Chapter 21

1. Chapter 21 21.4 Star Systems and Galaxies

2. Double and Triple Stars • Most solar systems have more than one star. • Solar systems with 2 stars are called binary systems. Solar systems with 3 stars are called triple stars • The stars orbit each other, and when one star passes in front of another star it is called an eclipsing binary system • The effects of gravity often clue astronomers to the presence of the 2nd star by a distinct wobble the star has when it orbits.

3. Extrasolar planets • The pull of gravity is one of the ways that astronomers have used to discover planets orbiting other stars. A different more subtle wobble indicates the presence of planets. • More than 400 planets have been found orbiting other stars. • Technology is not good enough to detect Earth like planets yet, but it is getting better all the time.

4. Galaxies • A galaxy is a collection of stars, dust, and gas that usually orbits a supermassive black hole and contains billions of stars. • There are 3 kinds of galaxies: • Spiral • Elliptical • Irregular

5. Spiral Galaxies • Have a pinwheel shape. Our galaxy, the Milky Way, is a spiral galaxy

6. Elliptical Galaxies • They are older galaxies that lack a lot of dust and gas. • Star formation has virtually ceased as a result.

7. Irregular Galaxies • Galaxies that have odd shapes are called irregular galaxies. • The odd shapes come from collisions with other galaxies.