Summary and Important Ideas from Discovery by Seeds, 10th Ed
for Astronomy 100 Classes
Summer 1009
Chapters 1 through 9
Chapter 1: The Scale of the Cosmos
It is often interesting to take a general, overall view of a subject that you are
about to study in detail, before you begin your comprehensive study. The first
chapter in Seed’s textbook is designed to give you such a thumbnail outline of the
structure of our universe. It also presents some ideas about the relative distances
between our home planet, Earth, and other areas within the universe.
We, therefore, begin our study of astronomy with a survey of our solar
system; that is, our Sun and its associated planets. We then sweep quickly past
some of the neighboring stars that surround our Sun. Even the nearest of these
other stars are really not very close to us. They do, however, make up a special
group of stars numbering about 400 billion that are called the Milky Way galaxy.
Looking beyond our own Milky Way galaxy, we find that additional stars
have formed other similar galaxies. Although the size and shape of these other
galaxies may differ from that of our home galaxy, the entire universe can be
thought of as a vast collection of hundreds of millions of such galaxies that extend
outward as far as our current detection systems allow us to see.
The universe also consists of gas and dust that exist in the open spaces
between these gigantic star groups, as well as a few strange objects such as neutron
stars, quasars, bursters, and black holes which do not fit easily into such a
simplified picture. These are some of the fascinating things we will learn about in
later chapters. Not only are these bizarre structures very exciting and interesting in
their own right, they will help us understand a lot about the history and evolution
of the universe as a whole.
It would be nice if we could perform hands-on experiments on the various
parts of the universe that we are to study, but unfortunately we are restricted almost
entirely to observing the data that comes to us from space in the form of
electromagnetic radiation. This passive mode of observation means that we must
learn to gather as much information from these data signals as we can and it is
important to study all of the types of EM radiation that comes to us from long
wavelength radio waves to extremely high frequence gamma rays. This means
that a large variety of telescopes must be used in our studies and we must use other
sophisticated instruments like
spectrometers and photometers to perform detailed inspections of the various
frequency components and spectra elements of these signals. If we do, we can
achieve phenomenal insights into the complex structure and evolution of things
that are billions of light years away from us in space and which we will never be
able to visit or experiment on first hand.
When we study the universe outside of our Earth’s atmosphere, only two
types of EM radiation make it through to the surface. Radio waves and visible
light supply nearly all of the data that we acquire from surface based telescopes.
The other types of electromagnetic radiation such as x-rays, infrared and
ultraviolet rays, and microwaves are now studied using satellites that can make
their observations above the atmosphere itself. Chapter 5 will aid us further in this
understanding, but for now let us concentrate on the way that light and radio waves
have given us our preliminary insight into the universe around us.
It is interesting to note that even though light, and all types of
electromagnetic radiation for that matter, travels at a very high rate of speed, it still
takes some time for light to reach us even from our nearest celestial neighbors.
Light emitted by our own Sun takes only about 8 minutes to reach the Earth, but
light from the next nearest star, Alpha Centauri, takes well over 4 years. The
farther away an object is from us in space, the longer it takes for the light from that
source to reach us.
Our current telescopes can pick up light from objects that we believe to be
about 10 billion light years away from us. Because of the finite travel time for
light and all other types of electromagnetic signals, we are seeing these objects, not
as they are today, but as they were billions of years ago. Since our best current
estimates of the age of the universe itself are only about 15 billion years, we are
actually looking back in time nearly 2/3 of the way to the very to birth of the
universe. This is a great advantage in unraveling the mysteries of the origin of the
universe and in trying to understand it evolution, but on the other hand, it makes it
difficult to study the characteristics of these distant wonders as they exist today.
We currently believe that the universe as we known it today began with a single,
stupendous explosion called the "Big Bang." We will look at the "Big Bang"
theory of cosmology more closely in later lessons.
Because of the extremely large, and sometimes extraordinarily small
numbers that must be used when describing the universe, scientists depend on a
system known as scientific notation in which the numbers used are expressed with
powers-of-ten notation. Although we will not require you to add, subtract,
multiple, or divide these numbers in complex mathematical calculations, it is still
advisable that you have a basic knowledge of the general size that these numbers
represent. The superscript on the 10 which follows the basic magnitude in this
notation tells you the number of times that
you must multiply this basic number by ten if the superscript is positive, or the
number of times you must divide it by ten if the superscript is negative. Because
you will see numbers written in this notation quite often during your study of
astronomy, you should take a few minutes now to familiarize yourself with
powers-of-ten notation in Appendix A (pages 478-479) at the back of the textbook
and in the “outline of important concepts” for this chapter that is also available as
part of this course to support the textbook and this study guide.
Measuring the distance from Earth to other objects within the universe is one
of the most basic and yet most difficult activities performed in astronomy. Any
measurements made require the use of units as well as the magnitude of the
number being expressed. In order to do this in a way that everyone in the world
will understand, a master set of units has been developed called the SI system,
(Systeme International d’Unites), see Appendix A. We will not study this system
in great detail but it is critical that you understand the basics of this set of units and
how they apply to the study of astronomy. Pay special attention to: 1. metric
prefixes, 2. SI metric fundamental units, conversion, and 4. Powers-of-10 notation.
As you study and review this material on pages 477 - 479 in the textbook. If you
are interested in studying the SI system of units or feel that you are having
difficulty with these concepts, you might wish to consider taking Physical Science
111, a one hour correspondence course offered by Ohio University that is devoted
specifically to this subject.
Several methods are used to determine the distance to far away objects in
the universe, such as radar, parallax, variable star analysis, etc., some of which
will be covered in detail in Chapter 8. For now simply realize the although radar is
a quite accurate way of measuring the distance to other objects in our solar system,
and parallax can measure the distance to some of the nearby stars in our own
galaxy, more indirect methods must be used for all of the more distant parts of the
universe. This is most generally done by comparing the brightness of objects as
seen from Earth with the intrinsic intensity or absolute brightness of these objects
as determined from other means of study. This means that the actual distances to
most parts of the universe are very difficult to ascertain and even when the best
scientific methods are employed there can still be some large uncertainties in these
determinations. As you will see as we go along, if really accurate distance
measurements to the most remote parts of the universe were known, we would
have a much better overall understanding of the structure, function and evolution
of the universe around us.
When working with the Known Universe, that is the portion that we can
actually observe and study, it is important that we relate the knowledge we have
gained here on Earth to what we see happening in the universe at large. In order to
extend our understanding to these far-away phenomenon we must make two rather
strong assumptions.
The first of these involves the PRINCIPLE OF UNIVERSALITY, which
states that all physical laws as we know them on Earth can be applied in the same
form everywhere in the universe. If the laws of nature behave differently in other
regions of space, we have no basis for comparison with our known experimental
knowledge gained over the years here on Earth. Remember that we can generally
only study the universe by observing the light, radio waves, x-rays, and other types
of electromagnetic radiation that come to us through space. Aside from our visits
to the Moon and a little space debris in the form of meteorites that have fallen
through the atmosphere to reach us on the Earth's surface, we can only learn about
outer space by decoding the information we gather from the various kinds of
electromagnetic radiation from luminous or reflective objects throughout the
universe. As we will see later, man has risen to the challenge and developed many
sophisticated ways of using this electromagnetic radiation to investigate the
properties of distant celestial objects.
