KEY CONCEPT
The universe is
expanding.
Sunshine State
STANDARDS
SC.H.1.3.1: The student
knows that scientific
knowledge is subject
to modifications as
new information challenges prevailing
theories and as a new
theory leads to looking at old observations
in a new way.
BEFORE, you learned
NOW, you will learn
• Galaxies contain millions or
billions of stars
• Electromagnetic radiation carries information about space
• How galaxies are moving apart
in the universe
• What scientists are discovering
about the development of
the universe
EXPLORE Large Numbers
How much is a billion?
VOCABULARY
Doppler effect p. 800
big bang p. 802
PROCEDURE
1
MATERIALS
Guess how thick a billion-page book would
be. Write down your guess.
• book
• ruler
• calculator
2 Count how many sheets of paper in a book
add up to a millimeter in thickness. Multiply
by 2 to calculate the number of pages.
3 Then divide 1 billion (1,000,000,000) by
that number to determine how many millimeters
thick the book would be. Divide your result
by 1,000,000 to convert to kilometers.
WHAT DO YOU THINK?
• How thick would a billion-page book be?
• How close was your guess?
Galaxies are moving farther apart in
the universe.
COMBINATION NOTES
You could record information about the expansion
of the universe in a combination notes table.
The universe is unbelievably huge. It consists of all space, energy, and
matter. The Milky Way is just one of about 100 billion galaxies. These
galaxies occur in groups that together form superclusters. Between the
superclusters are huge areas of nearly empty space.
Because the universe is so huge, you might think that the most
distant regions of the universe are very different from space near Earth.
However, by looking at the spectra of light from stars and galaxies,
astronomers have determined that the same elements are found
throughout the universe. Scientific observations also indicate that the
same physical forces and processes operate everywhere.
Chapter 22: Stars, Galaxies, and the Universe 799
Looking Back in Time
When we look far out into space, we see galaxies by the light they
gave off long ago. This light has traveled millions or even billions of
years before reaching telescopes on Earth. The
Andromeda Galaxy, for example, is the closest
large galaxy. The light of its stars takes over 2
million years to reach Earth. When we view this
galaxy through a telescope, we are seeing what
happened in it 2 million years ago. To see what
is happening there now, we would have to wait
2 million years for the light to arrive.
Light from the
Andromeda Galaxy
takes 2 million years
to reach Earth.
As astronomers look at galaxies farther and
farther away, they see how the universe looked
at different times in the past. These views are like
photographs in an album that show someone
at various stages of life. Astronomers can see how the universe has
developed over billions of years.
Check Your Reading
Why can astronomers learn about the past by looking
at distant galaxies?
The Motion of Galaxies
Have you ever noticed that the sound of an ambulance siren changes as
it travels toward and then away from you? The pitch of the siren seems
to be higher as the ambulance approaches. As the ambulance passes
you and starts moving away, the pitch of the siren seems to get lower.
The shifting pitch of the siren is an example of the Doppler effect,
which is a change in the observed wavelength or frequency of a wave
that occurs when the source of the wave or the observer is moving.
The Doppler effect occurs with light as well as sound. If a galaxy is
moving toward Earth, the light we receive will seem compressed to
shorter wavelengths. This change is called a blue shift because the light
shifts toward the blue end of the spectrum. If a galaxy is moving away
from Earth, the light we receive will seem stretched to longer wavelengths. This change is called a red shift because the light shifts toward
the red end of the spectrum.
In the early 1900s, astronomers discovered that light from distant
galaxies is stretched to longer wavelengths. This fact indicates that
the galaxies are moving apart. By analyzing the spectra of galaxies,
astronomers also discovered that the galaxies are moving apart faster the
farther away they are. These observations led astronomers to conclude
that the universe has been expanding throughout its history.
800 Unit 6: Space Science
Evidence of an Expanding Universe
The Doppler effect can show how galaxies are moving in relation to Earth.
moving away
moving toward
Earth
Light from a galaxy
moving away from Earth
will seem stretched to
longer wavelengths.
Light from a galaxy
moving toward Earth
will seem compressed to
shorter wavelengths.
What do the arrows on the light waves indicate?
The illustration of raisin-bread dough rising will help you imagine
this expansion. Suppose you were a raisin. You would observe that all
the other raisins are moving away from you as the dough expands.
