusters yield a similar age. White dwarfs cool
down at a predictable rate—the older the dwarf, the
cooler it is. The age of the white dwarfs was determined
to be 12 to 13 billion years, and since the first stars
formed around 400 million years after the Big Bang,
this method is consistent with the WMAP data.
How big is the universe? Astronomers measure the size
of the observable universe by considering the distance
that light has traveled in the time since the Big Bang.
The Hubble length, equal to the speed of light divided
by the Hubble constant (c/H0), serves as a characteristic
length scale of the universe. Using the accepted value
of H0, the Hubble length is found to equal about 14
billion light years.
A galaxy is an enormously large collection of
matter—stars, gas, dust, and black holes all
held together by gravitational attraction. The
galaxy in which we live is named the Milky
Way galaxy and is a spiral galaxy.
Many stars are held together in star clusters,
or groups of stars that were formed around the
same time. Globular clusters contain the oldest
known stars.
The interstellar medium is the matter, such as
gas and dust, that occupies the space between
stars. A concentration of gas and dust in space
is called a nebula.
We cannot look more than about a thousand
light-years in most directions into our galaxy
because interstellar dust absorbs light in the
visible spectrum. Instead, astronomers use
radio, infrared, and high-energy waves to
image the space beyond. Radio waves with 21cm wavelength are particularly important.
The Hubble classification scheme, still in use
today, identifies four distinct types of galaxies.
Elliptical galaxies are round with no disk;
spiral galaxies are disk-shaped with spiral
arms; barred-spiral galaxies have a bar-shaped
nucleus; and irregular galaxies have no regular
A group of galaxies held together by
gravitational attraction is known as a galaxy
cluster. Superclusters are clusters of clusters of
An active galaxy is a galaxy that radiates
exceptionally large amounts of energy,
primarily from its nucleus. Seyfert galaxies are
one type of active galaxy that emits strongly in
the infrared. Radio galaxies are active galaxies
that emit radio waves.
Cosmology is the study of the origin, present
structure, evolution, and destiny of the
universe. The cosmological principle states
that on a sufficiently large scale the universe is
homogeneous and isotropic.
According to Hubble’s law, the farther away
a galaxy is, the faster it is receding. The
Hubble constant describes the rate at which the
universe is expanding.
The Big Bang theory states that 13.77 billion
years ago our universe expanded rapidly from
an infinitely hot, dense state, and it has been
evolving ever since. Predictions vary about the
future of the universe, according to the open,
closed, or oscillating universe models.
The Big Bang theory predicts that the universe
should still be filled with cosmic microwave
background radiation, at a blackbody
temperature of 2.7 K. The detection of this
radiation provided strong evidence in favor of
the Big Bang model.
Our universe’s expansion is accelerating,
indicating that some force is acting to oppose
gravitational attraction. To explain this
accelerating expansion, cosmologists have
proposed that our universe is full of dark energy.
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greatly on the value of the (still imprecise) Hubble
constant, with a correction for the slowing down of the
universe in the past and more recent acceleration.
In Section I of the resource guide, we described some
of the principles of motion, force, and light that are
foundational to our understanding of astronomy. We
discussed the different forms of radiation that make
up the electromagnetic spectrum and examined how
telescopes collect and focus light to produce images of
distant astronomical objects.
In Section II, we explored the Sun and stellar processes.
We saw how stars undergo complex life cycles that
can span billions of years, from protostars to main
sequence stars to very massive stars that may collapse
to form extremely dense neutron stars. In this section,
we considered the properties and structure of the Sun,
including behaviors such as sunspots and the solar wind.
Section III focused on our Solar System. We examined
each of the eight planets in our Solar System in turn
and considered their unique sets of properties. Along
the way, we described landmark efforts to explore our
Solar System, through direct observations, satellites,
and surveying rovers. We also discussed smaller
objects such as dwarf planets, comets, and asteroids.
The final section of the resource guide provided an
overview of galaxies and cosmology, which took us
on a tour of the largest scales of time and distance.
We learned about the formation and classification of
galaxies, and we ended by describing the landmark
discovery that the universe is expanding. Having
reached the end of the guide, we hope that you feel
inspired and engaged to learn more about astronomy,
whether by stargazing at night or by following the
latest NASA spaceflight missions. May your newfound
understanding lead you to a deeper connection with
the universe around you.