The second of these great assumptions in the COSMOLOGICAL
PRINCIPLE, which states that from any location in the universe the general
features of the universe will appear the same in all directions. This means that we
must consider any local variations as irrelevant to the overall structure of the
universe. From this we must conclude that the Earth is "nothing special" on the
grand scale of the universe, and looking at the universe from the Horsehead Nebula
or the furthest galactic cluster that we can see would yield a very similar picture of
the universe to the one that we see from Earth. Remember, our home planet is
very important to us, but it is not the center of the universe in any sense of the
word.
Chapter 1 provides only a brief survey. If you do not yet know all that you
would like to know about our Milky Way galaxy, the stars around us, or any other
parts of the universe, don't despair. The rest of this course deals with most of the
ideas covered in this general overview in more detail. We will, for instance, look
closely at our Sun as a representative of all other stars because it is so much closer
to us that we can investigate it more easily than those other stars that are located
many light years away. We will also spend some time going over the grouping of
stars into complex galaxies, and even return to a basic study of the structure and
general composition of our Solar System. And we will accomplish this using data
gathered while riding on our moving observatory, the Earth as it hurtles through
space.
Chapter 2: The Sky
Stars make up the majority of the objects visible in the sky above our Earth.
Although the Sun, the Moon, and the planets appear brighter and usually capture
our initial attention, it is the patterns of stars that make up the bulk of the view that
we have when we observe the night sky. It is no wonder that some of the oldest
knowledge and records of scientific observation are comprised of information
about the appearance of the sky as we see it at night.
The stars are only visible in the night sky because the Sun is so bright that it
outshines all of the stars that are overhead during the daytime, making them
impossible to see. At night, however, the beauty of these pin points of light
becomes apparent and man has been fascinated by their grandeur since the earliest
times. Groups of stars form patterns in the sky, some of which are easily
recognized and some of which are quite obscure. There are 88 specific star
groupings as seen from Earth that are called CONSTELLATIONS and which
collectively make up the entire surface of the sky.
Within a given constellation the stars are designated according to their
apparent brightness as seen from Earth. In this classification system the brightest
star in a constellation is called the alpha star, the second brightest the beta star,
etc., until all of the letters in the Greek alphabet have been used. After that
additional stars in that constellation are simply given numbers. Historically the
constellations all have Latin names. Therefore, the brightest star in the
constellation Orion would be called Alpha-Orionis (the “is” being the possessive
ending for the constellation name), the second brightest star in the constellation
would be Beta-Orionis, etc.
All stars are part of one and only one official constellation and can be named
as part of that star group using the name of that constellation and the Greek letter
or number. Some stars, however, have specific names so we can refer to them
either as a member of a certain constellation or by the name given to that
individual star. Individual names are usually provided only for the brightest stars.
The example star cited above, Alpha-Orionis, is also known by the individual
name, Betelgeuse. Individual names for some stars were originally in Latin or
Greek, but many stars are now known today by their Arabic names that were
assigned during the Dark Ages when the science of astronomy was almost
nonexistent in Europe but was still quite active in the countries of the Middle East.
Sometimes even these two ways of identifying stars are not sufficient and
stars are then designated as the entries that appear in certain star catalogues, like
M-31, NGC-185, and X-1. These types of designation are also used for objects
like x-ray sources that are not visible to the human eye, very distant entire galaxies
that appear from Earth as single points of light, or even extremely distant groups of
galaxies. These sources of light thus should really not be named as component
stars of any specific constellation, but by convention some of these designations
are still used because they have been referred to in that way for many centuries.
Some patterns of stars that are quite easily recognized when seen from Earth
are made up of only some of the stars in a given constellation or of stars from two
or more of the official constellations. These patterns are known as ASTERISMS.
Names have been given to many of these, such as the Great Square in Pegasus or
the Big and Little Dippers. These star groupings are often confused with official
constellations by people not familiar with the true designations used in astronomy
but you should remember that they are not actual members of the official 88
constellations that now make up the entire sky, both in the Northern and Southern
hemispheres.
The differences in brightness of the stars have been apparent to observers for
centuries. The first scale to designate the brightness of the stars as they are seen
from Earth was devised by the Greek scholar, Hipparchus. In his system the
brightest stars in the sky were given the classification of first magnitude, the next
brightest as second magnitude, down to the dimmest stars that he could see on a
clear dark night which were classified as sixth magnitude. This scale was later
extended to even dimmer stars that can be seen only by using telescopes. The best
telescopes in use today can detect and record light from stars as dim as 28th
magnitude. When brighter objects such as the very brightest stars, the Moon and
the Sun are placed on this scale, negative numbers must be assigned for the really
bright ones. This means that some of the planets in our solar system can have
magnitudes as bright as -5th magnitude, a full Moon is about -12.5 magnitude, and
the noonday Sun is so bright that it rates as -27.7 magnitude. Even a few of the
brightest individual stars have a negative magnitude with the brightest star in the
sky, Sirius logging in at -1.47 magnitude.
This magnitude designation is really the APPARENT VISUAL
MAGNITUDE of the object because it tells us how bright the object appears to be
when seen from Earth’s surface. This brightness is greatly influenced by the
distance that the object is from Earth when it is being viewed. The actual amount
of energy given off by a star can be quite different from the brightness as it appears
from Earth. This energy output is referred to as the star’s LUMINOSITY and this
is closely related to the star’s ABSOLUTE VISUAL MAGNITUDE which
indicates how bright the star would be if it were located at a reference distance of
exactly 10 parsecs away from Earth in space. Most stars are much, much further
away so their apparent visual magnitude is much less than their absolute visual
magnitude. We will return to the study of these indicators of energy output and
brightness in chapter 8.
Magnitude may also be used to indicate differences in brightness. If two
celestial objects differ by 1 magnitude in brightness, the more luminous one is
actually 2.5 times brighter than the other. A difference in 5 magnitudes makes the
more luminous one 100 times brighter. Remember that the higher the apparent
visual magnitude of an object, the dimmer it appears to be in the night sky while
the lower the apparent visual magnitude, even into negative values, the brighter it
is. Also keep in mind that there are several different ways to use the designation
magnitude, so carefully check the context of the designation when you find it used
in future chapters.
When the position of objects in the sky as seen from Earth must be specified,
it is often convenient to use the idea of a transparent sphere which completely
encircles the Earth, upon which all celestial objects are positioned. Such an
imaginary transparent canopy is called a CELESTIAL SPHERE. This map of the
entire sky is pictured on pages 16 and 17 in the textbook. It is established with a
celestial equator that is directly above Earth’s equator, and N and S celestial poles
directly above the Earth’s North and South poles.
The exact location of any celestial object on this sphere is not, however,
determined by longitude and latitude as it would be using the locating system
applied to Earth’s surface. Instead a pair of coordinates known as RIGHT
ASCENSION and DECLINATION are used. Right ascension locates the position
of the object around the equator of the celestial sphere beginning at the VERNAL
or SPRING EQUINOX which is the point at which the Sun’s path across the
celestial sphere called the ECLIPTIC crosses the celestial equator in the Spring of
the year. Declination measures the angular distance north (designated +) or south
(indicated by -) of the celestial equator where the object may be found. Right
ascension in usually measured in hours and minutes, with one hour equal to 15
degrees of angular arc, but can be designated in degrees as well.. Declination is
always measured in degrees, with zero degrees referring to a point directly on the
celestial equator and 90 degrees indicating that the object is directly over the north
pole (+ 90 degrees) or the south pole (- 90 degrees).
