Chapter 28 Stars and the Universe

Chapter 28
Stars and the Universe
THE SEARCH FOR EXTRATERRESTRIAL LIFE
In 1996, a team of scientists announced that a meteorite recovered
from the ice of Antarctica might contain evidence of
life outside Earth. Microscopic studies revealed something
that could be fossils of bacteria. Organic compounds thought
to be the result of biological processes were identified near
this “fossil.”
Most scientists agree that the meteorite is from Mars.
However, no evidence of life on Mars has been detected by
remote observations or by spacecraft sent to Mars to photograph
surface features. If life exists there, it is probably in
the form of primitive microscopic organisms that live under
the planet’s surface. Some scientists suggest that both the
fossil-like shape and the compounds thought to be of biological
origin could be the result of inorganic processes. There is
still discussion among scientists about whether the features
in this meteorite are or are not the first direct evidence of life
outside Earth.
709

710 CHAPTER 28: STARS AND THE UNIVERSE
ET, Phone Earth
Another attempt to discover extraterrestrial life is project
SETI: the Search for Extra Terrestrial Intelligence. This program
seeks to identify radio transmissions coming from outside
the solar system. Considering the billions of stars in the
universe, it seems possible that planets with conditions similar
to those on Earth could be orbiting some stars. It also
seems possible that on some of these planets there may be
technological civilizations like our own.
The huge distances in space may prevent anyone from
traveling from one star system to another. However, radio
waves, which are relatively easy to generate, travel at the
speed of light. Large radio receivers, also known as radio telescopes,
can pick up very faint radio transmissions. Large
radio telescopes were built to explore the universe with long
wavelengths of electromagnetic radiation that are not visible
to our eyes. Radio waves can penetrate dust and clouds of gas
that prevent visual observations.
How would scientists know if they were receiving signals
from another civilization? It seems likely that the signals
would not be in a familiar language. If astronomers detect
patterns in radio transmissions that have no known sources
in natural sources, they may be listening to communications
from another civilization. Some of the largest radio telescopes
in the world have been used over the past several decades to
listen for intelligent transmissions.
At first, the job of analyzing these signals was a severe
limitation. How could scientists separate intelligent communications
from the great amount of radio noise generated by
stars? This is where computers came to the aid of scientists.
Computers can quickly analyze radio signals, looking for patterns.
The development of faster and more powerful computers
has enabled astronomers to scan far more observations
than humans could ever analyze. However, since SETI began
in about 1960 there have been no signals identified as likely
forms of intelligent communication.

What Would We Answer?
If scientists did detect intelligent communications, how could
it affect Earth? Perhaps the information in the data would
provide new insights into mathematics, science, or technology.
Perhaps people could learn more about the promise and
the dangers of a developing civilization. On the other hand,
there is always the danger of conquest by a distant power.
The future has always brought people into the unknown. Fortunately,
people have learned to apply discoveries to improve
their lives. Experience has taught that, in the long run, the
developments of science seem to benefit humans.
WHAT IS A STAR?
A star is a massive object in space that creates energy and radiates
it as electromagnetic radiation. The sun is a star. If you
compare the sun with the thousands of stars known to astronomers,
the sun appears to be a typical star. Actually, most
of the stars visible in the night sky are larger and brighter than
the sun. At the same distances as the visible stars, there are
more stars too dim to be visible from Earth. Observations of the
sun give astronomers insights into most other stars and their
observations of other stars help them understand the sun.
ACTIVITY 28-1 LIGHT INTENSITY AND DISTANCE
WHAT IS ASTAR? 711
Using a light meter that measures the intensity of light, you can
measure the change in the intensity with distance. Place a lightbulb
in a dark room. Measure the intensity of the light at different distances
from the lightbulb. Make a data table of the intensity of illumination
at various distances from the lightbulb. Graph your data.
What other factor shows a similar change of intensity with distance?

712 CHAPTER 28: STARS AND THE UNIVERSE
Starlight
For centuries, astronomers have wondered how the sun could
produce the great quantities of energy it radiates into space.
They knew that light sources on Earth produced light by
chemical changes such as combustion. Fuels such as wood and
coal rapidly combine with oxygen in the atmosphere to produce
heat and light. If the sun burned coal or wood to produce
energy, it would run out of fuel very quickly. As scientists became
aware of the age the solar system, it became clear that a
very different process was taking place in the sun.
Advances in science in the early twentieth century showed
that matter could be changed into energy.You may have heard
of Albert Einstein’s famous formula E mc 2, in which E is energy,
m is mass, and c is the speed of light. The speed of light
is a very large number. Therefore, the square of that number
is enormous. The point of this formula is that great quantities
of energy can be created by the loss of a small amount of mass.
In the nearly 5 billion years since the solar system originated,
it is estimated that the sun has only lost about one-third of 1
percent of its total mass.
Nuclear Fusion in Stars
Most of the mass of the sun is hydrogen, the lightest element.
When four hydrogen nuclei join to make a helium nucleus,
they lose about 1 percent of their mass. The process by which
light elements join to make heavier elements is called nuclear
fusion. (See Figure 28-1.) While 1 percent may seem
like a small loss of mass, it is enough to create a great amount
Figure 28-1 The sun generates
most of its energy by fusing
hydrogen to make helium
deep within the sun. The loss
of about 1 percent in mass
during this process creates
vast quantities of energy.

