The Universe Section 2

Section 2: The Milky Way and Other Galaxies

Preview• Key Ideas • Bellringer • Galaxies • Types of Galaxies• How Galaxies Change Over Time

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Key Ideas

〉What is a galaxy and what is it made of?

〉Why are galaxies divided into three major types?

〉How do scientists know that galaxies change over time?

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BellringerBased on what you have learned, as well as previous knowledge, answer the following questions.

1. What types of objects would you expect to find within a galaxy?

2. Does the same force that keeps our solar system together keep galaxies together?Explain your answer.

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Bellringer, continued

3. The word galaxy comes from the Greek word for “milk,” and the star-rich region of our own galaxy has long been called the Milky Way. Other galaxies, before they were recognized as containing stars, were called nebulas, from the Latin word for “clouds.”What do you think was the reason these starry regions were described in these terms?

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Galaxies〉What is a galaxy and what is it made of?

〉A galaxy is a collection of millions to billions of stars.

• galaxy: a collection of stars, dust, and gas bound together by gravity

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Galaxies

• Galaxies contain millions or billions of stars.– Because stars age at different rates, a galaxy may

contain many types of stars.

• Gravity holds galaxies together.– Without gravity, everything in space might be a veil of

gas spread out through space.– Just as Earth revolves around the sun, our solar

system revolves around the center of our galaxy.

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Galaxies, continued

• Gravity holds galaxies together in clusters.– Galaxies are not spread evenly throughout space.– The Milky Way galaxy and the Andromeda galaxy are

two of the largest members of the Local Group, a cluster of more than 30 galaxies.

– Clusters of galaxies can form even larger groups, called superclusters.

• cluster: a group of stars or galaxies bound by gravity

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Visual Concept: Contents of Galaxies

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Types of Galaxies〉Why are galaxies divided into three major types?

〉The three types of galaxies have many stars, but differ in structure.

• We live in the Milky Way galaxy.– Most of the objects visible in the night sky are part of

the Milky Way galaxy.– Scientists use astronomical data to piece together a

picture of the Milky Way galaxy.

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Types of Galaxies, continued

• The Milky Way is a spiral galaxy.– Our galaxy is a huge spiraling disk of stars, gas, and

dust, with several distinct spiral arms.– Our solar system is located within a spiral arm.– The nucleus of the galaxy is dense and has many old

stars.– The gas and dust located between stars is called

interstellar matter.

• interstellar matter: the gas and dust located between the stars in a galaxy

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Types of Galaxies, continued

• Elliptical galaxies have no spiral arms.– Elliptical galaxies are spherical or egg shaped.– They contain mostly older stars and have little

interstellar matter.– Because older stars are red, elliptical galaxies often

have a reddish color.

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Types of Galaxies, continued

• All other galaxies are irregular galaxies.– Irregular galaxies lack regular shapes and do not

have a well-defined structure.– Some irregular galaxies may be oddly shaped

because the gravitational influence of nearby galaxies distorts their spiral arms.

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How Galaxies Change Over Time

〉How do scientists know that galaxies change over time?

〉Because the light from ancient galaxies takes so long to reach Earth, scientists do not know what far-away galaxies look like currently. By studying closer galaxies that might be similar to ancient ones, they can slowly piece together the puzzle of how galaxies evolve.

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How Galaxies Change Over Time, continued

• Quasars may be infant galaxies.– In 1960, a faint object was matched with a strong

radio signal. This object was called a quasar.– Each quasar has a huge central black hole and a

large disk of gas and dust around it.

• quasar: quasi-stellar radio sources; very luminous objects that produce energy at a high rate and that are thought to be the most distant objects in the universe

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Visual Concept: Quasar

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• Galaxies change over time.– Galaxies change as they use up their stores of gas

and dust.– Galaxies also change as a result of collisions. – As galaxies approach each other, mutual gravitational

attraction changes their shape.– Collisions of gas and dust may cause new stars to

begin forming.

How Galaxies Change Over Time, continued