The Structure, Size, and
Origin of the Universe
Earth Science
Geology 007
Information about the Universe comes to us as
electromagnetic radiation (light) emitted and
reflected by events and objects in the cosmos.
Our knowledge of the Universe has always been
constrained by our ability to gather light.
The human eye is sensitive to a small region
of the total electromagnetic spectrum
Text
The unaided human eye is small
and does a poor job of gathering
faint light from distant objects.
For thousands of years our knowledge of the
Universe was limited to the objects and events
we could see with the naked eye.
Sun
Stars
Moon
Supernovae
Mercury, Venus,
Earth, Mars, Jupiter,
Saturn
Nebulae
Comets
Eclipses
Stars and nebula in Orion
The Moon and the Sun appear as disks.
Stars and planets are points of light.
Moon
Jupiter
Venus
How can you tell a star from a planet?
(Hint: planet is a Greek word meaning “wanderer”.)
Can you spot
the planet in
these images
taken over a
succession of
nights?
Planets move
against the fixed
starry
background.
Although the Sun and the Moon appeared
to circle the Earth, the motions of the
planets were more complicated.
Movement of Mars against the starry background.
Early astronomers also saw that the motions
of the Sun and Moon were complex.
Anelemma showing the changing position of the Sun in
the sky at the same time each day throughout the year.
Early astronomers also saw that the motions
of the Sun and Moon were complex.
Monthly cycle of lunar phases.
Telescopes
Telescopes gather light from a
large area using lenses and
mirrors and focus the light on
the eye. Distant objects appear
closer and brighter
Galileo built his own telescope
and used it to observe the moon
and planets in 1611. This is the
first recorded use of a telescope
for astronomy.
Galileo Galilei
Observed mountains and craters
of the Moon.
Observed rings of Saturn (but
mistook them for moons).
Observed Jupiter’s four largest
moons in orbit around the planet.
Observed phases of Venus.
Although most famous for his ill-fated rejection of
the Geocentric model of the Universe in favor of
the Heliocentric model, Galileo’s most profound
impact on astronomy may have been to
demonstrate that objects like the Moon and planets
were other worlds like the Earth.
Charles Messier
French astronomer and comet
hunter in the 1700’s.
Cataloged over 100 diffuse
objects in the sky that were
easily mistaken for comets.
These are now recognized as
“deep sky” objects such as star
clusters, nebulae, and spiral
nebulae (galaxies).
The nature of these objects was
poorly understood.
Messier Objects
Distances
How large is the Universe?
How far away are the Sun, Moon, planets,
and stars?
Fundamental yardstick for measuring
astronomical distances is the astronomical
unit (AU) = distance from the Earth to the
Sun.
Johannes Kepler
1571-1630
Developed 3 laws of planetary motion
"The orbit of every planet is an ellipse
with the sun at a focus."
"A line joining a planet and the sun
sweeps out equal areas during equal
intervals of time."
"The square of the orbital period of a
planet is directly proportional to the cube
of the semi-major axis of its orbit."
Kepler’s Laws
1st law
2nd law
3rd law
Kepler’s 3rd law allowed the relative
distances between planets to be
calculated
What was needed was one accurate
measure of the absolute distance between
a planet and the Sun - from one distance,
the rest could be calculated.
In the 1700s the “Holy Grail” of astronomy
became the determination of the EarthSun distance - the Astronomical Unit.
Distances of distant objects can be
measured using the Parallax method.
optical shift measured
in arcseconds
(1/3600 of a degree)
Using Parallax to Measure
Distances
p
parallax = 1/2 optical shift
distance = (B/2) / p (radians)
d
B
Measuring the parallax of Venus by observing a
transit of Venus across the disc of the Sun.
Transit of Venus across
the disk of the sun, 2004.
(Composite image by Johannes
Schedler)
By the late 1700’s parallax measurements had been
used to determine the Earth-Sun distance to be
app. 149.60×10^6 km (93 million miles) = 1 AU
Distances to all planets and minor planets
can be calculated by Kepler’s 3rd law.
Absolute size of Sun and planets can be
calculated knowing their apparent size and
distance.
Mass of the Sun can be calculated.
Can measure the distance to nearby stars.
Stars are very far away - need a long baseline to
observe parallax. Can observe the shift in position
of a star seen from opposite points in the orbit of
the Earth.
Parsec
Baseline of stellar parallax measurements is radius
of Earth’ orbit = 1 AU.
Define a unit of distance equal to 1 arcsecond of
parallax given a baseline of 1 AU.
1 parsec = 206,260 AU = 3.26156 light years = 31
trillion kilometers (19 trillion miles)
1838: Friedrich Bessel measures the distance to 61
Cygni to be 3 parsecs (parallax = .314 arcsecond)
Closest star is Proxima Centauri with a parallax
of .77 arcsecond = 1.3 pc = 4.3 ly distant.
Stars within
50 light years
of the Sun
http://www.atlasoftheuniverse.com/50lys.html
The parallax method for measuring
astronomical distances is limited by
the power of telescopes
Modern measurements of parallax obtained from
space-based instruments can be made for
objects up to a thousand parsecs away.
More distant objects require the use of other
methods to estimate distance.
“Standard Candles” - objects of known absolute
brightness that can be used as distance
yardsticks.
Apparent vs. Absolute Magnitude
Observed Brightness
Actual Brightness
10 pc
Brightness of a star is related to the inverse
square of its distance from an observer.
