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 MatterGravity 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 GalaxiesThere 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?
LESSON 4
CHAPTER 9
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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 GalaxiesGalaxies 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 UniverseMost 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 Check2. Locate Where is Earth in the Milky Way?
Key Concept Check3. Define What is the Big Bang theory?
Make a horizontal single-tab matchbook to describe the contents of the Milky Way.
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 ShiftYou 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 Check4. Contrast Describe how the sound waves in front of the car are different from the sound waves behind the car.
Car speeding away from observerLong wavelength, low frequency,low sound
Car speeding toward an observerShort wavelength, high frequency,high sound
Doppler Shift
Dark EnergyWill 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.
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 disagree. Did you change your mind?
What do you think
END OF LESSON
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ConnectED
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 out-ward from a thick central bulge
f. form from the gravitational pull of neighboring galaxies
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