of energy. However, nuclear fusion can occur only under extreme
conditions of heat and pressure. In the last chapter,
you learned that Jupiter, the largest planet in our solar system,
is too small to have enough internal pressure to support
fusion.
ACTIVITY 28-2 MAKING LIGHT
Make a list of the methods you can use to create light energy in
an Earth science lab setting. This can be a competitive activity
among lab groups with one point awarded for each method
to create light energy and two points if you can safely demonstrate
it. As in any other laboratory procedures, your teacher must
approve all materials and methods you plan to use before you
try them. Duplication such as burning two different substances
counts as a single idea. Remember that you are looking for ways
to create light energy and not methods to bring in light from another
source like the sun.
Energy Escapes from Stars
WHAT IS ASTAR? 713
Once the energy is created deep in the sun, it moves to the
sun’s visible surface by radiation and convection. Convection
is the same process of heat flow by density currents that distributes
energy through Earth’s atmosphere and oceans.
Slow convection currents within Earth also carry heat energy
from Earth’s interior to the surface. From the solar surface,
the energy escapes as electromagnetic radiation. The surface
temperature of the star determines the kind of electromagnetic
energy it radiates into space. The sun is a yellow star
because its roughly 6000°C surface radiates most intensely
as yellow light in the visible part of the spectrum.
Based on observations of other stars, astronomers predict
that the sun will continue to radiate energy as it now does for
approximately another 5 billion years. The next section will
tell you about the evolution of stars such as the sun.

714 CHAPTER 28: STARS AND THE UNIVERSE
HOW ARE STARS CLASSIFIED?
In the early twentieth century, astronomers in Denmark and
the United States discovered that they could classify stars on
the basis of the amount of electromagnetic energy they generate
and their temperature. The total energy output of a star
is called its luminosity, or absolute brightness. Apparent
brightness, or stellar magnitude, is how bright the star looks
as seen from Earth. The closer a star is, the brighter it appears
to us.
A good example is the sun. The sun is actually a smaller
star, and gives off less light than most of the stars you see in
the night sky. However, the sun is so close to Earth that during
the day its light drowns out the light of the other stars. If
we could see the sun at the same distance as the nighttime
stars, it would be dimmer than most of them. Therefore, the
brightness of a star depends on its absolute magnitude, or luminosity,
and its distance from the observer.
You may have noticed that when you turn off an incandescent
lightbulb the color of the hot wire briefly changes to
red before it goes dark. Red is the coolest color of light visible
to our eyes. If a material is heated beyond red-hot, it becomes
white and then blue. Continued heating would push the radiation
into the ultraviolet part of the spectrum and beyond.
These forms of electromagnetic energy are not visible to us.
However, they can affect us in other ways. Sunburn is caused
primarily by ultraviolet light, which is a part of the spectrum
of sunlight. Figure 28-2 compares the sun’s spectrum with
the spectrum of light radiated by hotter (blue) and cooler
(red) stars.
Hertzsprung-Russell Diagram
The graph used to classify stars is often called the Hertzsprung-Russell,
or H-R, diagram in honor of the two men who
developed it. This graph is printed below from the Earth Sci-

Figure 28-3 When stars are
plotted on a graph according
to their energy output (luminosity)
and surface temperature
(which determines the
star’s color), most stars fall
into groups. Nine stars of
special significance () are
labeled by name.
Figure 28-2 The solar spectrum
illustrates why the sun is
classified as a yellow star. It
gives off its most intense radiation
in the middle of the visible
part of the spectrum. Blue
stars are hotter and stronger
in short-wave radiation. Red
stars are cooler, and they radiate
less energy per square
meter of surface area.
Intensity of Radiation
HOW ARE STARS CLASSIFIED? 715
Visible light
ence Reference Tables, where it is labeled “Luminosity and
Temperature of Stars.” (See Figure 28-3.) The graph is usually
plotted with the temperatures decreasing to the right
along the bottom axis. This is contrary to the way most
graphs are made. (Usually, values increase to the right as
well as upward on the vertical axis.) This graph is different
because it is usually shown the way astronomers originally
developed it.
Luminosity (Relative to the Sun)
1,000,000
Massive
Stars
10,000
100
1
Blue
Supergiants
Blue
star
BLUE LIGHT
YELLOW
Sun
RED LIGHT
Red
star
0 5
Wavelength (× 10
10
–5 cm)
Supergiants
Rigel
Betelgeuse
+ +
Main Sequence
+ Sirius
Polaris +
Red Giants
+ Aldebaran
+ Alpha Centauri
+
Sun
0.01
White Dwarfs
+ Procyon B
Red
Small
Stars
0.0001
20,000 10,000
Temperature (°C)
Dwarfs
Barnard's
Star +
5,000 2,500
Blue Stars White Stars
Color
Yellow Stars Red Stars