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Over the course of this resource guide, we have worked
to develop an understanding of astronomy on many
different scales, from the scale of the Earth and the Solar
System to galactic clusters of incomprehensible size.
We have seen how the tools of astronomical observation
have evolved significantly over the course of thousands
of years—from the naked eye in ancient times, to optical
telescopes, to instruments that can detect radiation
invisible to humans, to probes that traverse the furthest
reaches of our Solar System. We use robotic rovers to
investigate the surfaces of other planets in our solar
“neighborhood,” and telescopes collect radiation that
was emitted millions of years ago by distant stars and
galaxies. The models that astronomers use to make
predictions are constantly being refined in response to
observations and data collected every single day. With
each new discovery, our understanding of the universe
grows just as rapidly as our determination to learn more.
c. 600 bce –
Philosophers, including Thales of Miletus, suggest that natural phenomena can be explained
and understood by humans.
c. 350 bce – Aristotle proposes a geocentric model of the Universe.
c. 280 bce – Aristarchus proposes an early heliocentric model of the Universe.
Ptolemy extends Aristotle’s model to incorporate existing data.
1543 –
Copernicus publishes a heliocentric model of the Universe.
1609 –
Kepler publishes his first two laws of planetary motion.
1610 –
Galileo observes the motion of sunspots across the surface of the Sun.
1619 –
Kepler publishes his third law of planetary motion.
1632 –
Galileo publishes support of the Copernican heliocentric model.
1687 –
Newton publishes the Principia, containing three laws of motion and the law of universal
1781 –
William Herschel discovers Uranus using a telescope.
1801 –
Sicilian astronomer Giuseppi Piazzi discovers Ceres, the largest known asteroid.
1868 –
Helium is detected as an unknown element in spectral lines from the Sun.
1872 –
Henry Draper photographs the stellar spectrum of Vega, the first of its kind.
1911 –
Hertzsprung and Russell introduce H-R diagrams.
1924 –
Edwin Hubble develops a galaxy classification scheme.
1930 –
American astronomer Clyde Tombaugh discovers Pluto.
1931 –
Karl Jansky discovers radio waves originating from the Milky Way.
1943 –
Seyfert galaxies, a type of active galaxy, are first described.
1957 –
October 4: The Soviet Union launches Sputnik 1, the first artificial satellite.
1958 –
January 1: The United States launches satellite Explorer 1.
1961 –
April 12: Soviet cosmonaut Yuri Gagarin becomes the first person in space.
1961 –
May 20: American astronaut Alan Shepard achieves spaceflight.
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150 ce –
May 25: President Kennedy proposes a Moon landing in a congressional address.
1962 –
February 20: American astronaut John Glenn orbits the Earth.
1962 –
The Mariner 2 spacecraft conducts a flyby of Venus.
1964 –
The binary star Cygnus X-1, an intense X-ray source, is discovered.
1964 –
The cosmic microwave background radiation is first detected by Penzias and Wilson.
1965 –
June: Edward White takes the first spacewalk by an American during the Gemini 4 mission.
1966 –
November: The final Gemini mission ends, paving the way for the Apollo program.
1967 –
February 21: Apollo 1 crewmembers die in a cabin fire during a prelaunch test.
1968 –
A pulsar is detected at the center of Crab Nebula, matching predictions.
1968 –
December: Apollo 8 becomes the first manned mission to orbit Earth and return.
1969 –
May: The Apollo 10 mission serves as a “dress rehearsal” for a Moon landing.
1969 –
July 16: The Apollo 11 mission launches from Kennedy Space Center.
1969 –
1970 –
July 20: American astronaut Neil Armstrong becomes the first person to walk on the Moon’s
The Soviet spacecraft Venera 7 lands on Venus, becoming the first spacecraft to land on
another planet.
1973 –
May 14: Skylab, the first American space station, launches.
1974 –
The Mariner 10 spacecraft photographs the surface of Mercury during a flyby.
1975 –
July 17: The Soyuz and Apollo spacecrafts successfully dock as part of a joint mission between
the U.S. and Soviet Union.
1979 –
U.S. Voyager 1 & 2 spacecraft explore the Jovian system.
1981 –
American physicist Alan Guth proposes inflation as a modification to the Big Bang theory.