Remember that although the celestial sphere is a convenient way to locate
the direction in which any object may be viewed from the Earth’s surface, it does
not help us to understand the three dimensional structure of the universe. Stars and
other cosmological objects are each located a different distances from Earth but the
celestial sphere depicts them as all being the some distance from our planet. Again
remember that the actual distance from Earth to objects within the universe is not
easy to determine and that when this distance is specified it is usually in
astronomical units, light-years, or parsecs, all of which are not units that are
familiar to most students in their everyday lives. Make sure that you know the
definitions of each of there specialized units of distance and know how they relate
to each other in actual distance measured.
When discussing astronomical locations, the point directly above the head of
any observers is referred to as their ZENITH point, and their HORIZON is always
90 degrees below the observer’s zenith even if there are buildings or mountains in
the way so that the actual sky cannot be seen down that far. The ALTITUDE of
any object seen in the sky is then defined as the angular distance that the object
appears above their horizon.
Look over the diagrams in the textbook to see how the celestial sphere is set
up directly over its corresponding geographic locations on the Earth and how the
concepts of Right Ascension, Declination, Zenith, Horizon, Altitude, Celestial
Equator, the Celestial N and S Poles, and Vernal Equinox are used in relation to
the location of stars and other objects on the celestial sphere. Since these terms
will be used in describing future subjects in our study of astronomy, you should
make sure that you are totally familiar with them now.
Also note that the discussion of the motion of the Earth through space
begins in this chapter. Not only does the Earth spin daily on its own axis
(producing night and day) while it travels around the Sun (giving us our yearly
calender), but its axis of rotation is tilted at a 23.5 degree angle to its plane of
revolution around the Sun (which is responsible for the seasonal changes in the
weather). This tilt does not always point in the same direction in space which
leads to subtle changes in the position of the stars as seen from Earth over a 26
thousand year cycle. This very slow change in the direction in which the tilted axis
in which Earth’s axis points is referred to as PRECESSION. It is caused by the
gravitational pull of the Sun and the Moon on an Earth that is not quite a perfect
sphere. These concepts will be studied in more detail in the next few chapters.
Chapter 3 - Cycles of the Sky
Making observations is crucial to our study and subsequent understanding of
the universe. It would be convenient if we had a stable observation platform from
which we might have an unobstructed and consistent view of the planets and the
stars, but this is not the case. Not only does the atmosphere of the Earth inhibit our
view into space (as we shall see in Chapter 5), but the Earth undergoes not one but
several complex motions that present ever-changing views of the universe. It is
not hard to imagine that any true understanding of the universe had to wait until
early observers figured out the details of the motion of our Earth through space, so
this is where we will begin our own study in this chapter.
People have always used observations of the stars as the basis for time
measurements. The rising and setting of the Sun determined the length of the days,
the phases of the Moon established the months, and the seasonal variations in
climate as well as changes in the background star patterns in the sky defined the
years. Time and calendars were also of great interest to early agricultural
civilizations. Understanding the seasons often meant the difference between life
and death, because in most parts of the world, crops have to be planted and
harvested in the proper seasons. Although modern calendars show some political
influence from Roman days, they are still strongly based on observations of the
Sun and Moon. It is no wonder that the shrewdest and wisest men became priests
and astronomers and because of this were often the most powerful and influential
people in ancient societies.
One other offshoot of the annual motion of the Earth around the Sun is the
changing position of the Sun, as seen from Earth against the fixed background of
stars. This means that the Sun can be located in a different “house” during each
month of the year leading to the development of the ideas of ASTROLOGY, the
non-scientific belief that the changing position of the Sun has a strong influence of
the daily lives of people and that their future can be predicted by the position of the
Sun and stars on important days of their lives, such as the day of their birth or the
day that they were married. The location of the Sun against the ZODIAC, the 18
degree wide band of stars that stretch out on either side of the ECLIPTIC, is still
used by some to guide them through their daily lives. The signs of the zodiac are
simply the 12 principle constellations along the ecliptic and is not generally
believed by scientists to have any influence of the everyday lives of the people
living on Earth. Even though this chapter contains a brief explanation of astrology
that may be of interest to some students, we should remember that there is no
proven scientific basis for the belief that men's lives are determined by the relative
positions of the planets and stars and since this is a course concerned with
ASTRONOMY, we will not consider these behavior theories that are based on
celestial alignments any further.
The seasonal effect of the Earth and the Sun’s annual motion, however, is of
great importance because it affects the growing seasons for crops and influences
the migrational habits of birds and animals. This is because of annual changes in
the weather and hours of daylight for various locations on Earth. Note that the
resulting seasonal temperatures are not always higher when the Earth is closest to
the Sun. The effects of the Sun that produce Spring, Summer, Fall and Winter, are
explained in detail on pages 26 and 27 of the textbook. Make sure that you go over
these and understand them fully.
The motion of the other planets around the Sun also causes them to appear to
move against the background of fixed stars. You will learn more about this and
about retrograde motion in the next chapter. For now realize that the inner two
planets, Venus and Mercury, are constrained in their motion so that they are never
very far from the Sun, as seen from Earth. This means that they often appear as
morning or evening “stars” in the sky, and can never be seen directly overhead at
or around midnight as the other planets can.
The motion of the Moon around the Earth is the basis for the division of the
year into 12 nearly equal segments designated as months. During each month the
lighted surface of the Moon as seen from Earth cycles from nearly zero to fully
lighted and back again in a sequence that is referred to as the PHASES of the
Moon. This cycle, new moon - waxing crescent -first quarter - waxing gibbous full moon - waning gibbous - third quarter - waning crescent - new moon, takes a
little less than 30 days so that the major phases of the Moon are separated by about
one week (7 days).
Note that the major phases, (underlined in the last paragraph), only occur
once a month for an instant in time when the Earth, Moon, and Sun are in precisely
the correct relative positions. Most of the time the Moon is in either the crescent or
the gibbous phases, that is it spends most of its time between the major phases
designated above.
It is also sometimes possible for the Earth to travel through the shadow of
the Moon as light from the Sun is blocked or for the Moon to pass through the
shadow of the Earth. These rare occurrences are known as ECLIPSES. When
Sunlight is blocked by the Moon, people on Earth can experience a solar eclipse.
When the light that normally strikes the Moon is blocked by Earth we can observe
a lunar eclipse. Whether such an event is seen as a total or a partial eclipse depends
on whether the penumbra or umbra portion of the shadow is involved. See pages
32 - 35 in the textbook. Note that solar eclipses can only occur during the time of
the new moon phase, and lunar eclipses can only be seen at or near the full moon
phase.
Also remember that the new moon is overhead as seen from Earth at 12
o’clock noon and the full moon phase is seen overhead at or near 12 o’clock
midnight so at sometime during each month the moon will be visible in the sky at
all possible times of day and night, although it is harder to see the moon in the
daytime and so we do not always notice its presence. Solar eclipses, especially
total solar eclipses, are much more infrequent and more difficult to observe than
lunar eclipses and because the Moon does not orbit around Earth exactly on the
ecliptic plane, eclipses do not occur every time the moon goes through the new or
full phase.