716 CHAPTER 28: STARS AND THE UNIVERSE
Main Sequence Stars
When plotted on this graph, most stars fall into distinct
groups. The greatest number of stars fall into an elongated
group that runs across the luminosity and temperature diagram
from the upper left to the lower right. This region of the
graph is known as the main sequence. The position of a star
along the main sequence is primarily a function of the mass
of the star.
RED DWARF STARS The smallest stars, such as Barnard’s Star,
are red dwarf stars, which are barely large enough to support
nuclear fusion. They are red in color because they are relatively
cool. These stars are so dim that even the relatively
close red dwarfs are difficult to see without a telescope. In
fact, about 80 percent of the night stars closest to Earth are
too dim to be visible to the unaided eye. This leads astronomers
to infer that red dwarf stars are more numerous
than all other groups of stars. However, we do not see them because
they are so dim.
Small stars last longer than larger stars. The lower temperature
and pressure in these stars allow them to conserve
hydrogen fuel and continue nuclear fusion much longer than
larger stars. The combination of small size and slow production
of energy makes them very dim.
BLUE SUPERGIANT STARS At the other end of the end of the
main sequence are the blue supergiants. These massive stars
do not last as long as the smaller stars. The extreme conditions
of temperature and pressure at the center of these stars
cause rapid depletion of their large quantities of hydrogen.
Some of them are a million times brighter than the sun. They
are also much hotter than the sun, giving them a blue color.
These largest stars are not nearly as common as the smaller
stars, in part because they burn out quickly. The most massive
stars last less than one-thousandth of the life of the sun.
Rigel, a bright star in the winter constellation Orion, is
10,000 times as luminous as the sun. The blue color of Rigel

is apparent if you compare it with Betelgeuse, another bright
star in Orion. Betelgeuse is a red giant star on the opposite
side of the same constellation.
Most other stars fall into one of the three groups on the
temperature-luminosity chart. White dwarfs, red giants, and
the supergiants are the most common star groups outside the
main sequence.
HOW DO STARS EVOLVE?
Different sizes of stars have different life cycles. However,
the evolution of stars can be illustrated by considering a star
about the size of the sun.
Birth of a Star
HOW DO STARS EVOLVE? 717
Star formation begins when a cloud of gas and dust (mostly
hydrogen) begins to draw together under the influence of
gravity. There are two sources of this material. Some of it is
hydrogen and helium left over from the formation of the universe
about 14 billion years ago. The rest is the debris from
the explosions of massive stars that formed earlier in the history
of the universe. This initial phase takes place over a period
on the order of 50 million years. (The process is faster for
larger stars and slower for smaller stars.)
As the material draws together, heat from the collapse of
the matter and from friction causes the temperature to increase
until there is enough heat and pressure to support nuclear
fusion. At this time, the star becomes easily visible since
it produces and radiates great quantities of energy. The condensation
process can be observed with binoculars or a small
telescope in the constellation Orion. Several young stars
below the belt of Orion can be seen shining through a giant
cloud of gas that surrounds them.

718 CHAPTER 28: STARS AND THE UNIVERSE
Middle Age
The star becomes less luminous after it fully condenses, and
it spends most of its life on the main sequence region of the
luminosity and temperature chart. (See Figure 28-3 on page
715.) Gravitational pressure balanced by heat from nuclear
fusion prevents the star from further shrinkage. This is the
longest and most stable phase of stellar evolution.
Death of an Average Star
After about 10 billion years, a star the size of the sun runs
low on hydrogen. Fusion slows, and the core of helium collapses,
causing the outer part of the star to expand quickly,
becoming a red giant. Fusion of helium and other heavier elements
replaces the hydrogen fusion process. The outer shell
of gases expands and cools in the red giant stage, leaving behind
a dense, hot core, which is a white dwarf star.
Death of a Massive Star
Stars with more than about 10 times the mass of the sun end
their period in the main sequence more violently. These stars
create a variety of heavier elements before they collapse. The
collapse process of larger stars generates so much energy
that these stars end their life in an explosion known as a supernova.
They briefly generate more energy than the billions
of stars that make up the whole galaxy. Most of the mass of
the star is blown into space. The core of the star may form an
extremely dense object called a neutron star. Some stars are
so massive that they form an object with gravity so strong
that not even light can escape. This is called a black hole.
Black holes cannot radiate energy, but they can be detected
because energy is given off by matter that falls into the black
hole. They can also be located by their gravitational effects on
other objects.