1986 –
The European Giotto spacecraft makes the first closeup observations of a comet.
1989 –
The Hipparcos precision astronomy experiment is launched.
1995 –
The U.S. Galileo spacecraft reaches Jupiter.
1997 –
The U.S. Pathfinder lander and Sojourner rover explore the surface of Mars.
1999 –
The Chandra X-ray Observatory is launched.
2001 –
The Shoemaker probe lands on the surface of the asteroid Eros.
2004 –
The Cassini spacecraft reaches Saturn, beginning a thirteen-year observational mission.
2006 –
The International Astronomical Union reclassifies Pluto as a dwarf planet.
2008 –
The U.S. Phoenix lander discovers subterranean ice on Mars.
2011 –
The U.S. Messenger probe maps the surface of Mercury.
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1961 –
2012 –
The U.S. Curiosity rover lands on Mars.
2014 –
The European space probe Rosetta successfully lands on a comet.
2015 –
The U.S. New Horizons probe becomes the first spacecraft to explore Pluto during a flyby.
2018 –
The Mars InSight lander launches, with a mission of studying the interior and seismic activity
of Mars.
October: Published research concludes that a black hole exists at the center of the Milky Way
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2018 –
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absolute magnitude– the apparent magnitude a celestial
object would have if it were placed at a standard
distance of ten parsecs from Earth
absorption line– a dark band on an otherwise continuous
spectrum, caused by absorption of a particular
wavelength of light
absorption spectrum– a continuous spectrum interrupted
by dark bands, observed when light is absorbed at
specific wavelengths as it passes through a substance
active galactic nucleus– the center of an active galaxy,
which acts as the source of virtually all the galaxy’s
energy emission
active galaxy– a galaxy that emits exceptionally large
amounts of energy in the form of radiation
active region– a region of the Sun’s surface where
eruptions of energetic particles are most frequent
amplitude – the maximum displacement from the
equilibrium position of a wave
apparent brightness– the brightness that a celestial
object appears to have, as observed from Earth
apparent magnitude– the apparent brightness of a star,
expressed as a magnitude
asteroid – an irregularly shaped body of rock and/or
metal, also known as a minor planet
asteroid belt– a region between the orbits of Mars and
Jupiter containing many asteroids
astrometry– the measurement of the positions, parallaxes,
and motion of stars
atom – the building block of the world around us, made
up of a positively charged nucleus surrounded by
electrons to make a neutral particle
atomic number– a characteristic number for each
element, equal to the number of protons in the nucleus
of an atom of that element
atomic spectroscopy– the process of analyzing spectral
lines to identify the atomic makeup of an excited
barred-spiral galaxy – a spiral galaxy in which the
spiral arms unwind from the ends of a bar-shaped
concentration of material
Big Bang– an abrupt expansion event that marked the
beginning of time and space as we know it
black hole– an extremely dense celestial object with a
gravitational field strong enough to prevent anything,
including light, from escaping
blackbody radiation– the spectrum of thermal radiation
emitted by an opaque object
Bohr atomic model– a model of the hydrogen atom
containing discrete energy levels corresponding to
fixed electron orbits about a central nucleus
Cassegrain telescope – a form of reflecting telescope that
uses a smaller secondary mirror to reflect light back
through a hole cut in the primary mirror
chromosphere – a thin, transparent layer of the Sun that
resides just above the photosphere
closed universe– a model of the Universe that predicts
that the Universe’s expansion will eventually slow
until it collapses
coma– the glowing halo surrounding a comet
comet – an icy, small celestial object that orbits the Sun
and produces a distinctive tail as it passes near the Sun
convection zone– a region of the Sun’s interior where
energy is transferred to the outer layers through
circulating currents of gas
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21-centimeter radiation– a specific wavelength of radio
waves emitted by a particular transition in hydrogen
atoms, which can be used to map the spiral structure
of our galaxy
Copernican revolution– a groundbreaking insight by
Copernicus that the Earth was not the center of the
dwarf planet– a celestial body that orbits the Sun and is
massive enough to be nearly round but does not clear
the region around its orbit
core – the most interior region of a star or planet
eccentricity – the distance between the foci of an ellipse
divided by the length of the major axis
coronal hole– a region in the Sun’s corona where gases
have a much lower density than neighboring areas;
solar wind tends to be emergent from these regions.
