1. Announcements Reading for next class: Chapter 20 Cosmos Assignment 2, Due Wednesday, April 14  Angel Quiz

2. Questions: Black Holes Star-Gas-Star cycle Halo, halo stars, halo vs. bulge, halo vs disk Spiral arms Do dying stars come back as the same star? superbubbles

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

4. What do your classmates see? To answer this need to know a little of Einstein’s theory of Motion and Gravity: Gravity is Motion in Warped Space - Time You can’t tell the difference between acceleration by gravity and any other constant acceleration E = mc 2, energy and mass are same thing measured in different units

5. Mass warps Space - Time Warped Space - Time tells Mass how to Move Forget time, think just about warped space

6. Orbits in Warped Space - Time c = circular, e = elliptical, u = unbounded

7. Elevator & Rocket

8. Gravity = Acceleration Light Beam in an Elevator or Gravity

9. Gravity Attracts Light Light generates Gravity Reasonable since E = mc 2 Black Holes Gravity attracts light Light loses energy escaping from environs of a Black Hole. Convert radiation energy to gravitational potential energy. Escaping Light is redshifted to longer wavelengths and periods

10. Your classmates would see you slow down as you approached the BH event horizon Can use period of light as a clock Redshifted light oscillates with a longer period Time appears to run slower near event horizon You would appear to stop and hover (& fade out) as you approached the Event Horizozn

11. What would you notice as you passed the Event Horizon Nothing special For you time does not slow down in a BH. You quickly crash into the previous matter inside the BH (But you couldn’t tell us about it)

12. What can we know about Black Holes? Nothing can escape from inside an Event Horizon Long range forces can exert influence outside Event Horizon 1.Gravity 2.Electric Force Can determine: 1.Mass 2.Charge 3.Spin

13. Mini Black Holes can Evaporate Mini BH produce strong tides (stellar BH don’t have strong enough tides)  Lose energy by work of tidal gravity on material outside the event horizon  Since energy = mass, they lose mass and get smaller  Evaporate

14. The Milky Way, our galaxy Beginning of Unit IV: Cosmology Milky Way - chapter 19

15. What does my building look like?

16. Milky Way small portion from the winter sky

17. First Idea: Count stars in different directions, more stars -> larger extent. What Assumption is made?  apteyn Model of Milky Way, 1922 Sun

18. Question 1: What Assumptions were made? A.Stars are clustered in a disk B.Stars are evenly distributed in space C.Stars are clustered near the Sun D.We see all the stars in the Milky Way

19. Question 1: What Assumptions were made? A.Stars are clustered in a disk B.Stars are evenly distributed in space C.Stars are clustered near the Sun D.We see all the stars in the Milky Way

20. What was missing? We don’t see all the stars, because some are hidden by interstellar clouds of gas & dust The stars are not evenly distributed in space (but this is not as important)

21. All-Sky View Mosaic View, does give good idea of MW structure

22. Familiar Constellations

23. Dusty gas clouds obscure our view because they absorb visible light This gas is the interstellar medium that makes new stars

24. Infrared light passes more easily through dusty gas clouds This gas is the interstellar medium that makes new stars

25. Infrared Light

27. Milky Way has DISK shape Stars are concentrated into a disk, but some stars above and below the disk Neutral Hydrogen gas is concentrated in disk

28. Milky Way Cartoon

29. Stars in the disk orbit in the same direction with a little bobbing up & down

30. Stars in the bulge & halo have randomly oriented orbits

32. Question 2: Why do orbits of disk stars bob up and down? A. They’re stuck to the interstellar medium B. Gravity of disk stars pulls toward disk C. Halo stars knock them back into disk

33. Question 2: Why do orbits of disk stars bob up and down? A. They’re stuck to the interstellar medium B. Gravity of disk stars pulls toward disk C. Halo stars knock them back into disk

34. Life of a Galaxy: Gas -> Star -> Gas cycle  Gas clumps together by gravity -> stars  Stars produce heavy elements by fusion  Stars die and return processed gas to space  Enriched gas clumps together by gravity  New stars

35. Life of a Galaxy: Gas -> Star -> Gas cycle Hot, ionized gas - one million K Warm neutral gas - 10,000 K (most is here) Cool neutral gas - 100 K Molecular clouds - 30 K Molecular cores - 6 K STAR FORMATION Gas Cools

36. X-ray map of sky shows hot gas high above and far below galactic disk

37. Hot gas eventually cools to form atomic hydrogen and settles into disk X-ray map of sky shows hot gas high above and far below galactic disk

38. Hot gas eventually cools to form atomic hydrogen and settles into disk Additional cooling makes the gas cold enough to form molecules

39. Darkest regions of Milky Way correspond to these dense clouds. Additional cooling makes the gas cold enough to form molecules  hat is where stars form!

40. Hot, Blue, Massive MS stars heat and ionize the gas around them

41. Massive, hot, blue MS stars ionize the gas around them Produce ionization nebula

42. Recycling Stellar Material- Low mass stars blow off winds & eject envelopes

43. Recycling stellar Material - High mass stars explode as supernova

44. Recycling Stellar Material Stellar Winds and Supernova return stellar material, as hot gas, to the Interstellar Medium (gas between the stars) ENRICHED in Heavy Elements produced during nuclear fusion & supernova explosions

45. Gas -> Star -> Gas cycle

46. Disk: ionization nebula & blue stars = star formation Halo: no ionization nebula or blue stars = no star formation

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

48. Much of star formation in disk happens in spiral arms Whirlpool Galaxy Where do stars form?

49. Question 3: How can we study the Center of the Milky Way? A.By observing it in x-rays B.By observing it in ultraviolet light C.By observing it in visible light D.By observing it in infrared light E.By observing it in radio light Choose all that apply

50. Question 3: How can we study the Center of the Milky Way? A.By observing it in (very energetic) x-rays B.By observing it in ultraviolet light C.By observing it in visible light D.By observing it in infrared light E.By observing it in radio light Choose all that apply

51. Center of the Milky Way

52. Question 4: Will stars move faster closer to the center or farther away from it? A.Closer B.Farther C.The same at all distances

53. What happens in the Solar System?

54. Star Motions near the Center of the Milky Way

56. What did we observe? Stars moved fastest closest to the center Similar Solar System Speed decreases as 1/  D  Point mass at Center of Galaxy From velocity & distance can determine the Mass M ( inside distance D ) = V 2 D / G  Supermassive Black Hole, M ~3-4x10 6 Msun