1. Galaxies

2. It has been said that a million galaxies are found in the part of the sky bounded by the cup of the Big Dipper!

3. So, what is a galaxy?
It is a giant structure that contains hundreds of billions of stars.

4. Galaxies have been classified in four groups.
Spiral
Barred spiral
Elliptical
Irregular

5. Spiral galaxies Disk shaped with more stars near the center
Have arms that extend from the center
Stars in arms rotate slowly
Usually very large
light years across
20% of all galaxies are spirals
Mix of old and young stars,
make new stars in arms

9. Barred Spiral galaxies
Stars arranged in the shape of a bar which rotates together
Outer stars rotate faster that inner ones
10% of all galaxies are barred spirals

11. The Milky Way is a barred spiral galaxy
light years wide
Disk is light years thick in the center
More than three arms
The Sun is on the Orion arm
Orion arm takes 200 million years to rotate once.

13. Elliptical Galaxies Round, football, or squished football in shape
Most are small or dwarf in size
60% of all galaxies are elliptical
No spiral arms
Contain mostly old stars with very little gas and dust, no ingredients to make new stars

16. Irregular galaxies
Galaxies that have no regular shape and do not look like ellipticals or spirals
Large and Small Magellanic Clouds are examples
10% of all galaxies are irregular
Composed of bright young stars, and many recent bursts of star formation

18. Galactic Clusters
We now know that galaxies, like stars, are grouped. We call the groups galactic clusters.
Our group is called the Local Group and contains at least 28 galaxies, 3 spirals, 11 irregulars and 14 ellipticals

19. Superclusters
We call groups of galactic clusters superclusters!