LESSON 4 Stars and Galaxies...percentages of old, red stars than spiral galaxies do. They contain...

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GalaxiesMost stars exist in galaxies. Galaxies are huge collections of

stars. The universe has hundreds of billions of galaxies. Each galaxy can contain hundreds of billions of stars.

Dark Matter

Gravity holds stars together. Gravity also holds galaxies together. Astronomers study how galaxies rotate and how gravity affects their movement. They have found that most matter in galaxies is invisible. Matter that emits no light at any wavelength is dark matter.

Scientists hypothesize that more than 90 percent of the universe’s mass is dark matter. Scientists do not know what material this invisible dark matter contains.

Types of Galaxies

There are three major types of galaxies. Galaxies can be spiral, elliptical, or irregular.

Spiral Galaxies The stars, gas, and dust in a spiral galaxy exist in spiral arms that begin at a central disk. Some spiral arms are long and symmetrical. Others are short and stubby. Spiral galaxies are thicker near the center, a region called the central bulge. A spherical halo of globular clusters and older, redder stars surrounds the disk.

What do you think? Read the two statements below and decide

whether you agree or disagree with them. Place an A in the Before

column if you agree with the statement or a D if you disagree. After

you’ve read this lesson, reread the statements to see if you have

changed your mind.

Before Statement After

7. Most of the mass in the universe is in stars.

8. The Big Bang theory is an explanation of the

beginning of the universe.

Key Concepts

• What are the major types

of galaxies?

• What is the Milky Way,

and how is it related to

the solar system?

• What is the Big Bang

theory?

Identify the Main Ideas To

help you learn about

galaxies and the universe,

highlight each heading in

one color. Then highlight

the details that support and

explain it in a different

color. Refer to this

highlighted text as you

study the lesson.

Key Concept

Check

1. Name What are the

major types of galaxies?

Stars and Galaxies

Galaxies and the Universe

LESSON 4

CHAPTER 20

646 Statrs and Galaxies Galaxies and the Universe

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Elliptical Galaxies Unlike spiral galaxies, elliptical galaxies do not have an internal structure. Some are spheres, like basketballs. Others are shaped like footballs. Elliptical galaxies have higher percentages of old, red stars than spiral galaxies do. They contain little or no gas and dust. Scientists hypothesize that many elliptical galaxies form as the gravity of two or more spiral galaxies comes together.

Irregular Galaxies Galaxies that are oddly shaped are called irregular galaxies. Many form from the gravitational pull of neighboring galaxies. Irregular galaxies contain many young stars and have areas of intense star formation.

Groups of Galaxies

Galaxies are not distributed evenly in the universe. Gravity holds them together in groups called clusters. Some clusters of galaxies are enormous. The Virgo Cluster contains about 2,000 galaxies.

Most clusters exist in even larger structures called superclusters. Between superclusters are voids, which are regions of nearly empty space. Scientists hypothesize that the large-scale structure of the universe resembles a sponge.

The Milky WayThe Milky Way is a spiral galaxy that contains gas, dust, and

almost 200 billion stars. The Sun, Earth, and the rest of our solar system is inside the disk of the Milky Way. The Milky Way is part of the Local Group, a cluster of about 30 galaxies.

The Andromeda Galaxy is the largest galaxy in the Local Group. Scientists expect the Milky Way will begin to merge with the Andromeda Galaxy in about 3 billion years. Because stars are far apart in galaxies, few stars are likely to collide during this event.

The Big Bang TheoryIs there a beginning to time? According to the Big Bang

theory, the universe began from one point billions of years ago and has been expanding ever since.

Origin and Expansion of the Universe

Most scientists agree that the universe is 13–14 billion years old. The universe was so dense and hot at first that even atoms didn’t exist. After a few hundred thousand years, the universe cooled enough for atoms to form. Eventually, stars formed, and gravity pulled them into galaxies.

Key Concept

Check

2. Locate Where is Earth

in the Milky Way?

Key Concept

Check

3. Define What is the Big

Bang theory?

Make a horizontal single-tab

matchbook to describe the

contents of the Milky Way.