The raisins are being moved apart by the expanding dough. Furthermore, you would observe that distant raisins are moving away faster
than nearby raisins. They move away faster because there is more
dough expanding between you and those raisins.
As the dough rises, the raisins are
pushed farther apart from each other.
The more distance there is between
raisins, the faster and farther they
move apart.
8 cm
4 cm
4 cm
2 cm
before dough rises
after dough rises
Like the dough that expands and moves raisins apart, space in the
universe is expanding and moving galaxies apart. The universe does
not expand into anything, since there is nothing outside the universe.
Rather, the universe itself is expanding.
Check Your Reading
How are galaxies moving in relation to each other?
Chapter 22: Stars, Galaxies, and the Universe 801
Galaxies
How does the universe expand?
SKILL FOCUS
Measuring
PROCEDURE
1
Spread the cut rubber band against the ruler without stretching it.
Mark off every centimeter for 6 centimeters.
2 Align the first mark on the rubber band with the 1-centimeter mark on the
ruler and hold it in place tightly. Stretch the rubber band so that the second
mark is next to the 3-centimeter mark on the ruler.
3 Observe how many centimeters each mark has moved from its original
location against the ruler.
MATERIALS
• thick rubber
band cut open
• ballpoint pen
• ruler
TIME
20 minutes
WHAT DO YOU THINK?
• How far did each mark on the rubber band move from its
original location?
• What does this activity demonstrate about the
expansion of the universe?
CHALLENGE How could you calculate the rates at
which the marks moved when you stretched the
rubber band?
Scientists are investigating the origin of
the universe.
After astronomers learned that galaxies are moving apart, they developed new ideas about the origin of the universe. They concluded that
all matter was once merged together and then the universe suddenly
began to expand. The evidence for this scientific theory is so strong
that almost all astronomers now accept it.
VOCABULARY
Add a description wheel
for big bang in your
notebook.
The big bang is the moment in time when the universe started
to expand out of an extremely hot, dense state. Astronomers have
calculated that this event happened about 14 billion years ago. The
expansion was very rapid. In a tiny fraction of a second, the universe
may have expanded from a size much smaller than a speck of dust to
the size of our solar system.
Evidence of the Big Bang
Evidence for the big bang comes from various sources. One important
source of evidence is microwave radiation. Astronomers predicted in
1948 that the universe would still be filled with microwaves emitted
shortly after the big bang. In 1965 researchers detected this kind of
radiation streaming through space in all directions.
802 Unit 6: Space Science
Besides the presence of microwave radiation and the motions
of galaxies, scientists have found other evidence of the big bang by
observing space. For example, images of very distant galaxies provide
information about the universe’s development. Additional evidence
of the big bang has come from experiments and computer models.
Development of the Universe
Immediately after the big bang, the universe was incredibly dense and
hot—much hotter than the core of the Sun. Matter and energy
behaved very differently than they do under
present conditions. As the universe rapidly
expanded, it went through a series of changes.
Scientists do not fully understand what conditions were like in the early universe. However,
they are gaining a clearer picture of how the
universe developed. One way that scientists are
learning about this development is by performing
experiments in particle accelerators. These huge
machines expose matter to extreme conditions.
Scientists have found that the earliest stages
in the universe’s development occurred in a tiny
fraction of a second. However, it took about
300,000 years for the first elements to form. Stars,
planets, and galaxies began to appear within the next billion years.
Some evidence suggests that the first stars formed only a few hundred
million years after the big bang.
Check Your Reading
What happened to the universe shortly after the big bang?
KEY CONCEPTS
CRITICAL THINKING
1. How are distant regions of the
universe similar to space near
Earth?
4. Apply If a star 100 light-years
from Earth is beginning to
expand into a giant star,
how long will it take for
astronomers to observe this
development? Explain.
2. What does the Doppler effect
indicate about the motion of
galaxies?
3. How do scientists explain the
origin of the universe?
This Hubble telescope
image of very distant
galaxies has helped
scientists learn what the
universe was like about
13 billion years ago.
CHALLENGE
5. Analyze Why do scientists
need to perform experiments
to learn about the earliest
stages of the universe?
6. Infer Galaxy A and galaxy B
both give off light that appears
stretched to longer wavelengths. The light from galaxy
B is stretched to even longer
wavelengths than the light
from galaxy A. What can you
infer from these data?
Chapter 22: Stars, Galaxies, and the Universe 803