HOW DO ASTRONOMERS STUDY STARS?
Stars are extremely hot and have no solid surface. Scientists
can send instruments and cameras to land on Mars or
other solid objects. However, these methods cannot be used to
investigate stars. Any devices scientists build would melt and
probably vaporize long before reaching the visible surface of
a star. Furthermore, the night stars are too distant to reach
with spacecraft. With our present technology, it would take
tens or even hundreds of thousands of years for a spacecraft
to reach even the nearest star beyond the sun. Therefore,
most of the information astronomers have about stars comes
from light and other electromagnetic energy they radiate into
space.
Optical Telescopes
HOW DO ASTRONOMERS STUDY STARS? 719
Astronomers use telescopes to concentrate the light of stars.
Telescopes allow them to observe objects that are too dim to
be visible to unaided eyes. Some people think that the most
important feature of a telescope is how much it magnifies.
However, the stars are so distant that even the most powerful
telescopes show nearly all of them as points of light. When
an image is magnified, it will become dim, unclear, or fuzzy if
the object is too far away.
Other factors are more important than magnification in
telescope construction and use. The size, or diameter, of the
front lens (or light-gathering mirror) of the telescope determines
the dimmest object that can be observed. The farther
astronomers look into space, the dimmer the objects become.
The second factor is the quality of optics of the telescope.
If the lenses or mirrors that gather the light are not made
with great precision, magnified images will not be sharp.
Earth’s atmosphere is also a limiting factor. This is why
major observatories are built on high mountains, where the
atmosphere is thin and has less effect on the light. Figure
28-4 on page 720 shows several buildings containing large

720 CHAPTER 28: STARS AND THE UNIVERSE
telescopes on a mountaintop in Arizona. The Hubble Space
Telescope, which orbits Earth above the distorting effects of
Earth’s atmosphere, was a major step forward in observational
astronomy.
ACTIVITY 28-3 MAKING A TELESCOPE
You can construct a simple telescope using two convex lenses. A
tube to hold the lenses at the proper distance and alignment is
helpful but not essential. By using lenses with more or less curvature,
you can change magnification. Moving the lens that is
closest to your eye adjusts the focus.
Radio Telescopes
Figure 28-4 The telescopes
of the Kitt Peak Observatory
are located on a mountaintop
to reduce problems
associated with light passing
through the atmosphere.
The higher the observatory
and the farther it is from
atmospheric pollution and
artificial lights, the better the
quality of the observations
and images.
Some telescopes gather long-wavelength radio energy rather
than visible light. Radio telescopes, like those in Figure 28-5,
are not blocked by clouds of dust and gas in space that block
visible light. They are also useful in detecting objects that do
not produce radiation in the visible part of the electromagnetic
spectrum. Radio telescopes do not make sharp images,
and it is difficult to tell the exact positions of a radio source.
However, radio telescopes allow astronomers to make observations
that would not be possible with telescopes that work

Figure 28-5 Objects that do
not give off visible light can
be investigated with radio
telescopes. Radio signals
penetrate clouds of dust
and gas that block visible
light. They have been especially
useful in mapping the
Milky Way Galaxy.
in the visible part of the spectrum.
Other Telescopes
HOW DO ASTRONOMERS STUDY STARS? 721
Other kinds of telescopes allow astronomers to use electromagnetic
wavelengths shorter than visible light, such as X
rays and gamma rays. These instruments must be located in
orbit above Earth’s atmosphere, which filters out these forms
of radiation.
Technology has changed the ways astronomers use telescopes.
The first telescopes were used for direct observations.
If astronomers wanted a permanent record of their observations,
they had to draw by hand what they observed through
the telescope. Chemical photography enabled astronomers to
take pictures through their telescopes. Today, the more advanced
telescopes use electronic sensors like those in digital
cameras along with computers to create better quality images
than ever before possible.

