1. Chapter 19 Our Galaxy
All-Sky View

2. The Milky Way galaxy appears in our sky as a faint band of light
Dusty gas clouds obscure our view because they absorb visible light

3. Edge-on view, primary features: disk, bulge, halo, globular clusters

4. From above the Milky Way has spiral arms

5. We’re about 28,000 ly from center, in one of the spiral arms

6. Stars in disk all orbit in same direction (with a little up & down motion)
Stars in the bulge and halo have random orbits

7. Star-gas-star cycle Stars make new elements by fusion
Dying stars expel gas and new elements, producing hot bubbles (~106 K)
Hot gas cools, allowing atomic hydrogen clouds to form (~100-10,000 K)
Further cooling permits molecules to form, making molecular clouds (~30 K)
Gravity forms new stars (and planets) in molecular clouds
Gas Cools

8. Observations of Milky Way’s disk using many different wavelengths of light

9. Where are the star forming regions

10. Ionization nebulae - found around high-mass stars, means active star formation
Orion nebula

11. Reflection nebulae - scatter the light from stars
Why do reflection nebulae look bluer than nearby stars? They scatter blue light the most (same as sky!)Chamaeleon 1 complex (VLT UT1+FORS1) V, R, and I bands

12. Halo: No ionization nebulae, no blue stars
 no star formation
Disk: Ionization nebulae, blue stars  star formation

13. Most star formation in disk happens in spiral arms.
Ionization Nebulae
Blue Stars
Gas Clouds
Whirlpool Galaxy

14. Gas clouds get squeezed as they move into spiral arms
Squeezing of clouds triggers star formation
Young stars flow out of spiral arms

15. How did our galaxy form?

16. Models of formation assume 2 things:
Matter originally distributed almost uniformly
Gravity of denser
regions pulled in
surrounding
matter

17. Denser regions contracted, forming protogalactic clouds
H and He gases in these clouds formed the first stars

18. Halo stars formed first as gravity caused cloud to contract

19. Supernova explosions from first stars slowed collapse & kept much of the gas from forming stars
Leftover gas settled into spinning disk

20. Stars continuously form in disk as galaxy grows older

21. This cosmological simulation follows the development of a single disk galaxy over about 13.5 billion years, from shortly after the Big Bang to the present time. Colors indicate old stars (red), young stars (white and bright blue) and the distribution of gas density (pale blue); the view is 300,000 light-years 

22. What lies in the center of our galaxy?
Infrared light from center (left) , radio emission from center (right)
Swirling gas near center
Orbiting star near center

23. Stars are orbiting something massive but invisible.
Orbits indicatethe object has a mass of about 4 million MSun
Download a great movie of star motions from:

24. Stellar Orbits Around Galaxy Center