One more thing about the orbital motion of Earth around the Sun. The
Earth’s climate is not exactly the same each year. Because of slight variations in
the orbital distance of Earth from the Sun, along with other subtle factors, the Earth
can go through hotter periods (global warming) and cooler periods (ice ages) on
about a 100,000 year cycle. Realizing and understanding these cycles are quite
important to the long term survival of humans as residents of Earth and although
the Milankovich hypothesis gives us some insight into this cyclic process, much
more study is needed to determine how these variations will effect how we as a
species can survive in the future on our planet, Earth.
Chapter 4 - The Origin of Modern Astronomy
The human race has always been conceited and considered itself very
important in the overall structure of the universe. Even today, when our theories of
the cosmos place very little, if any, special significance to our position among the
stars, many people still feel our self-importance. Is it any wonder then, that early
theories of astronomy placed the Earth at the center of the universe and considered
mankind as the most important part of creation? These ideas have often put the
doctrine of the church at odds with the theories of science.
If planets and other celestial objects moved through space as even our
earliest observations clearly showed that they did, it must be everything else that
did the moving. It certainly could not be the Earth itself. Because of the daily
passage of the Sun and stars overhead, it was originally thought that all objects in
the sky could be assigned a specific location on a gigantic moving backdrop called
the CELESTIAL SPHERE, which revolved in such a way that all of these lighted
objects circled around the Earth once each day. Any theory with the Earth at the
center is known as a GEOCENTRIC theory. Although this model is now known to
be incorrect, it is still a useful concept that can be helpful when we try to visualize
the locations of the so-called fixed stars (those so far from the Earth that their
positions are virtually the same from year to year). The paths of the Sun, Moon,
and planets can then be traced against this relatively unchanging background
keeping the Earth stationary at the center. Even today, if you are interested in
observational astronomy or astrology, this is still a very useful model, but we must
realize that it has its limitations and lacks a certain quantity of scientific simplicity.
Fortunately as time progressed, learned men began to unravel the actual motions of
the heavens.
Were it not for the "wanderers of the night sky", the planets, the true nature
of our cosmos would probably have taken a much longer time to understand. As it
was, the motions of these moving points of light across the sky were very difficult
to explain using the geocentric celestial sphere idea. Complicated geometrical
configurations were envisioned by early scholars such as Ptolemy to keep the Earth
stationary in the center of the universe but they were still not the final solution.
We know today that the planets move in nearly circular orbits around the
Sun (the orbits are really elliptical in shape but, in most cases, can still be
considered as very nearly circular). This is called the HELIOCENTRIC or Sun
centered theory. It took very intuitive and farsighted men like Copernicus and
Kepler to develop this concept over the objection of the Catholic Church and to
make it popular with the common people. It also took many dedicated observers
like Tycho Brahe to gather the data necessary to prove that these new theories were
correct.
Kepler's Laws are still the basis for our understanding of our solar system
and other collections of massive objects throughout the universe that move about
one another under the influence of mutual gravitation. Each of the three laws has
its important insights, from the first law which states that all planets move on
elliptical orbits, not circular orbits around the Sun, the second law which holds that
a line between the planet and the Sun will sweep out equal areas in equal amounts
of time, to the third law that relates to the distance a planet is from the Sun and
how that relates to its period (time) of revolution around its elliptical path.
Additional support was given to the heliocentric theory by work done by
Galileo using a newly invented device called a telescope. His observations of
mountains on the Moon, the rings of Saturn, the moons of Jupiter, and sunspots
gave much needed scientific credibility to the idea that the Sun is really the orbital
center of the solar system and that the heavens were not perfect and did not revolve
around the Earth as the church believed that they must be. The relationship
between gravity and celestial motion was explained somewhat later by Sir Isaac
Newton. The theory of gravity is the key to understanding why celestial objects
obeyed Kepler's Laws and thus helped to show that these ideas are, in fact, true
explanations for the motion of the planets within our solar system as well as of all
celestial objects in orbit around each other throughout the universe.
Newton also developed three very important laws of motion at about this
same time which help us understand how objects move and why forces are
necessary to produce any changes in this motion. For example, any massive object
can only be accelerated if a suitable unstable force is applied to it. These laws also
help us understand the concepts of orbital motion and free fall and enable us to
visualize why objects in orbit can be in stable motion that goes on unchanged for
many thousands or even millions of years.
It is fun to look over the time line for the lives of these important men of
science and to see how their work and discoveries overlapped with historical
events of their day. This information is nicely summarized in figure 4-12 on page
61 in the textbook. Look it over carefully and you will have a much better
understanding of the human factors involved in this period of scientific
development.
Finally, there is a section in this chapter dealing with the effect of gravity on
the waters in the seas on Earth. The gravitational pulls of both the Moon and the
Sun produce TIDES in our oceans. Exceptionally high tides, spring tides, and low
tides, neap tides, are the results of the gravitational effects of the Sun and the
Moon when they either work together or oppose each other. We will see later in
the course that similar gravitational forces also produce distortions in the crusts of
planets and even in the outer layers of stars. This will help to explain some of the
more subtle anomalies that we observe through modern telescopes even today.
Chapter 5 - Astronomical Telescopes
Even with all that we know about the structure and functions of the universe,
almost all of the information that we have about things outside of the atmosphere
of the Earth comes to us from one and only one source, ELECTROMAGNETIC
RADIATION. We have traveled to the Moon to study its composition in detail
and have also collected a few samples of extraterrestrial material in the form of
meteorites that have reached the surface of the Earth from outer space, but all other
data on the universe has come from extensive and often exhaustive analysis of the
light, radio waves, x-rays, gamma-rays, and other types of electromagnetic
radiation that we have been able to gather with our eyes and with the various kinds
of telescopes that have been developed specifically for this purpose.
Electromagnetic radiation to which the human eye is sensitive is called
LIGHT, and this was the first type of astronomical data to be studied. OPTICS
refers to the way that such visible radiation is influenced by its surroundings, such
as when light rays interact with lenses and mirrors. There are, however, many
other types of electromagnetic radiation that make up what is commonly called the
electromagnetic spectrum. All of these types of radiation are similar in their
characteristics, such as their basic electric and magnetic interactions with their
surroundings and the velocity with which they travel through space. This velocity
is known as the speed of light no matter what type of electromagnetic radiation is
being considered.
Viewed as waves, each type of electromagnetic radiation has its own range
of wavelengths (and related frequencies), and when considered as particles
(photons) each has its own range of energies. The modeling of light as both a
wave and a particle is called the "dual nature of light." Both the wave and particle
models are necessary because of the complex nature of light and the ways in which
it interacts with its surroundings. Using two models helps the human mind grasp
some of the characteristics of the behavior of electromagnetic radiation in different
situations, even though the actual structure and behavior of this radiation does not
change. Since light, for example, does not change from one moment to the next,
the dual nature examples simply help us to better visualize and to understand the
subtle behavior of light under varying conditions. During normal everyday
interactions with large-scale objects such as lenses or mirrors, light behaves as a
wave, but when the interaction of light with individual atoms or subatomic
particles is studied, the particle model must be used.
