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Lesson Outline for Teaching
Lesson 4: Galaxies and the Universe
A. Galaxies
1. Huge collections of stars are called galaxies, each of which can contain hundreds of
billions of stars.
2. The force that holds stars and galaxies together is gravity.
a. Most matter in galaxies is dark matter, which emits no light at any wavelength.
b. More than 90 percent of the universe’s mass is thought to be dark matter, but
scientists do not know what type of material it contains.
3. The three main kinds of galaxies are spiral, elliptical, and irregular galaxies.
a. A(n) spiral galaxy has a bulging central disk of stars with spiral arms that radiate
from the central disk.
b. A(n) elliptical galaxy can have the shape of a basketball or a football but lack
internal structure.
c. A(n) irregular galaxy does not have a clear shape or structure.
4. Gravity pulls galaxies together in groups called clusters.
a. Clusters clump into larger groups called superclusters.
b. Regions of empty space between the superclusters cause the universe to have a
structure like a sponge.
B. The Milky Way
2. The Milky Way is a(n) spiral galaxy that contains gas, dust, and almost 200 billion
3. The Milky Way is part of a(n) cluster of about 30 galaxies called the Local Group.
4. Earth and the other planets of our solar system are located in a spiral arm of the
Milky Way.
C. The Big Bang Theory
1. By looking into space, astronomers can look back in time.
2. Based on the Big Bang theory, the universe began from one point billions of years
ago, and it has been expanding since then.
Stars and Galaxies
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
1. Our planet, Earth, is part of a(n) solar system that is in the Milky Way galaxy.
Lesson Outline continued
3. Scientists think that the universe began 13 to 14 billion years ago as a dense, hot
form that was too hot even for atoms to exist.
a. When the universe cooled, stars began to form and gravity pulled the stars into
b. The universe is expanding, and galaxies are moving away from one another.
c. Scientists can measure the speed at which galaxies move away from Earth by
observing wavelengths of light.
4. Sound waves and light waves compress when the source emitting those waves moves
toward you. In contrast, sound waves and light waves spread out when the source
emitting those waves moves away from you.
a. When a wavelength shifts to a different wavelength, the shift is called the
Doppler shift.
b. In our expanding universe, galaxies are moving away from us, and the light coming
from those galaxies is red-shifted.
c. The farther a galaxy is from us, the faster the galaxy is moving away from us and
the greater the shift is toward the red end of the spectrum.
5. Scientists have observed that, over time, galaxies are moving away from Earth at an
increasingly faster pace.
6. Scientists are researching a force called dark energy that could be pushing galaxies
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Discussion Question
Compare and the contrast the three main types of galaxies. What similarities do they share?
How is each different from the others?
All galaxies are formed and affected by gravity, and all contain billions of stars. The three most
common kinds of galaxies have different shapes. Only spiral galaxies have an internal
structure. Elliptical galaxies have more old stars than the other types of galaxies, and irregular
galaxies contain more young stars than the other types of galaxies. In comparison with the
other types of galaxies, elliptical galaxies have very little gas and dust.
Stars and Galaxies