coronal mass ejection– a large burst of plasma that
escapes from the solar atmosphere into space
cosmic microwave background radiation– a radio
signal that permeates the universe as a result of the Big
Bang; matches the radiation emitted by a blackbody at
a temperature of 2.7 K
cosmological principle– the set of assumptions that form
the basis of cosmology, namely that on a sufficiently
large scale the universe is homogeneous and isotropic
cosmological redshift– a redshift of light from distant
galaxies that results from the expansion of the universe
cosmology – the study of the origin, evolution, and
structure of the universe
crater – a bowl-shaped indentation on the surface of a
planet or moon
critical density– the minimum average density of matter
and energy required to make the universe flat
crust – the thin, outermost, solid layer of Earth (or any
dark energy – an as-yet unknown source of gravitational
repulsion that has been proposed to explain the
accelerating expansion of the universe
dark matter– matter in the Universe whose presence
can be inferred from its gravitational effects but
is otherwise undetectable, i.e., matter that appears
electromagnetically “dark”
deferent – a component of the geocentric model; a large
circle encircling the Earth on which an epicycle moves
Doppler effect– a perceived change in frequency of a
wave due to relative motion between the source and
dwarf galaxy– a small galaxy a few thousand light-years
in diameter
ecliptic– the mean plane of Earth’s orbit around the Sun
electromagnetic radiation– waves made up of oscillating
electric and magnetic fields
electromagnetic spectrum– a classification of
electromagnetic waves by frequency or wavelength
element– a substance composed of only one kind of atom
ellipse – a curved geometric figure for which the sum of
the distances from any point on the curve to two fixed
points (the foci) is a constant
elliptical galaxy – a galaxy that has no disk, no spiral
arms, and very little visible gas and dust
emission line– one of several bright lines that make up an
emission spectrum
emission nebula – a cloud of ionized gas that glows by
absorbing and then re-emitting starlight from very
hot, young stars nearby
emission spectrum – bands of light at specific frequencies,
emitted by a source of electromagnetic radiation
empirical – describes a physical law that is based on
observations of a phenomenon but does not explain
that phenomenon from first principles
energy level– one of a discrete number of states an
electron may occupy within an atom
epicycle– a component of the geocentric model; a circular
path that planets follow, which itself follows a larger
path (deferent)
event horizon– the imaginary “surface” of a black hole,
the boundary beyond which no light can escape
excitation – the process of an electron moving from a
lower to a higher energy level
excited state – any state of an atom that is higher in energy
than its lowest (ground) state
eyepiece – the lens of a telescope through which an
observer looks; serves as a magnifier for the final
flat universe – a model of the universe that predicts a
continuing but slowing expansion that approaches
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corona – the outermost atmosphere of the Sun just above
the chromosphere
flatness problem – a cosmological question of why
the earliest density of the universe must have been
extraordinarily close to the critical density
focal length– the distance from a lens to its focal point,
the point at which it will focus light rays from a distant
frequency – the number of oscillations per unit time of a
wave or other periodic behavior
galactic bulge – a thick distribution of gas and stars near
the center of the galactic disk
galactic disk– a flattened circular region that contains
most of a galaxy’s stars and interstellar matter
galactic halo – a spherical region surrounding the galactic
disk containing faint old stars
galaxy – an enormously large collection of matter,
including stars, gas, dust, and black holes, which are
all held together by gravitational attraction
galaxy cluster– a collection of galaxies held together by
gravitational attraction
Galilean moons – the four largest moons of Jupiter, first
discovered by Galileo
geocentric model– a model of the solar system that places
Earth as the center of the universe
giant– a star with a radius between 10 and 100 times that
of the Sun
globular cluster– a roughly spherical cluster of 100,000
to 1 million stars, found in the galactic halo and
containing some of the oldest known stars
gravitational lens – a concentration of mass that bends
the path of light
Great Red Spot – a large storm system in Jupiter’s
atmosphere that has been observed for centuries
greenhouse effect – a process in which thermal energy
becomes trapped by a planet’s atmosphere
ground state– the lowest possible energy state of an atom
heliocentric (Sun-centered) model – a model of the solar
system that places the Sun at the center and the planets
in orbit around it
Hertzsprung-Russell (H-R) diagram – a plot of
luminosity versus temperature (or spectral class) for
a group of stars
highlands – elevated regions of the Moon’s surface that
appear bright when viewed from Earth
Hubble classification scheme– a method of classifying
galaxies into groups according to their structure,
developed by Edwin Hubble
Hubble constant– a fundamental value in cosmology,
equal to the rate at which the galaxies are receding, or
the rate at which the universe is expanding
Hubble time – an upper limit on the age of the universe
since the time of the Big Bang
hydrostatic equilibrium – the condition of an object in
which the outward pressure balances the inward pull
of gravity
inflation – a brief phase of incredibly rapid expansion
shortly after the Big Bang
interstellar dust – very tiny solid particles that populate
the space between stars
interstellar medium – the matter that exists between the
stars, made up of gas and dust
inverse-square law – a law describing a field that
decreases in strength with the square of the distance
from the source
ion – an atom that has gained or lost electrons; ions have
an overall electric charge.