Although the first studies of the sky were made using optical telescopes to
gather incoming visible light from various objects in the universe, we have learned
to use the other types of electromagnetic radiation in our astronomical studies as
well. Radio telescopes and a limited number of microwave telescopes placed on
very high mountain tops can be used from the surface of the Earth, but almost all
other types of electromagnetic radiation are blocked to a great extent by the
atmosphere and can only be studied if suitable telescopes can be operated high up
in the atmosphere itself, such as in high-altitude balloons of high-flying aircraft, or
with telescopes placed outside of the atmosphere using satellites. In the last few
decades, x-rays, gamma rays, infrared, and ultraviolet studies and even microwave
technology have been used to greatly increase our knowledge of the universe
through detailed analysis of the data now made available by the capture of these
other types of electromagnetic radiation in satellite telescopes.
The behavior of light can be understood by using light rays to show the path
of light through lenses and as it is reflected from mirrors. If you study the ray
diagrams for optical telescopes you will get a better idea of how refracting
telescopes (which use lenses as the primary objective elements) and reflecting
telescopes (which use mirrors as their primary objective elements) operate to
gather visible light for more detailed study and analysis. Notice that lenses can
distort the images formed by telescopes so special achromatic lens systems must be
used in high quality telescopes to overcome this chromatic aberration. You will
also learn about resolving power and magnifying power in the chapter. Spherical
mirrors can also cause distortion so really high quality reflecting telescopes must
be made with parabolic mirrors which are much more difficult and expensive to
make but which are absolutely critical when fine resolution pictures are required.
In order to see faint or very distant objects with any telescope, the objective
element of that instrument must be quite large. Since refracting telescopes, which
use a lens for their objective elements, are limited in size to about 1 meter in
diameter, the largest optical telescopes are reflecting units in which the primary
mirror can be as large as 6 meters in diameter giving them much greater light
gathering power. To get even larger light gathering surfaces, the newest
telescopes use multiple mirrors that are computer controlled to focus all of the
mirrored elements at the same point. The twin Keck telescopes in Hawaii are good
examples of this multiple mirror technology. Many other modern telescopes use
this multiple mirror construction as well.
Since it is important to get as much information as possible from the limited
amount of light that we receive in our astronomical telescopes, there are several
special instruments that are used to gather and analyze light signals. Chargecoupled devices (CCDs) and high resolution photographic plates enhance our
ability to gather weak light signals and spectrographs with their ability to break
down signals into their complex frequency components have given us a very
powerful tool for the analyzation of all types of electromagnetic data. Without
such capability we would not be able to determine the composition, pressure and
temperature conditions, or the complex motions of the various celestial objects that
we observe throughout the universe.
Radio telescopes are similar the optical reflecting telescopes but are much
larger and are designed to receive long wavelength electromagnetic radiation in the
radio portion of the spectrum. Radio telescopes can also be operated on the surface
of Earth because radio waves, like light signals, can easily penetrate the
atmosphere. Because of the longer wavelength of radio signals, the resolution of
radio telescopes in not as good as that of optical telescopes. This resolution can be
improves by connecting two or more widely separated radio telescopes together
using computerized electronics to form a radio interferometer. Radio telescopes
used in this way can be several kilometers apart as in the VLA (Very Large Array)
in New Mexico, or may be many thousands of kilometers apart as when the radio
telescopes in Chile and those in Hawaii are used in combination.
The largest single-dish radio telescope is located at the Arecibo Observatory
in Puerto Rico. This dish is more than 1000 feet in diameter. It is so large that it is
built right on the contoured ground of an existing mountain valley and is not able
to be aimed at any specific desired location in the sky. This telescope simply
observes whatever is directly overhead at the time, but because of its incredible
size, it is still one of the most powerful additions to the ranks of high performance
astronomical telescopes in use throughout the world today.
The advent of satellite technology has given astronomers the ability to
extend their observations of celestial objects into other regions of the
electromagnetic spectrum. A great deal of information can be obtained by using
infrared, ultraviolet, x-ray, microwave, and gamma ray telescopes that have been
boosted into space where they operate on satellites that continually orbit the Earth.
The data gathered is send back to scientists on the surface for analysis. Even
optical observations are much better when they are made above the distortions
produced by the Earth’s atmosphere, so the Hubble Space telescope is also satellite
mounted. The primary mirror on the Hubble is only 2.4 meters in diameter, but it
gives resolution and clarity that is as good, if not better, than the best large
diameter ground based multi-mirror optical telescopes in use today. There are
several plans in place today to expand our use of satellites and even to expand our
use of space probes sent out of Earth’s orbit to visit other areas of our solar
system. Data gathered from such instruments promises to expand our knowledge
and understanding of the universe to even greater levels in the years to come.
Chapter 6 - Atoms and Starlight
What is so special about electromagnetic radiation? First of all, it can come
to us through empty space from various parts of the universe. It can travel over
large distances and retain its basic information intact, yet under certain wellunderstood circumstances, it can be changed by interactions on its way to us, and
this can tell us much about the trip and the materials that have been encountered as
this radiation finds its way to us on Earth. The most important reason to study
electromagnetic radiation in detail, however, is the fact that there is nothing else
available for us to study. There simply is no other type of data that reaches us
from outside of our atmosphere that can give us any of the important information
that we need to understand our celestial surroundings or the history of our
universe.
Even though it may sound like we are at a great disadvantage in only having
electromagnetic radiation as our one primary source of data from the stars, we have
learned to acquire vast amounts of information from this source. Light is the best
known form of electromagnetic radiation because human eyes are designed to
serve as very sensitive receptors for light. This does not mean that light is easily
understood or that the more complex information carried by light is easy to gather
and analyze. The light from the stars and other celestial objects does not have
much intensity left when it reaches the Earth's surface, because most of it comes
from so very far away. Scientists have, however, developed special techniques to
process these weak signals and use them to enhance our understanding of the
cosmos.
Observation of the sky with the naked eye reveals only tiny dots of light,
except for the visual images of our Sun and Moon. Even these closest neighbors in
our solar system cannot be studied in detail with the unaided eye, so the first great
advancement in modern astronomy was the development of the telescope. A
telescope is basically a device for gathering the faint light signals from distant
objects and makes them available for analysis. Telescopes also have the ability to
magnify the object under observation, giving us greater access to the visual details
of the universe. Very distant objects, however, still appear as simply dots of light
even with the largest and most powerful of our modern optical telescopes.
Fortunately when we study light (and other types of electromagnetic
radiation) we can use spectrometers to split light into its component frequencies
and look at the color, and intensity, of the various spectral lines in great detail. The
color tells us about the temperature of the incandescent source that produced the
light and sometimes about the gas and dust clouds that the light has passed through
on its way to our telescopes. The intensity of the incoming light, coupled with
other observation, often is the only clue as to how far away the light source is from
the Earth. Study of the frequency spectrum informs us of the kind and abundance
of the chemical elements in the stars, the pressure and density within these sources,
and even the speed with which such objects are moving toward or away from us
through space. This last information is provided by a process called the Doppler
Effect and this type of analysis is the key to our understanding of the expanding
nature of the entire universe around us.
