1. Star Systems & Galaxies
Chapter 15 Section 4
Star Systems & Galaxies

2. Star Systems Most stars are members of a group Binary Star: Two Stars
Alpha Centauri A & B
What is so cool about this star system?
Eclipsing Binary: Star eclipses
Remember: What’s an eclipse?

3. Star Clusters Open Clusters Globular Cluster Loose and disorganized
A few thousand stars
Globular Cluster
Round and densely packed
Very old stars
Millions of stars

4. Galaxies Spiral Galaxies 30 percent of all galaxies Disk-shaped
Large: 20, ,000 light-years
Contain both young and old stars
More stars at the center
Barred-spiral galaxies: Stars form a bar through the nucleus with arms extending outward

5. More Galaxies. . . Elliptical Galaxies 60 percent of galaxies
Can be round or oval
Small: Largest is 200,000 light-years
No arms
More 3D shape
Very little gas and dust

6. And More Galaxies. . . Irregular Galaxies
10 percent of the known galaxies
Irregular shape (Not spiral or elliptical)
Usually small 1, ,000 light years
No symmetry
Contain young stars
Were once spiral or elliptical, but gravity has deformed the shape

7. Milky Way Galaxy Large Spiral Galaxy 100,000 light-years wide
10,000 light-years thick
Three major arms

8. Milky Way Where is our sun? Sagittarius Arm Orion Spur
2/3 away from nucleus
30,000 light-years
Revolves around nucleus
Orbit = 200 million years

9. Quasar “quasi” stars – “like stars” Active young galaxies
Giant black hole centers
Extremely bright!
Very strong gravity

10. The Size of our Universe
How big are we compared to the universe?
How big is Earth?
The sun?
What’s bigger?

11. The Size of our Universe