The Milky Way

1. The Milky Way

2. Three Major Components • Bulge – young and old stars • Disk – young stars located in spiral arms • Halo – oldest stars and globular clusters • Components are chemically, kinematically, and spatially distinct

3. Chemical Composition • Disk • Large range in metallicity • Lots of gas and dust • Bulge • Mixed populations • Halo • Inner halo has very low metallicity stars • Outer halo has older stars

4. Kinematics

5. Disk • Stars orbit the center of the galaxy in circular orbits (vr ~ 0) • Stars concentrated into orbital plane • Vertical motions of stars give disk its 1,000 lyr thickness • Near the sun, stars have orbital period of ~200 Myrs

6. Halo and Bulge • Stars not concentrated in plane • Randomly oriented elliptical orbits • Appear spherical in shape

7. Why does that matter? • We can use orbital velocities to determine mass • The orbital velocity of a star is entirely dependent on the mass enclosed inside its orbit

8. Rotation Curves

9. Rotation Curves

10. Rotation Curves • Orbital velocity increases as more mass is enclosed • Velocity should decrease once edge of visible material is reached • Stays constant to large r • There must be a lot of mass we can’t see • Dark Matter

11. Spatial Distribution • Disk • 0.3 kpc thick • 15 kpc radius • M ~ 8*1010 Msun • Contains spiral arms • Location of Interstellar Medium (ISM) • Bulge • Spherical distribution • 1 kpc radius • M ~ 2*1010 MSun • Halo • Slightly flattened spheroid • ~45 kpc radius • M ~ 109 MSun

12. Spatial Distribution • Disk • 0.3 kpc thick • 15 kpc radius • M ~ 8*1010 Msun • Contains spiral arms • Location of Interstellar Medium (ISM) • Bulge • Spherical distribution • 1 kpc radius • M ~ 2*1010 MSun • Halo • Slightly flattened spheroid • ~45 kpc radius • M ~ 109 MSun

13. The Star-Gas-Star Cycle Universal Recycling

15. Star Birth • Giant molecular cloud gives birth to cluster • Cloud is very cold (10-30 K) • Stellar winds prevent rest of cloud from forming stars

16. Blowing Bubbles • Stellar winds blow material off of star and return it to the ISM • Large stars have very strong winds, which can blow big bubbles • Smaller stars have weaker winds

17. Supernovae • O and B stars live short lives and SN after only a few million years • SN explosion creates a shock wave • Shockwave sweeps up material compresses gas, heating it to about 106 K • Shockwave loses momentum when it sweeps up other material

18. Superbubbles • Clusters create superbubbles when SN explosions merge • Cannot be contained in disk • Bubble can extend up to 1 kpc above disk

19. Atomic H • Material cools and falls back to the disk • T = 100-10,000 K • Forms atomic H (HI) • Brings metals to disk • Cooling gas allows gas to form • Dust absorbs visible light • Observe HI with 21 cm line • HI exists throughout the Galactic disk

20. Molecular Clouds • Atomic H cools and combines to form H2 • T ~ 10-30 K • Molecular clouds very dense, so they settle to the center of the disk • Clouds too cold to emit, so we use other gasses • CO, H2O, NH3, C2H5OH

22. The Cycle Doesn’t Last • Mass locked up in the end products of stellar evolution • WD, NS, and BH • Mass also locked up in brown dwarfs • Eventually Milky Way will completely run out of material necessary for the S-G-S Cycle to continue, and star formation will cease

23. Distribution of Gas in the Milky Way

24. Star Forming Regions – HII Regions • O and B stars in cluster heat and ionize the surrounding gas • T~104 K • Gas cools from atomic emission lines • HII regions are found primarily in spiral arms • Emits Hα light – 656 nm (red) • Dust reflects blue light

29. Spiral Arms • Where all the star formation occurs • Red spots are HII Regions • Blue spots are new O and B stars • Dark areas have lots of dust

30. Spiral Arms • Spiral arms caused by spiral density waves • Gas is attracted to density wave • Waves condense and induce star formation • VIDEO

31. Formation of Spherical Population • Old, metal-poor stars in the spherical parts of the Galaxy (Bulge and disk) are called Pop. II • Formed by collapse of big cloud • Actually probably several clouds

32. Disk Population • Younger, more metal-rich stars located in the disk are called Pop. I • Ongoing star formation makes young stars and drives up metallicity • Galactic cannibalism builds up galactic disk • VIDEO

33. Galactic Center

34. Sgr A* • Strong radio source – indicates strong magnetic field • Stars and gas swirl around it

35. Sgr A* • Star motions and Newton’s Kepler’s 3rd law indicate mass of about 4*106 MSun concentrated in tiny region • Must be a supermassive black hole