The interaction of light with individual atoms involves the absorption and
emission of photons (light particles), as the electrons surrounding the nucleus jump
up or fall down between the allowable energy levels associated with each specific
type of atom or molecule. These transitions can be detected ad analyzed by a
spectrometer to reveal the detailed composition and structure of even very distant
light sources, so these processes are very important to modern astronomy and are
the basis for our understanding of distant parts of the universe. Many aspects of
our universe would never have become known to us if these atomic and molecular
interactions with light had not been discovered and were not well understood.
To give a better basis for the understand these photo-electric interactions,
the first part of this chapter gives a generalized explanation of the structure of
atoms. The basic components of atoms, protons and neutrons make up the nucleus
of all atoms and the variations in the numbers of protons and neutrons in each
nucleus defines which element the atom represents and also which isotope of that
element any individual atom is. Electrons orbit this nucleus and can occupy
several different energy levels around any given nucleus. Transition between these
allowed energy levels either absorb or emit photons of electromagnetic radiation,
often in the visible portion of the spectrum. You should look this material over
carefully and learn the various parts of the atom, how they interact with light, and
what they can tell us about the composition and structure of even the most distant
light sources we can observe in space. Pay particular attention to understanding
emission and absorption spectra and how they relate to the transitions of orbital
electrons between allowed energy levels within ordinary atoms. You should also
learn how heated objects radiate electromagnetic radiation and how this emitted
radiation can be used to accurately determine the temperature of the source of this
radiation using what has come to be referred to as the Balmer thermometer.
The spectrum produced by prisms and diffraction gratings in spectrographs
are so important to our understanding of the stars that one of the first things
astronomers do when they study a star is determine its spectral class. This spectral
class designation not only immediately tells us the temperature and size of the star,
but as we will see in later chapters, can also help us plot the location of the star on
an H-R diagram. An H-R diagram, on the other hand presents us with information
about a great many other interesting characteristics of that star and also about the
overall structure and history of stellar evolution through out the entire universe.
Make sure that you understand the significance of the spectral classes of
stars. These classes are O, B, A, F, G. K, M, with the O class stars being the
hottest and the M class the coolest. We can also subdivide each class into 10
subclasses using the numbers 0 - 9 as indicators. Our own Sun is a G2 star that has
a surface temperature of about 5800 degrees Kelvin. Other stellar classes also
correspond to specific temperatures for the stars that they represent.
The shifting of the position of the spectra lines in the spectrum of starlight is
an indication of the relative motion of that star toward us or away from us. A shift
toward the blue end of the spectrum indicates that the object emitting the light is
moving toward us, a shift toward the red end shows that it is moving away. Since
our observations show that the spectra lines for most stars and even entire galaxies
are red shifted, that is moving away from us, the basic model of cosmology today
is based on the concept of a gigantic initial explosion called the Big Bang which
marked the beginning of the universe as we know it. This model assumes that all
parts of the universe are currently moving away from all other parts of the universe
at extremely high rates of speed.
Another important thing that the line spectrum of a star or other celestial
object tells us is the composition of that object. Each chemical element has its own
distinctive spectral pattern. This means that the presence or absence of any given
element can be determined by a careful analysis of its spectrum. The relative
intensity of the lines characteristic of a given element can also tell astronomers the
amount of each chemical element present in that celestial object even though it is
many thousands or even millions of light years away. It is generally accepted that
this method is so accurate that we actually know more about the composition and
structure of stars millions of light years away from us than we do about the interior
of the Earth only a few thousand kilometers below our feet.
As you carefully read this chapter, try to acquire a general feel for the way
that light and the other types of electromagnetic radiation behave and how line
spectra can serve as the key to understanding many of the properties of the
universe around us. We will refer to these concepts often during the rest of the
course, so you must fully understand the basic ideas presented here if you hope to
keep up with the information presented later in our study of astronomy. For
example, in the next chapter we will use many of the techniques described here to
delve into the composition and structure of the closest star to our home planet, the
Sun.
Chapter 7 - The Sun
Our Sun is a gigantic nuclear furnace that, like all stars, is slowly converting
millions of tons of it’s primary component, hydrogen, into helium each day. Even
with this extraordinary use of fuel, this process has been going on for billions of
years and is expected to continue at its present rate for billions of years more. This
is fortunate because nearly all energy used on Earth is either directly or indirectly
the result of the energy produced deep in the core of our Sun and life as we know it
would soon cease to exist if this energy was no longer available. Since the Sun is
so important to us, it seems only natural that we learn as much as possible about its
structure and the processes that produce and maintain it’s energy output.
The conversion of hydrogen into helium (known as HYDROGEN FUSION
or the proton-proton cycle because it involves the combination of 4 hydrogen
nuclei (protons) to form one helium atom) is an exothermic process, that is the
reaction is self-sustaining and gives off energy as it progresses. This energy is
produced in the Sun’s core, deep within the star itself, and gradually works its way
outward as heat by two heat-transfer methods known as radiation and convection.
Although it takes hundreds of years for this energy to reach the Sun’s surface, there
is always a large quantity arriving that can be radiated into space as light and other
forms of electromagnetic radiation. Hydrogen fusion takes place continuously
over billions of years because there is a nearly perfect balance between this
outward flowing energy and the tremendous pull of gravity caused by the Sun’s
very large mass. The exact balance of these two factors is known as
HYDROSTATIC EQUILIBRIUM.
It is believed that the release of energy by the Sun is nearly constant so that
the energy received on Earth does not change greatly from year to year. This is
very important because the evolution of life and its continued existence on Earth
would not be possible without this stable environment made possible by this
constant incoming energy flux. This is why the study of the Sun is so important
and has such a high potential to effect the lives of all of us if even very small
variations occur in the operation of the Sun’s nuclear furnace.
According to our everyday standards on Earth, the core of the Sun is an
extreme place with very high pressure and density, and a temperature of about 15
million degrees Kelvin. As the heat from the Sun’s core works its way outward,
the general properties of our nearest star become less exaggerated so that by the
time the heat gets to the surface, the conditions are not so radical. The pressure
and density are nearly that of gases as they are found on Earth but the temperature
is still quite high, about 6000 degrees Kelvin. The Sun’s visible yellow surface,
called the PHOTOSPHERE, has a very large area and so is able to radiate
extremely large amounts of energy into space. As was previously mentioned, this
energy is critical to the continued existence of our Earth as we know it today.
The continual out-flux of energy from the Sun’s core produces convection
cells at the Sun’s surface which result in a GRANULATION effect consisting of
regions of hot boiling gases. Because the entire Sun is a gaseous material, the
motion of this gas is quite complex and this leads to not only heat gradients but
also twisting and distortion of the Sun’s magnetic field lines which leads to
gigantic surface storms that disrupt the photosphere and throw plumes of hot gases
out into space as SOLAR PROMINENCES. This tremendous surface agitation
also causes the emission of large amounts of electromagnetic radiation along with
charged particles like protons and electrons which comprise the SOLAR WIND.
The twisting of the Sun’s magnetic field lines deep beneath the solar surface tends
to inhibit the flow of heat outward in certain localized regions resulting in cooler
areas on the photosphere surface known as SUN SPOTS.
There are two more important outer layers of the Sun.