722 CHAPTER 28: STARS AND THE UNIVERSE
Source of
White Light
Spectroscope
Glass
Prism
The spectroscope is one of the most important tools that astronomers
use. This instrument separates light into its component
colors (wavelengths), like the glass prism shown in
Figure 28-6. When starlight is passed through a spectroscope,
dark lines appear in certain parts of the spectrum. These
dark lines are produced when certain wavelengths of light
are absorbed by gaseous elements within the outer parts of
the star.
Each element has its own characteristic absorption lines.
Since stars are composed primarily of hydrogen and helium,
white light that passes through these elements shows dark
lines in the orange, yellow, green, and blue colors that characterize
hydrogen and helium. These spectral lines correspond
to the energy that electrons absorb when they move to
higher energy levels within the atoms. The atoms give off the
same colors when the electrons fall to lower or inner energy
levels. Each element has a unique set of energy levels. Therefore,
these “spectral fingerprints” allow astronomers to identify
the composition of distant stars.
ACTIVITY 28-4 MAKING A SPECTRUM
Long Wavelengths
Red
Orange
Yellow
Green
Blue
Violet
Short Wavelengths
Figure 28-6 When white
light passes through a glass
prism, the light separates into
the spectrum of colors, or
wavelengths, of which it is
composed.
You can separate sunlight into its spectrum with a glass prism. This
works best in a darkened room where windows face the sun.
Close the shades so that a narrow slit of direct sunlight enters the
room. Place the prism near the narrow opening that admits sunlight.
The prism will bend the light beam and separate it into its

colors. You may need to rotate the glass prism to project a visible
spectrum. The spectrum can be projected onto a sheet of white
paper. The stronger the light and the closer the paper is held to the
prism, the brighter the spectrum will be. To increase the size of
the spectrum, move the paper screen away from the prism. What
two changes in the spectrum do you observe as the paper screen
is moved away from the prism?
WHAT IS THE STRUCTURE OF THE UNIVERSE?
Early astronomers noticed fuzzy objects in the night sky.
They called these objects nebulae (singular nebula). The
word nebula comes from the Latin word for cloud. Unlike the
stars, these objects looked like dim fuzzy patches of light.
Nebulae and Galaxies
Telescopes revealed that some nebulae are regions of gas and
dust where stars are forming. In addition, some nebulae were
at greater distances than any known stars. Astronomers
eventually realized that some nebulae are huge groups of
stars held together by gravity. These objects are called galaxies.
The whole Andromeda galaxy is visible as a small, faint
patch of light high in the autumn sky. Powerful telescopes revealed
that the Andromeda galaxy, like thousands of other
galaxies, is a gigantic group of billions of stars. Galaxies are
separated by vast distances that contain relatively few stars.
Figure 28-7 on page 724 is a typical spiral galaxy.
The Milky Way
WHAT IS THE STRUCTURE OF THE UNIVERSE? 723
Astronomers realized that all the individual stars visible to
us in the night sky are a part of the group called the Milky
Way Galaxy. The sun and solar system are part of the Milky

724 CHAPTER 28: STARS AND THE UNIVERSE
Figure 28-7 Galaxy NGC 4414 is a typical spiral galaxy composed of billions of
stars. Both the Milky Way Galaxy and its relatively nearby twin, the Andromeda
Galaxy, are spiral galaxies.
Way Galaxy. This name came from observations of a faint,
white band of light that can be seen stretching across the sky
on very dark, moonless nights. (The Milky Way is not visible
in urban areas where light pollution prevents the night sky
from being dark enough to make it visible.) This broad band
is actually made of thousands of stars.
Radio telescopes enabled astronomers to map the Milky
Way Galaxy and estimate that it is composed of roughly 100
billion stars. Clouds of dust and gas that are also a part of our
galaxy obscure most of them. The center of our galaxy is located
in the direction of the summer constellation Sagittarius.
The shape of the Milky Way Galaxy, like the Andromeda
Galaxy, is a flattened spiral. The sun and solar system are located
about two-thirds of the way from the center to the outer
edge, as shown in Figure 28-8.
As stars orbit the core of the galaxy, inertia keeps gravity
from drawing them together. Orbiting the core of the galaxy

Sun
100,000 light-years
is an additional cyclic motion of Earth in space. Our planet
rotates on its axis in a 24-hour cycle. It also revolves around
the sun each year. The solar system revolves around the center
of the Milky Way Galaxy in about 220 million years. Although
this is a long time, the Milky Way Galaxy is so large
that this motion is actually about 10 times faster than
Earth’s revolution in its orbit around the sun.
Clusters and Superclusters
WHAT IS THE HISTORY OF THE UNIVERSE? 725
10,000 light-years
Figure 28-8 Earth and the solar system are located about two-thirds of the way
from the galactic center to the outer edge of the Milky Way.
The structure of the universe does not stop at galaxies. The
Milky Way and Andromeda galaxies are two of about 30
galaxies known as the local group. Astronomers are now
mapping superclusters of galaxies and even larger structures
of matter. Why the matter of the universe is so unevenly distributed
is one of the most important questions that astronomers
are investigating today.
WHAT IS THE HISTORY OF THE UNIVERSE?
When you look at very distant objects in the universe, you
are looking back in time. This is because light has a limited
speed. You learned in an earlier chapter that you could estimate
the distance to a lightning strike by counting the seconds
between seeing the flash and hearing the thunder. In
this procedure, you see the flash at essentially the same time
it occurred. Light travels so fast that it could circle Earth
about seven times in a single second.