If you know the absolute magnitude of a star
and you measure its apparent magnitude, you
can calculate its distance.
1 pc
2 pc
4 pc
8 pc
Standard Candles
Objects of known absolute brightness
Main sequence stars: star color gives a rough
estimate of absolute brightness.
- Not reliable for individual stars, but useful for
clusters of stars
Variable stars
- periodicity related to absolute brightness
Novae and Supernovae
Hertzprung Russell
Diagram
Note
relationship
between color
and brightness
for main
sequence
stars.
Cepheid Variable
Stars
•Stars that brighten and
dim on a regular cycle.
Period
of the cycle is
•
directly related to the
absolute brightness of
the star.
A
• three-day period
Cepheid has a
luminosity 800 times
that of the Sun. A
thirty-day period
Cepheid is 10,000 times
as bright as the Sun.
Cepheid Variable Stars
• Discovered by Henrietta Swan Leavitt in 1908.
• She established the relationship between
period and brightness.
• Edmund Hubble use observations of cepheid
variables in the Andromeda “spiral nebula” to
show that this was actually far outside of the
Milky Way.
• Proved that spiral nebulae were actually other
galaxies, greatly expanding the size of the
Universe.
Cepheid Variable
Stars
• Observations of cepheid
variable stars in distant
galaxies allow for
accurate measurements
of distances in
extremely far away
objects.
• Allows for calibration of
brighter supernova
standard candles.
Hubble Space Telescope image of a cepheid
variable star in a distant galaxy 16 million light
years (5 Mpc) away.
Supernova 1994D in the outskirts of the galaxy NGC 4526
approximately 20 Mpc
Hubble Space Telescope Ultra Deep Field Image
•High resolution image of most distant
galaxies ever photographed.
•Image size is 1/10 diameter of the full
moon.
Over
10,000 objects - most are galaxies.
•
•One thirteen-millionth of the total area of
the sky
•13 million X 10,000 = 130 Billion Galaxies!
•How far away are these galaxies?
Redshift / Blueshift
Light waves emitted by a
source moving away from the
observer are seen to increase
in wavelength, shifting visible
light toward the red end of
the spectrum.
Light waves emitted by a
source moving toward the
observer are seen to decrease
in wavelength, shifting visible
light toward the blue end of
the spectrum.
normal
red-shifted
Stars exhibit characteristic
lines in the spectra of light
being emitted by the star.
These line occur at specific
frequencies which are
absorbed by elements such as
hydrogen present in the star.
In some stars these spectral lines
are shifted to longer wavelengths,
indicating that these stars are
moving away from the Earth.
Hubble’s Law
• In addition to stars, galaxies also exhibit
redshifts and blueshifts.
• Edmund Hubble and colleagues demonstrated
that there was a direct relationship between
increasing distance and increasing redshift in
galaxies.
• More distant galaxies appear to be moving away
from the Milky Way faster than closer galaxies.
• This observation is best explained by an
expanding universe in which the space between
galaxies is increasing, stretching out the
wavelength of light as it travels.
Redshift due to expansion of the Universe
Because light travels at a finite speed, an expanding universe means that the farther
out in space you look, the farther back in time you are seeing.
Galaxies in the Hubble Space Telescope Ultra Deep Field view
include distant red-shifted galaxies the formed between 400 and 800
million years after the Big Bang.
The Big Bang
• Belgian cosmologist (and Jesuit Priest) Georges
Lemaitre predicted based on Einstein’s Theory of
General Relativity that the Universe was
expanding and that observations of galaxies
would show this to be true (Hubble published his
work two years later).
• Expanding space implies that in the distant past,
space was smaller and everything in it was closer
together.
Monsignor Georges Lemaître,
priest and scientistt
• Proposed that a “Creation-like” event brought
forth the Universe from a “primordial atom” in
the distant past.
• Ironically, “Big Bang” was coined by Sir Fred Hoyle, an
opponent of the theory, on a radio broadcast in 1949.
Confirmation of the Big Bang Theory
• Theory predicts that the light echos from the Big Bang explosion
should be detectable as a background of microwave radiation that
appears to come from all points in the sky equally.
• Cosmic Microwave
Background (CMB) was
discovered accidentally in
1965 by Wilson and
Penzias.
• Both were awarded a
Nobel Prize for their
discovery.
CMB is the light released 380,000 years after the Big Bang when the
Universe first became transparent.
Confirmation of the Big Bang Theory
• Initial observations of the CMB from Earth showed that the
“afterglow” of the Big Bang seemed to come from all points of the
sky with equal intensity, suggesting a homogeneous early Universe.
• But the modern
Universe is very
clumped with groups of
stars, galaxies, and
clusters of galaxies.
• Theory predicts that the
CMB should also be
“clumpy”.
Detailed satellite measurements of the CMB in the 1990’s and
2000’s confirmed the prediction that the early Universe was not
completely homogeneous - giving rise to the structure of the
Universe to come.
Detailed satellite measurements of the CMB in the 1990’s and
2000’s confirmed the prediction that the early Universe was not
completely homogeneous - giving rise to the structure of the
Universe to come.
Timeline of the
Big Bang
The pre-CMB history
of the Universe is
modeled based on
high-energy physics
and quantum
mechanics.
James Webb Space Telescope
Launch date: 2013
Mission:
To image stars
and galaxies
from the very
beginning of the
Universe, less
than 500 million
years after the
Big Bang.