The
CHROMOSPHERE is a thin layer composed of SPICULES of hot gases that erupt
outward just above the photosphere. This layer can only be seen when the main
surface of the Sun is obscured from view as it is during a total solar eclipse. At
this time the chromosphere appears as a thin pinkish region surrounding the
darkened photosphere. The temperature of the chromosphere is only a little hotter
than the Sun’s photosphere, ranging from about 7 to 15 thousand degrees Kelvin.
The second distinct outer layer above the photosphere is the CORONA.
This layer extends millions of miles out into space and again can best be seen
during solar eclipses when it appears as a ghostly-white halo or crown around the
Sun. This layer is so hot that it emits tremendous amounts of energy, mostly in the
x-ray and ultraviolet regions of the spectrum. Variations in the density of this layer
known as CORONAL HOLES allow large quantities of charged particles to escape
from the Sun, thus enhancing the emission of the solar wind.
We explore the Sun’s surface using special telescopes that project the image
of the Sun’s surface onto a screen using concave mirrors. Looking directly at the
Sun with even a small telescope could seriously damage the eyes of an observer.
The interior of the Sun is more difficult to study but this can be done using
techniques similar to the ways that we study earthquakes on our own planet, Earth.
This study called Helioseismology, has shown us that the interior of the Sun
vibrates much like a bell that has been struck with a hammer. Examination of this
data can show us not only the structure but also the physical conditions such as
pressure and density variations deep within the interior of the Sun.
Fluxuations in the Sun’s energy output, although small, are important to us
and can be studied by observing the changes in sunspot activity on the Sun’s
surface. There is a predictable pattern to these variations that occurs over a 22year cycle. These variations are related to changes in the solar magnetic field lines
as the Sun’s internal gases are mixed around because different latitude regions of
the Sun rotate at different rates. This DIFFERENTIAL ROTATION causes a new
set of sunspots to appear every 11 years, but the magnetic properties of these
sunspots are reversed from North to South poles and back again with every cycle.
This leads to the definition of the complete sunspot cycle as 22 years, twice that of
the periodic observations of large numbers of sunspots on the surface which occur
regularly every 11 years.
The study of sunspots also enables us to determine the rate at which various
parts of the Sun rotate about its own internal axis. The equator of the Sun makes
one rotation in about 25 days but the polar regions turn more slowly, making one
rotation in around 35 days. This differential rotation is the basis for the variations
in the magnetic fields within the Sun that not only produces sunspots but also leads
to the production of large scale magnetic storms on the Sun’s surface that have
serious effects on the weather patterns and long range radio and television
communications here on Earth. Variations in the solar wind also lead to periods of
higher charged particle influx which produce beautiful electronic discharges in the
Earth’s atmosphere near the poles known as the AURORA, known in the Northern
hemisphere as the Aurora Borealis (or the Northern Lights) and in the Southern
hemisphere as the Aurora Australis (or Southern Lights).
Chapter 8 - The Family of Stars
Now that we have some idea about the structure and composition of our Sun,
we can turn our attention to other stars. Are they nearly the same or quite different
from our Sun? In actual practice, all stars are quite similar to our Sun, but stars
like people are born, live and die. They also vary in size and somewhat in
composition. In this chapter and the next two chapters, we will look into these
similarities and differences and learn that our Sun is actually quite typical for a
relatively small star that is about half way through its life cycle.
The first steps in understanding the family of stars in our universe are to
learn to determine how far each star is from us, how much energy the star
produces, the relative size of different types of stars, the masses of stars, and then
construct a general model for a typical star. None of these concepts are easy to do,
mainly because all of the stars other than our Sun are very, very far away from us,
so distant that we measure how far they are from us in terms of how many years it
takes light emitted from them to reach us here on Earth, hence the term LIGHTYEAR as a measure of distance rather than a unit of time itself.
Most stars are so far away that we can only measure the distance to a very
small percentage of them using a triangulation process known as the stellar
parallax measurement. In this determination we carefully measure the angle to the
star at two different times of year, six months apart, so that our view of each star is
from opposite sides of the Earth’s orbit around the Sun. This gives us the largest
possible base line on which to perform our calculation but even so, the angles that
we measure are extremely small, on the order of a fraction of one second of arc.
Comparing this angle to our base line we can then calculate the distance to the star.
The result gives the distance in a unit called parsecs but this can easily be
converted to light-years. (One parsec = 3.26 ly.) Using this method, the distances
to several hundred thousand stars can be obtained but these are only a few of the
closest stars in our own Milky Way galaxy. Millions of other stars exist at even
greater distances, so what other methods can we use to determine how far these
stars are from us?
Fortunately the farther away a star, or any other light source, is from us the
dimmer it appears. There is a very definite rule known as the inverse square law
that gives us the exact relationships of brightness to distance. If we know the
actual brightness of the star to begin with we can use the inverse square law to
calculate the distance to that star. The actual brightness is more properly known as
the INTRINSIC BRIGHTNESS of the star and this is closely related to the
LUMINOSITY of the star, that is the actual total energy emitted by that star each
second. There are several ways of determining this intrinsic brightness, some of
then involving standard candles (objects in space that we can identify and for
which we already KNOW the luminosity) or we can use an H/R diagram, a plot of
the spectral class of a star as a function of its luminosity. As we have seen in a
previous chapter, we can quite easily determine the spectral class of any star from
which we can gather even small amounts of starlight, so this method of estimating
the intrinsic brightness of a star is usually possible. Once we know the intrinsic
brightness we can compare this to the APPARENT BRIGHTNESS (how bright the
star actually appears to be in a telescope) and use this comparison along with the
inverse square law to calculate the distance to the star.
There is also a specific relationship between the luminosity, the temperature,
and the radius of a star. Again the temperature of a star can be quite easily
determined once the spectral class of a star has been established and the size of the
star can also be found from its location on the H-R diagram. This makes the H-R
diagram an extremely valuable tool in studying stars, so you should take a few
minutes to learn as much about it as possible.
There are several examples of the H-R diagrams in the textbook of pages
151 through 154. Note that the primary region where stars as found on an H-R
diagram is called the main sequence, a narrow diagonal region that runs from the
lower right (low luminosity, low temperature area) to the upper left of the diagram
(high luminosity, high temperature area). Above and to the right of the main
sequence is a region in which GIANT and SUPERGIANT stars are plotted, below
and to the left are the white dwarf stars. (We will learn much more about these
types of stars later in the course.) Also note that the size of the main sequence
stars increases as we progress upward (from lower right to upper left) on the
diagram.
The masses of stars are also very difficult to determine. Fortunately there
are many stars that exist as binary pairs throughout the universe. BINARY
STARS are two or sometimes 3, 4 or more, stars that are so close together that
their mutual gravitational attraction causes them to continually orbit their common
center of mass. By measuring the orbital periods and orbital diameters of these star
pairs, the ratio of their individual masses can be determined thus giving us a firm
handle on the masses of even very distant stars. Without binary stars it would be
quite difficult to measure the masses of any stars, especially those at great
distances form Earth. All three type of binary stars can be used in such
calculations. Make sure that you know the definitions and characteristics of
VISUAL BINARY STARS, SPECTROSCOPIC BINARY STARS, and
ECLIPSING BINARY STARS, each of which has its own unique way of helping
to solve the mystery of solar mass and define other stellar properties throughout the
universe.