Galaxies and the Universe Statrs and Galaxies 647


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Stretching Space As the universe expands, space stretches and galaxies move away from one another. The same thing happens in a loaf of unbaked raisin bread. As the dough rises, the raisins move apart. Scientists observe how space stretches by measuring the speed at which galaxies move away from Earth. As the galaxies move away, their wavelengths lengthen and stretch out. How does light stretch?

Doppler Shift

You have probably heard the siren of a speeding police car. As the figure below illustrates, when the car moves toward you, the sound waves compress. As the car moves away, the sound waves spread out. Similarly, when visible light travels toward you, its wavelength compresses. When light travels away from you, its wavelength stretches out. It shifts to the red end of the visual light path of the electromagnetic spectrum. The shift to a different wavelength is called the Doppler shift. Because the universe is expanding, light from galaxies is red-shifted. The more distant a galaxy is, the faster it moves away from Earth and the more it is red-shifted.

Visual Check

4. Contrast Describe

how the sound waves in

front of the car are different

from the sound waves

behind the car.

Doppler Shift

Dark Energy

Will the universe expand forever? Or will gravity cause the universe to contract? Scientists have observed that galaxies are moving away from Earth faster over time. To explain this, they suggest that a force called dark energy is pushing the galaxies apart.

Dark energy, like dark matter, is an active area of research. There is still much to learn about the universe and all that it contains.

648 Statrs and Galaxies Galaxies and the Universe


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Mini Glossary

Big Bang theory: the theory that the universe began from one point billions of years ago and has been expanding ever since

dark matter: matter that emits no light at any wavelength

Doppler shift: the shift to a different wavelength

galaxy: a huge collection of stars

1. Review the terms and their definitions in the Mini Glossary. Write a sentence that explains why the light from galaxies has a Doppler shift toward the red end of the electromagnetic spectrum.

2. The information below describes galaxies. Match each letter and its information to the type of galaxy it describes. Write the letter in the box.

3. What is one question that research on dark energy might help scientists answer?

a. many young stars and areas of intense star formation

b. spherical halo of globular clusters and older, redder stars

c. probably form when the gravity of different spiral galaxies merges

d. little or no gas and dust and no internal structure

e. arms of stars, gas, and dust spiral outward from a thick central bulge

f. form from the gravitational pull of neighboring galaxies

elliptical galaxies: spiral galaxies:irregular galaxies:

Types of Galaxies

Reread the statements at the beginning of the

lesson. Fill in the After column with an A if you

agree with the statement or a D if you dis-

agree. Did you change your mind?

What do you think

END OF LESSON

Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson’s resources.

Galaxies and the Universe Statrs and Galaxies 649


650 Stars and Galaxies

Lesson 1 The View from EarthC

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Predict three facts that will be discussed in Lesson 4 after reading the headings. Record your predictions in your Science Journal.

Galaxies

I found this on page .

Relate the number of stars in galaxies and the universe.

Galaxy Universe

hundreds of billions of hundreds of billions of

Contrast scientific knowledge about dark matter.

Dark Matter

Scientists

know that…

Scientists

hypothesize

that…

Scientists don’t

know…

Categorize details about types of galaxies. List at least four details about each type.

Spiral Elliptical Irregular

•

•

•

•

•

•

•

•

•

•

•

•



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Stars and Galaxies 651

Lesson 4 | Galaxies and the Universe (continued)

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Explain why scientists liken the large-scale structure of the universe to a sponge.

Characterize the Milky Way.

Milky

Way

made of includes our

member of will probably

type of galaxy:

Illustrate the Milky Way, and describe Earth’s position in it.

Earth’s position:

Restate the Big Bang theory.

The Milky Way




The Big Bang Theory



652 Stars and Galaxies

Lesson 4 | Galaxies and the Universe (continued)

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Sequence the expansion of the universe.

13 to 14 billion

years ago, the

universe was

dense and hot.

It cooled for

.

formed.

Galaxies move

farther apart as

the universe

expands.

Gravity pulled

.

formed.

Differentiate sound waves in the Doppler shift.

To an observer in front of the car’s motion:

To an observer behind the car:

Relate dark energy to the rate the universe is expanding.

Scientists theorize that dark energy is pushing galaxies apart

because .



Predict what will happen to the Sun, the solar system, and the Milky Way in billions of years.



LESSON 4 Stars and Galaxies...percentages of old, red stars than spiral galaxies do. They contain...

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