1. The Milky Way II What do halo stars tell us about our galaxy’s history? How did our galaxy form?

2. What do halo stars tell us about our galaxy’s history?

3. Halo Stars: 0.02–0.2% heavy elements (O, Fe, …), only old stars Disk Stars: 2% heavy elements, stars of all ages

4. Halo Stars: 0.02–0.2% heavy elements (O, Fe, …), only old stars Disk Stars: 2% heavy elements, stars of all ages Halo stars formed first, then stopped. Disk stars formed later, and kept forming.

5. Halo Stars: 0.02–0.2% heavy elements (O, Fe, …), only old stars Disk Stars: 2% heavy elements, stars of all ages Halo stars formed first, then stopped. Disk stars formed later, and kept forming. Disk Population: Spheroidal Population

6. How did our galaxy form?

7. Our galaxy probably formed from a giant gas cloud.

8. Halo stars formed first (where the cloud became very dense) as gravity caused the cloud to contract.

9. The remaining gas settled into a spinning disk.

10. Stars continuously form in the disk as the galaxy grows older.

11. Warning: This model is oversimplified.

12. Detailed studies: Halo stars formed in clumps that later merged.

13. The Galactic Center

14. What lies in the center of our galaxy?

15. Infrared light from centerRadio emission from center

16. Swirling gas near center

17. Orbiting stars near center

18. Stars appear to be orbiting something massive but invisible … a black hole? Orbits of stars indicate a mass of about 4 million M sun.

19. X-ray flares from galactic center suggest that tidal forces of suspected black hole occasionally tear apart chunks of matter about to fall in.