1. 25.3 The Universe

2. Vocabulary

3. Galaxy A group of stars, dust, and gases held together by gravity.

4. Galaxy Cluster A system of galaxies containing from several to thousands of member galaxies.

5. Hubble’s Law A law that states that the galaxies are retreating from the Milky Way at a speed that is proportional to their distance.

6. Big Bang Theory The theory that proposes that the universe originated as a single mass, which subsequently exploded.

7. Key Concepts

8. The Milky Way is a large spiral whose disk is about 100,000 light-years wide and about 10,000 light-years thick at the nucleus.

9. In addition to shape and size, one of the major differences among different types of galaxies is the age of their stars.

10. The red shifts of distant galaxies that the universe is expanding.

11. The big bang theory states that at one time, the entire universe was confined to a dense, hot, supermasentire ball. Then about 13.7 billion years ago, a violent explosion occurred, hurling this material in all directions.