1. Galaxies
Chapter 30
Section 3

2. What is a Galaxy?
A collection of stars, gas and dust held together by gravity.
Major building block of the universe.
Estimated that there are hundreds of billions of galaxies in our universe.

3. Size/Distances to Galaxies
Astronomers use stars detected within galaxies to estimate the distance.
Size ranges from a few thousand light years across to several hundred thousand light years across.
Example the Milky way is about 100,000 light years across.
Equal to 20 million solar systems.

4. Types of Galaxies Classified by Shape Spiral
The Milky Way is a large spiral galaxy.
Composed of a nucleus of bright stars
Arms spin around the nucleus
Contain young stars, gas, and dust

5. Types Continued Elliptical Galaxies
Vary in shape from nearly spherical to very elongated
Very Very Bright center
Do not have spiral arms
Contain little dust and gas
Few young stars

6. Irregular Galaxy Has no particular shape Total mass is small
Contain a large amount of gas and dust
Very few are observed