726 CHAPTER 28: STARS AND THE UNIVERSE
Using Light as a Yardstick
Redshift
Distances in space are so vast that light cannot reach Earth
instantaneously. For example, to reach Earth, electromagnetic
energy takes about 3 seconds to travel from the moon
and 8 minutes from the sun. Light takes more than 4 years
to arrive from the nearest night star, Proxima Centauri. The
most distant object visible to the unaided eye is the Andromeda
Galaxy. Light from the Andromeda galaxy takes
about 2 million years to reach us.
In fact, light provides a good method to measure distances
in the universe. A light-year is the distance that
any form of electromagnetic energy can travel in 1 year:
about 6 trillion miles, or 10 trillion km. Although the light
year may sound like a measure of time, it is a measure of distance.
When astronomers look at distant objects in space, they
see them as the objects were when the light started its long
journey toward Earth. The farther away astronomers look
into space, the farther back in time they see. At present, the
most distance objects visible to astronomers are estimated
to be about 13 billion light-years away. Astronomers can now
look at the universe about a billion years after its origin,
which is estimated to be 14 billion years ago.
Astronomer Edwin Hubble examined the spectra of distant
galaxies in the early 1900s. He compared the dark absorption
lines, or spectral lines, of light from these far away galaxies
to the absorption lines of nearby stars. Nearby stars had
spectral lines similar to those produced in the laboratory. The
light from the distant galaxies did not show the dark lines in
the same colors as the light from the nearby stars. However,
the dark lines in the spectra of distant galaxies were shifted
toward the red end of the spectrum. Hubble reasoned that the
motion of distant galaxies away from Earth causes the redshift
of spectral lines. The redshift of spectral lines is illustrated
in Figure 28-9.

Figure 28-9 The sun and
nearby galaxies show spectral
lines similar to those produced
in a laboratory. However,
distant galaxies show
these characteristic lines
shifted toward the red and of
the spectrum. Astronomers
interpret this as evidence that
the universe is expanding.
Blue
WHAT IS THE HISTORY OF THE UNIVERSE? 727
Short Waves Long Waves
Blue
If the galaxies were moving toward Earth, the spectral
lines would shift toward the blue end of the spectrum. The
shift toward the red end of the spectrum indicates that the
galaxies are moving away from Earth.
You can observe a similar change with sound. If you stand
next to a racetrack, the high-pitched sound of the approaching
car changes to a lower pitch as the car speeds past you.
This apparent change in frequency and wavelength of energy
that occurs when the source of a wave is moving relative to
an observer is called the Doppler effect. It was named for
Christian Johann Doppler, the scientist who explained it in
1842. The change in the frequency and wavelength of sound
waves is similar to the changes that Hubble observed with
light. The greater the redshift, the faster the object is moving
away. Astronomers have found that the most distant galaxies
are moving away the fastest.
ACTIVITY 28-5 DEMONSTRATING THE DOPPLER EFFECT
This procedure should be done only under teacher or adult supervision.
This activity requires a noisemaker that can be tied to a
strong cord, or string, and swung around your head. A noisemaker
such as an alarm clock or a small battery-operated device from a
Red
Short Waves Long Waves
Blue
Reference
Wavelength
Red
Red
Short Waves Long Waves
Sun
Nearby
Galaxy
Distant
Galaxy

728 CHAPTER 28: STARS AND THE UNIVERSE
science supplier works well. Swing the noisemaker in a circle
around your head as people at a safe distance listen for changes
in the pitch of the sound. Can the person swinging the device also
hear the pitch of the sound change? For observers outside the circle,
what part of the swing best represents redshift, and what part
of the swing represents a “blueshift.”
Two other factors supported Hubble’s hypothesis of an expanding
universe. The redshift of light of distant galaxies
could be observed in all directions. In addition, the dimmer
galaxies, which are thought to be dim because they are farther
from Earth, showed greater redshift. Hubble explained
that the redshift is caused by motion of distant galaxies away
from Earth. He also reasoned that this kind of motion is a
characteristic of an explosion.
You might think that the motion of distant galaxies away
from Earth in all directions means that we are at the center
of the expansion. However, from any position within the expanding
matter of an explosion, matter is moving away in all
directions.
In the 1960s, Arno Penzias and Robert Wilson were working
on long-distance radio communications for the Bell Telephone
Company. They constructed a special outdoor receiver
to detect weak radio signals. However, the device picked up
annoying radio noise that they were not able to eliminate. As
they investigated the source of these radio waves, they realized
that the energy they were picking up was billions of
years old. They were actually listening to the origin of the
universe. These radio signals, known as cosmic background
radiation, are weak electromagnetic radiation left
over from the formation of the universe.
The Big Bang
The outward motions of distant galaxies and the cosmic background
radiation are evidence that the universe began as an
event now called the big bang. The name was first proposed
as a joke to make fun of the theory, but the name stuck. This