irregular galaxy– a galaxy that has no regular geometric
shape or structure
Kuiper Belt – a vast region of the Solar System beyond
Neptune containing icy bodies and larger masses
known as Kuiper Belt objects
laws of planetary motion – three laws describing the
motion of planets about the Sun, developed by
Johannes Kepler
lens – a piece of curved glass that bends the pathway of
light passing through it
limb – the apparent edge of the disk of a celestial object
such as a planet or star
Local Group – the galaxy cluster containing the Milky
Way galaxy and about forty other galaxies
luminosity – the total amount of energy a star radiates
into space each second
magnetic field– a region of magnetic forces
magnetosphere – the region around a planet where its
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zero as time approaches infinity
magnitude scale – a system of ranking stars by apparent
main sequence – a well-defined band running from the
top left to bottom right on an H-R diagram containing
most stars
mantle– the layer of Earth just below the crust
mare (pl. maria) – low-lying, dry lava beds made of
basalt on the Moon’s surface
mesosphere – region of Earth’s atmosphere lying above
the stratosphere
meteor – a streak of light created by meteoroids that
plunge through Earth’s atmosphere at high speeds,
also called a “shooting star”
meteor shower – an event during which many meteors
can be seen, caused when the Earth passes through a
field of meteoroids left behind by a comet or asteroid
meteorite – a meteoroid that passes through Earth’s
atmosphere to land on its surface
incoming ultraviolet radiation is absorbed by ozone
and other molecules
parallax– an apparent shift in an object’s position relative
to a distant background as the position of an observer
parsec – standard unit of astronomical distance, defined
as the distance to an imaginary star whose parallax is
exactly one arc second
period – the amount of time that elapses between
consecutive oscillations of a wave or other repeating
photosphere– the visible surface of the Sun
planet– a celestial object that is (1) in direct orbit around
its star, (2) massive enough to be a spheroid in shape,
and (3) has cleared the neighborhood around its orbit
of other material
plate tectonics– the gradual reshaping of Earth’s surface
features as caused by motion between large sections
of the crust
meteoroid – a piece of stone or metal in space
plutoid – a term for a dwarf planet that orbits beyond
Milky Way galaxy – the galaxy to which our Sun and all
the visible stars in our sky belong
Population I star – a relatively young star that is rich in
heavy elements, usually found in the galactic disk
moon– a small body in orbit around a planet
nebula– a concentration of gas and dust in space
Population II star – a relatively old star composed of
mostly hydrogen and helium, usually found in the halo
or nucleus of a galaxy
nebular theory – a model of solar system formation, in
which the Sun and planets formed from a cloud of gas
collapsing under its own gravity
prominence – a fiery arch of ionized gases that moves
upward through the corona under the influence of the
Sun’s magnetic field
neutron star– an extremely dense star composed almost
entirely of neutrons
protostar – a star in its earliest observable phase of
nuclear fusion– a reaction in which lighter atomic nuclei
combine to form heavier ones, releasing energy in the
Ptolemaic model – a geocentric model proposed by Greek
astronomer Ptolemy around 150 ce
nucleus – the dense core of an atom, made up of protons
and neutrons
objective lens– the lens of a telescope positioned closest
to the object to be viewed
open cluster– a loose, irregular star cluster, generally
found in the plane of the Milky Way
open universe– a model of the Universe that predicts that
the universe will continue to expand indefinitely
ozone layer – a layer within Earth’s atmosphere where
pulsar – a rapidly rotating, highly magnetic neutron star
quasar – a distant source of radio waves with a star-like
radiation zone – a region of the Sun’s interior where
energy propagates outward in the form of photons
radio galaxy – an active galaxy that emits large amounts
of radiation in the form of radio waves
radio lobes– two large regions of energy at radio
wavelengths that appear on the radiograph of a typical
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magnetic field is influential
radio galaxy
States and the Soviet Union
radiograph– a false color picture made from observations
at radio wavelengths
spectral class– a classification system for stars according