Finally, if we take a general survey of the types of stars that make up the
entire universe, we find that small, low mass, low temperature M class stars (RED
DWARF STARS) are by far the most common, followed by WHITE DWARF
STARS (hot but still very small), and then slightly larger and hotter main sequence
stars in the G and K classes. Compared to these types, all of the other main
sequence stars, red giant, giant, and supergiant stars are not very common at all.
See the diagram on the top right hand side of page 165 in the textbook for a good
visual representation of this overall stellar survey.
Chapter 9 - The Deaths of Stars
Stellar formation and evolution is a fascinating and important aspect of
astronomy. Much of the data used in setting up models for those process has come
from observations of our nearest star, the Sun. There are, however, recent
indications that some of the details of these theories may not be as solidly based in
fact as we have been led to believe. Do not be surprised if in the next few years
some surprising and interesting things are published on this subject. What we will
study here are the best current ideas about stellar evolution. Most of the basics are
pretty firmly established, but it is always best to keep an open mind. New and
changing explanations that are formulated as more and better data is collected and
analyzed should not take us by surprise.
Stars begin their existence as clouds of gas and dust within the
INTERSTELLAR MEDIUM. The composition of these clouds or NEBULA
determines what types of star will be formed. Current data indicates that no new
Population II stars (stars low in heavy metal content) are now forming, so all
young stars are Population I stars (slightly higher in heavy metal content) very
similar to our Sun. We will study Population I and II stars in more detail in
chapter 12. Large gas and dust clouds of about 100 million solar masses are the
usual size to begin contraction in the formation of a galaxy within this larger cloud.
Smaller clouds of from 0.02 to 80 solar masses later continue their own contraction
to form individual PROTOSTARS. It is surprising that such a small range of
masses exist for stars when such an enormous, generally homogeneous cloud
collapses to begin their existence. As the smaller clouds continue to contract, they
eventually become full-fledged stars.
The contraction of gas and dust nebula is the result of mutual gravitation
between the individual particles within the cloud. This contraction can be
triggered by SHOCK WAVES which can be produced by nearby supernova
explosions or by density waves in these swirling clouds. Once the contraction
begins it is simply gravity that continues to pull the gas and dust together to form
hotter and denser regions. The densest of these gas and dust clouds are called
GIANT MOLECULAR CLOUDS. These clouds must be very cool if stars are
going to form within them because if the temperature is too high the internal
kinetic energy of the individual particles will be too great to allow gravity to
succeed in collapsing the clouds into protostars. We can see the formation of new
stars and protostars in dense regions of gas and dust such as the one found in the
famous Orion Nebula.
What is it that marks the birth of a star from a protostar? As the original
cloud of gas and dust contracts, the density and temperature of the central region of
the cloud increase. When the temperature of the core reaches about 10 million
degrees Kelvin, the process of nuclear fusion begins. This process begins to turn
the main component of the cloud, hydrogen gas, into helium. The fusion process
also releases large quantities of energy, the radiated energy given off from the
star's surface is what causes the star to "shine." The beginning of the nuclear
fusion process marks the birth of a true star.
The fusion process stems from Einstein's famous equation, Energy = mass
times the speed of light squared (E = mc2). When four protons (hydrogen atom
nuclei) combine into helium, the mass of the helium is a tiny bit less massive than
the combined masses of the original protons. This "missing mass" is what has
been converted into energy. After working its way out from the core, this energy
can be released as electromagnetic radiation from the star's surface. Study the
explanation of this process in the text, noting that sometimes carbon acts as a
catalyst if the star’s core is hot enough. A strange fact is that the more massive the
star, the hotter its core becomes due to the force of mutual gravitational attraction
and the faster its hydrogen fuel is used up. Heavy stars shine brighter and have a
shorter lifetime than do smaller Population I stars and thus stars like our Sun have
a stable hydrogen-burning lifetime of about 10 billion years, only about half of
which has thus far occurred. So our Sun can be expected to send consistent light
and heat to our Earth for many generations to come.
If gravity were the only force acting on stars, they would completely
collapse soon after the fusion process began in their cores. Fortunately there is an
opposing process produced within the cores of stars called RADIATION
PRESSURE. This outward pressure is created as the heat and fast-moving
particles from the core migrate outward toward the star's surface. Stars are highly
compressed gas, so their surfaces are not really as solid as that of the Sun appears
to us from Earth. The visible surface of a star is defined as a narrow layer where
photons have an equal chance of escaping into space or being reabsorbed back into
the star's interior. When the radiation pressure and the force of gravity are exactly
balanced, we say that the star is in a state of HYDROSTATIC EQUILIBRIUM.
The heat generated in the core of the star works it way outward by the heat-transfer
processes of radiation and convection.
Fortunately for life on Earth, hydrostatic equilibrium within stars can and
does last for millions and even billions of years. The period during which the
hydrogen fuel is being "burned" in the core of a star is called the star's MAIN
SEQUENCE LIFETIME. As we have already seen, an interesting plot of the
luminosity of a star versus its temperature (called a HERTZSPRUNG-RUSSELL
or "HR" DIAGRAM) can be drawn, upon which some of the general properties of
stars can be shown. Review the section in the text dealing with HR diagrams again,
this time paying attention to the evolutionary track followed by newly forming
stars as they condense out of gas and dust and finally settle on the main sequence
portion of the H/R diagram.
Notice that the path taken across the H/R diagram as the star forms and the
final location of the main sequence star that forms is almost entirely determined by
the mass of gas and dust available to form the star in the first place. The mass of a
star is also responsible for the overall luminosity of the star while it is on the main
sequence. This is referred to as the mass-luminosity relationship. One other thing
that should be pointed out here is that in some situations the mass of a star is not
sufficient to produce the amount of gravitational pull needed to raise the core
temperature of a collapsing gas and dust cloud to a high enough level to cause
hydrogen fusion to begin. In such cases the star never actually makes it to the
main sequence and must spend the rest of its short life simply cooling off again.
Such a protostar has an overall mass of less than 0.80 that of our Sun and is
referred to as a BROWN DWARF.
The inner working of a star are quite complicated. To combine all of the
laws of physics that are involved like; conservation of mass, energy transport,
nuclear fusion, hydrostatic equilibrium, and energy conservation, STELLAR
MODELS are formulated. These models help us to understand how the overall
mass of a star is the primary determinant in formation, mains sequence life, and the
death of stars and how the condition within the star vary with the depth below the
stars surface. This chapter begins our study of what has come to be known as the
LIFE CYCLE OF A STAR.
Most stars spend about 90% if their entire lifetime on the main sequence but
they eventually evolve into other forms. The overall time a star exists on the main
sequence is again primarily determined by its mass with the LOWER mass stars
having longer lifetimes. This is because more massive stars tend to accelerate their
rate of hydrogen fuel consumption since their cores become much hotter due to the
increased gravity. For example the Sun is expected to stay a main sequence star of
about 10 billion years, but a more massive star with an overall mass 25 times that
of our Sun would have a main sequence life of only about 7 million years. The
study of this cycle will take us through most of the next two chapters in this course
where we will learn what happens to a star when it finally leaves the main
sequence portion of its life..