theory proposes that at the time of its origin, the universe
was a concentration of matter so dense that the laws of nature
as we know them today did not apply. This matter expanded
explosively, forming the universe. Even the most
extreme conditions that exist within the largest stars could
not compare with the beginning of the universe.
Experiments and the theories of physics show that the
greatest possible velocity for matter or energy is the speed of
light: about 300 million meters per second. Like the temperature
of absolute zero (0 K), this is one of the absolute limits
known to science. Astronomers reason that the universe is
expanding at this rate.
By working backward, astronomers estimate that the universe
began in the big bang 14 billion years ago. Expanding
outward in all directions, the universe today could be as much
as 28 billion light years across. This is so gigantic that it is
nearly impossible for humans to comprehend how large this is.
Earth, the solar system, and even the huge Milky Way Galaxy
are incredibly small compared with the size of the universe.
ACTIVITY 28-6 A MODEL OF THE BIG BANG
WHAT IS THE FUTURE OF THE UNIVERSE? 729
Inflate a round balloon into a small ball. Draw several small dots
on the balloon’s surface. Notice that as the balloon is inflated
more, the dots always move apart. If observers were located anywhere
on the surface of the balloon or even inside the balloon, as
the balloon is inflated, they would see the dots moving away in
all directions. No matter what location is chosen, it would appear
that the observer is at the center of the expansion. Therefore, there
is no way that astronomers can find the center of the universe.
WHAT IS THE FUTURE OF THE UNIVERSE?
From an astronomical point of view, Earth is in a reasonably
unchanging state. Earth’s orbit around the sun is stable and
scientists expect the sun to continue its energy production on

730 CHAPTER 28: STARS AND THE UNIVERSE
the main sequence for billions of years. Collisions with large
objects from space, such as those thought to mark the ends of
past geologic eras, are possible. Fortunately, these events are
becoming less likely as the age of the solar system increases.
However, the very-long-term future of the universe is not
clear.
Three Possible Futures
The universe seems to have three possible futures. Some scientists
propose that the expansion of the universe may be
slowing due to gravity. However, it is possible that there is
not enough gravity to stop the expansion. In this case, the
universe will continue to expand without limit. Other scientists
propose that there may be enough gravity to just stop
the expansion, leading to a steady state. If the universe has
enough gravity to reverse its expanding phase, it could fall
back together in a very distant event that some astronomers
call the “big crunch.”
A good way to understand this is to consider a baseball
thrown straight up. Gravity brings the ball back to the
ground. However, if you could propel the ball fast enough, it
would continue upward into space and never return to Earth.
In recent years, astronomers have found evidence that the
universe is not only expanding, but that it is expanding at an
increasing rate. What force could work against the force of
gravity to cause this? It is as if you threw a ball up into the
air and it did not fall back to Earth. In fact, it is as if the ball
flew upward faster and faster with time. This would be surprising,
indeed. Astronomers find these observations just as
surprising.
Astronomers have named the mysterious cause of this accelerating
expansion “dark energy.” However, they cannot explain
it. Nor can they explain the source of gravitational force
that holds the rapidly spinning galaxies from breaking apart.
This force is attributed to the gravitational attraction of
“dark matter,” which astronomers think makes up about 90
percent of the matter in the universe. Dark matter and dark
energy, the mysteries of science just keep coming.

TERMS TO KNOW
Therefore, the ultimate future of the universe depends
upon the balance between the expansion of the big bang,
gravity, and dark energy. To date, astronomers have not been
able to determine which process will dominate. This remains
one of many questions that guide scientific investigation.
big bang luminosity
cosmic background radiation Milky Way Galaxy
Doppler effect nuclear fusion
galaxy redshift
light-year star
CHAPTER REVIEW QUESTIONS
Base your answers to questions 1–4 on the Earth Science Reference Tables or
Figure 28-3.
1. Which star has about the same surface temperature as the sun?
(1) Betelgeuse (3) Sirius
(2) Polaris (4) Procyon B
2. Which star is cooler, yet many times brighter than Earth’s sun?
(1) Barnard’s Star (3) Rigel
(2) Betelgeuse (4) Sirius
3. According to the “Luminosity and Temperature of Stars” graph in the
Earth Science Reference Tables, the sun is classified as
(1) a main sequence star. (3) a blue supergiant.
(2) a white dwarf. (4) a red giant.
4. What is the color of a main sequence star that gives off about 100 times as
much light as the sun?
(1) blue (3) yellow
(2) white (4) red
CHAPTER REVIEW QUESTIONS 731

