1. The Milky Way II AST 112

2. Interstellar Medium The space between stars is not empty! – Filled with the Interstellar Medium (ISM) Star formation is not random – Gas-rich regions: MORE – Gas-poor regions: LESS The ISM doesn’t just sit there!

3. Star-Gas-Star Cycle Matter in the Milky Way goes in cycles – Matter is taken out of the ISM when stars are born – Matter is altered and returned to the ISM throughout the star’s life and when it dies – This is called the Star-Gas-Star Cycle

4. Composition of the Milky Way Heavy elements seem to have mixed thoroughly into the Milky Way Supernovae eject heavy elements at greater than the Milky Way’s escape velocity – They’re somewhat localized So how do we get good mixing?

5. Star-Gas-Star Cycle

6. Stars return material to ISM by: – Stellar wind – Blowing up

7. Star-Gas-Star Cycle Material from a supernova collides with ISM material – Forms hot shock waves – Gradually slows, cools

8. Bubbles and Superbubbles Supernova expands, ISM “piles up” at the edge Can grow to 100 LY before slowing, merging with ISM “Superbubbles” happen when cluster stars die at the same time – Can grow to thousands of LY

9. Galactic Fountain What happens when the bubble becomes larger than the disk? – Blasts out of the Milky Way’s disk – Gas cools, rains back down – “Galactic fountain” mixes heavier elements into entire galaxy

10. Galactic Fountain

11. Galactic Fountains Credit: ESA

12. Hydrogen Clouds Milky Way is full of warm (neutral) hydrogen gas – Occasionally find cold molecular hydrogen clouds – Often find hot, ionized hydrogen clouds How do we map hydrogen in the Milky Way?

13. Warm Hydrogen Clouds: 21 cm Line Consider a hydrogen atom. – The proton is spinning and so is the electron. – Parallel spins have a higher energy than anti- parallel spins – Parallel spins can suddenly switch to anti-parallel spins and emit radio waves Wavelength of these waves is 21 cm

14. Warm Hydrogen Clouds Distribution of atomic H gas is shown by mapping the 21 cm hydrogen line.

15. Atomic Hydrogen Clouds The disk is full of hydrogen gas. The bulge doesn’t have much. We see hydrogen clouds at high latitudes that are falling down onto the disk – This supports the galactic fountain idea! Hydrogen stays in clouds for a long time, cools

16. Aside: Magellanic Stream The Magellanic Stream is a trail of hydrogen gas that starts at the Magellanic clouds and ends at the Milky Way

17. Star-Gas-Star Cycle Hydrogen cools and forms H 2 (molecular clouds) Heavy, dense; settle to middle of Milky Way Can see this!

18. Molecular Hydrogen Clouds But as we know… Molecular clouds continue to collapse – They heat – They fragment – They form stars!

19. Star Formation in the Milky Way Star forming regions are indicated by: Open clusters of young blue stars – Hot massive stars don’t last long – Don’t have time to stray far from their “brothers and sisters” Ionized hydrogen that is glowing from collapse and newborn stars – Glows red – Called HII (“H two”) regions

20. Star Formation in the Milky Way HII Region (Rosette Nebula) NGC 265 (Open Cluster)

21. Star-Gas-Star Cycle So the enriched gas has turned into stars. They will explode, supplying enriched material and gas to the ISM. This material expands, cools, and collapses back into stars… … and on and on and on.

22. Star-Gas-Star Cycle

23. It can’t go on forever. – Brown dwarfs don’t explode – White dwarfs, neutron stars and black holes aren’t turning into gas anytime soon (i.e. EVER) – The Milky Way is therefore running out of the gas that stars are born from

24. Star formation in the Milky Way will decrease over the next 50 billion years, at which point it will cease entirely.

25. At the center of our galaxy lurks…

26. … a teapot!

27. The Center of the Milky Way The bulge looks like it’s at the center – So, just more bulge stars? Use radio, infrared and see!

28. The Center of the Milky Way IR: stars and gas within 1000 LY of center

29. The Center of the Milky Way Radio: magnetic fields and turbulence

30. The Center of the Milky Way More radio: gas is swirling around a strong, unusual radio source Source is called Sagittarius A*

31. The Center of the Milky Way IR: stars shown within 1 LY of Sagittarius A* (arrows) Several hundred stars in this area

32. The Center of the Milky Way We can again use Kepler’s 3 rd Law to measure the mass of the object that these stars are orbiting Assume a mass of 100 suns and a distance from the Milky Way’s center of 0.017 LY (about 1000 AU) What is the orbital period?

33. The orbital period of the star in question (called S2) has been precisely measured to be 15.8 years!

34. Comparison of S2 and Pluto Orbits So this star is 30x farther from the center of its orbit than Pluto… And completes its orbit about 16x faster!

35. Center of the Milky Way So there are several hundred stars in the center of the Milky Way Stars orbit Sagittarius A*: – NOT consistent with a central mass of a few hundred stars! – ARE consistent with a central mass of a few MILLION stars – but they aren’t there!

36. Center of the Milky Way A supermassive black hole (SMBH) is thought to live at the center of the Milky Way. Its mass is calculated at 3-4 million solar masses.

37. Center of the Milky Way The circled dot is an x-ray flare. Any ideas why these might occur here?