to relative strength of spectral lines
red dwarf – a relatively small, cool main-sequence star
that can be found in the lower right corner of the H-R
spectroscope – a measurement device that separates
light emitted by a collection of excited atoms into
component wavelengths, used for viewing emission
refracting telescope– an optical instrument that uses
a combination of lenses to collect and focus light,
thereby forming an image of a distant object
refraction – the bending of light as it passes from one
transparent medium to another
retrograde motion– an apparent reversal in the direction
of planetary trajectories as viewed from Earth
revolution– the motion of one body orbiting about another
scarp – a cliff on Mercury’s surface, believed to have
formed when the crust cooled and shrank
Schwarzschild radius – the critical radius at which a
spherical massive body becomes a black hole
Seyfert galaxy– a type of active galaxy with a spiral
appearance; its emission comes from a very small
shield volcano – a broad, dome-like volcano built up over
long periods of time by successive eruptions and lava
singularity – a point at the center of a black hole at which
mass density and gravitational field strength are
solar activity – unpredictable, violent events on or near
the Sun’s surface, caused by magnetic phenomena
solar flare– a sudden, explosive outburst of
electromagnetic radiation and material from the Sun
solar nebula– a rotating cloud of interstellar gas and dust
from which the Solar System formed
Solar System– the Sun and all objects gravitationally
bound to it
solar wind – a stream of energetic, electrically charged
particles that flows outward from the Sun at all times
Space Race – a period of technological competitiveness in
spaceflight during the Cold War between the United
speed of light – the speed at which all electromagnetic
waves propagate in a vacuum, equal to 299,792,458
spiral galaxy– a galaxy shaped like a flattened disk that
contains spiral arms winding outward from a central
galactic bulge
standard candle– an object of known brightness that
serves as a reference for astronomical distance
star – a celestial object that is made up of gas, is held
together by gravity, emits light, and is powered by
nuclear fusion reactions
star cluster – a group of stars that was formed from the
same parent cloud
starburst galaxy– a galaxy in which stars are forming
at a rate hundreds of times greater than in our Galaxy
stratosphere – the region of Earth’s atmosphere lying
above the troposphere, extending upward to an altitude
of 40–50 km; contains the ozone layer
sunspot – a temporary, dark, relatively cool blotch that
exists on the Sun’s photosphere
sunspot cycle– an approximately eleven-year cycle in
which the number of sunspots regularly rises to a
maximum and falls to a minimum
supercluster– the term for a cluster of clusters of galaxies
supergiant – a star with a radius between a hundred and a
thousand times that of the Sun
supernova – a large, violent explosion that takes place at
the end of a star’s life cycle
synchronous orbit – the motion of an object whose
rotational period about its axis equals its orbital
period, such as the Moon’s orbit around Earth
synchrotron radiation– radiation produced by electrons
spiraling around at nearly the speed of light in a strong
magnetic field
telescope – an instrument that uses a combination of
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reflecting telescope– an optical instrument that uses
a combination of mirrors to collect and focus light,
thereby forming an image of a distant object
lenses or mirrors to collect light from a distant object
and concentrate it for analysis
temperature – a measure of the average speed of the
constituent particles of an object
thermosphere– the region of Earth’s atmosphere beyond
80 km
tides – daily fluctuations in the ocean level, caused by
gravitational interactions between the Earth and Moon
wave– the motion of a disturbance between two points
wave speed– the speed at which a wave propagates, equal
to the frequency times the wavelength
wavelength – the distance between successive points on
a wave (e.g., the distance between consecutive peaks)
white dwarf– a star with a high temperature but low
luminosity due to its relatively small size
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troposphere– the region of Earth’s atmosphere extending
from the surface to about 15 km
voids– regions in space where few galaxies are observed
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