732 CHAPTER 28: STARS AND THE UNIVERSE
5. How do stars like the sun create energy that is later radiated away into
space?
(1) nuclear fusion changing hydrogen into helium
(2) burning of carbon fuels
(3) changes in state such as melting and evaporation
(4) absorbing electromagnetic radiation from space
6. What instrument uses long-wave electromagnetic radiation to help astronomers
make celestial observations?
(1) radio telescopes (3) X-ray telescopes
(2) optical telescopes (4) binoculars
7. According to the Earth Science Reference Tables, in what property do ultraviolet,
visible, and infrared radiation differ?
(1) half-life (3) wavelength
(2) atomic mass (4) wave velocity
8. The Milky Way Galaxy is best described as
(1) a type of solar system.
(2) a constellation visible to everyone on Earth.
(3) a region of space between the orbits of Mars and Jupiter.
(4) a spiral-shaped formation composed of billions of stars.
9. In which list are celestial features correctly shown in order of increasing
size?
(1) galaxy→solar system→universe→planet
(2) solar system→galaxy→planet→universe
(3) planet→solar system→galaxy→universe
(4) universe→galaxy→solar system→planet
10. What causes the spectral lines of light from distant galaxies to be shifted
toward the red end of the spectrum?
(1) the gravitational field of Earth
(2) the gravitational field of the sun
(3) motion of the galaxies toward us
(4) motion of the galaxies away from us

11. The diagram below illustrates three stages of a current theory of the formation
of the universe.
Stage 1 Stage 2 Stage 3 (present)
A ball of hydrogen
exploded
A huge hydrogen
cloud moved cutward
with cloud parts condensing
to form galaxies
A major piece of scientific evidence supporting this theory is the fact that
wavelengths of light from galaxies moving away from Earth in stage 3 are
observed to be
(1) shorter than normal (a redshift).
(2) shorter than normal (a blueshift).
(3) longer than normal (a redshift).
(4) longer than normal (a blueshift).
12. In the diagram below, the spectral lines of hydrogen gas from three galaxies,
A, B, and C, are compared to the spectral lines of hydrogen gas observed
in a laboratory.
Blue Red
Laboratory
Hydrogen
Spectral Lines
Earth
The galaxies continue
to move outward
Galaxy A
Spectral Lines
Galaxy B
Spectral Lines
Galaxy C
Spectral Lines
CHAPTER REVIEW QUESTIONS 733
Blue
Blue
Blue
Red
Red
Red

734 CHAPTER 28: STARS AND THE UNIVERSE
What is the best inference that can be made concerning the movement of
galaxies A, B, and C?
(1) Galaxy A is moving away from Earth, but galaxies B and C are moving
toward Earth.
(2) Galaxy B is moving away from Earth, but galaxies A and C are moving
toward Earth.
(3) Galaxies A, B, and C are all moving toward Earth.
(4) Galaxies A, B, and C are all moving away from Earth
13. Because of the Doppler redshift, the observed wavelengths of light from
distant celestial objects appear closer to the red end of the spectrum than
light from nearby celestial objects. The explanation for the redshift is that
the universe is presently
(1) contracting, only.
(2) expanding, only.
(3) remaining constant in size.
(4) alternating between contracting and expanding.
14. How can we best describe the general pattern of motion that we observe
for distant galaxies in the universe?
(1) Most galaxies are moving toward the Milky Way Galaxy, and the closer
galaxies are generally approaching faster.
(2) Most galaxies are moving toward the Milky Way Galaxy, and the more
distant galaxies are generally approaching faster.
(3) Most galaxies are moving away from the Milky Way Galaxy, and the
closer galaxies are generally moving faster.
(4) Most galaxies are moving away from the Milky Way Galaxy, and the
more distant galaxies are generally moving faster.
15. What could cause the expansion of the universe to slow?
(1) energy production by nuclear fusion
(2) energy production by nuclear fission
(3) gravitational force
(4) electromagnetic radiation

Open-Ended Questions
The graph below shows the inferred stages of development of the sun. Use this
graph to answer questions 16 and 17.
1,000,000
Luminosity
10,000
100
1
0.01
0.0001
Inferred Stages of Development
Sun
White Dwarf
stage
Dust
and
gases
20,000 10,000 5,000 2,500
Surface Temperature (°C)
CHAPTER REVIEW QUESTIONS 735
16. Describe the change in luminosity of the sun that will occur from its current
Main Sequence stage to its final White Dwarf stage.
17. Which star shown on the “Luminosity and Temperature of Stars” graph in
the Earth Science Reference Tables is currently in the sun’s final predicted
stage of development?
18. According to the “Luminosity and Temperature of Stars” graph in the
Earth Science Reference Tables, what is the surface temperature of the
sun?
19. According to the Earth Science Reference Tables, what kind of electromagnetic
radiation has a wavelength of about 1 meter (100 cm)?
20. Name one characteristic that X rays, visible light, and radio waves have in
common.