1. Homework #9 will be posted later today: due Tuesday, April 14, 2:30 pm.
Solar telescope out-of-class activity will be posted shortly. Check website

2. Unless otherwise indicated, all out-of-class activities are due no later than Thursday, April 23 at class time (they can be turned in anytime before this date).
Turn them in during class time or deliver them to Dr Dash or Tara outside of class.

3. Out-of-class activity opportunity.
Check website for details.

4. If the Sun shrank into a black hole, its gravity would be different only near the event horizon
Black holes don’t suck!

5. Light waves take extra time to climb out of a deep hole in spacetime, leading to a gravitational redshift
Time passes more slowly near the event horizon
Tidal forces near the event horizon of a 3 MSun black hole would be lethal to humans
Tidal forces would be gentler near a supermassive black hole because its radius is much bigger

6. Verifying the existence of Black Holes:
Need to measure mass
Use orbital properties of companion
Measure velocity and distance of orbiting gas
It’s a black hole if it’s not a star and its mass exceeds the neutron star limit (~3 MSun)

7. Some X-ray binaries contain compact objects of mass exceeding 3 MSun which are likely to be black holes

8. One famous X-ray binary with a likely black hole is in the constellation Cygnus

9. Gamma-Ray Bursts
Brief bursts of gamma rays coming from space were first detected in the 1960s

10. Observations in the 1990s showed that many gamma-ray bursts were coming from very distant galaxies
They must be among the most powerful explosions in the universe—could be the formation of a black hole

11. What causes gamma-ray bursts?

12. Supernovae and Gamma-Ray Bursts
Observations show that at least some gamma-ray bursts are produced by supernova explosions
Some others may come from collisions between neutron stars

13. Supernovae and Gamma-Ray Bursts
Some others may come from collisions between neutron stars

14. Reviewing: What is a black hole?
A black hole is a massive object whose radius is so small that the escape velocity exceeds the speed of light
What would it be like to visit a black hole?
You can orbit a black hole like any other object of the same mass—black holes don’t suck!
Near the event horizon time slows down and tidal forces are very strong

15. Do black holes really exist?
Some X-ray binaries contain compact objects too massive to be neutron stars—they are almost certainly black holes

16. Where do gamma-ray bursts come from?
Most gamma-ray bursts come from distant galaxies
They must be among the most powerful explosions in the universe, probably signifying the formation of black holes
What causes gamma-ray bursts?
At least some gamma-ray bursts come from supernova explosions

17. Where do stars live? Galaxies!

18. Spiral galaxies
Elliptical galaxies
Irregular galaxies

19. A case-study of a spiral galaxy: The Milky Way

24. What does our galaxy look like?
The Andromeda Galaxy

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

26. All-Sky View

27. Because of our location, we see our galaxy edge-on
Primary features: disk, bulge, halo, globular clusters

28. If we could view the Milky Way from above the disk, we would see its spiral arms

29. Everything within a galaxy orbits about its center

30. Stars in the disk all orbit in the same direction with a little up-and-down motion. They move within the disk.

31. Orbits of stars in the bulge and halo have random orientations

33. Sun’s orbital motion (radius and velocity) tells us mass within Sun’s orbit:
1.0 x 1011 MSun

34. Orbital Velocity Law
The orbital speed (v) and radius (r) of an object on a circular orbit around the galaxy tells us the mass (Mr) within that orbit

35. What have we learned? What does our galaxy look like?
Our galaxy consists of a disk of stars and gas, with a bulge of stars at the center of the disk, surrounded by a large spherical halo
How do stars orbit in our galaxy?
Stars in the disk orbit in circles going in the same direction with a little up-and-down motion
Orbits of halo and bulge stars have random orientations

36. How is gas recycled in our galaxy?

37. Star-gas-star cycle: Recycles gas from old stars into new star systems

38. High-mass stars have strong stellar winds that blow bubbles of hot gas

39. Lower mass stars return gas to interstellar space through stellar winds and planetary nebulae

40. X-rays from hot gas in supernova remnants reveal newly-made heavy elements

41. Supernova remnant cools and begins to emit visible light as it expands
New elements made by supernova mix into interstellar medium

42. Multiple supernovae create huge hot bubbles that can blow out of disk
Gas clouds cooling in the halo can rain back down on disk

43. Atomic hydrogen gas forms as hot gas cools, allowing electrons to join with protons
Molecular clouds form next, after gas cools enough to allow to atoms to combine into molecules

44. Gravity forms stars out of the gas in molecular clouds, completing the star-gas-star cycle

45. Radiation from newly formed stars is eroding these star-forming clouds

46. Summary of Galactic Recycling
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

47. We observe star-gas-star cycle operating in Milky Way’s disk using many different wavelengths of light

48. Radio (atomic hydrogen)
Visible
21-cm radio waves emitted by atomic hydrogen show where gas has cooled and settled into disk

49. Radio (CO)
Visible
Radio waves from carbon monoxide (CO) show locations of molecular clouds

50. IR (dust)
Visible
Long-wavelength infrared emission shows where young stars are heating dust grains

51. Infrared
Visible
Infrared light reveals stars whose visible light is blocked by gas clouds

52. Visible
X-rays
X-rays are observed from hot gas above and below the Milky Way’s disk

53. Gamma rays show where cosmic rays from supernovae collide with atomic nuclei in gas clouds

54. Where do stars tend to form in our galaxy?

55. Ionization nebulae are found around short-lived high-mass stars, signifying active star formation
Orion nebula

56. Reflection nebulae scatter the light from stars
Why do reflection nebulae look bluer than the nearby stars?
Chamaeleon 1 complex (VLT UT1+FORS1) V, R, and I bands

57. Reflection nebulae scatter the light from stars
Why do reflection nebulae look bluer than the nearby stars?
For the same reason that our sky is blue!
Chamaeleon 1 complex (VLT UT1+FORS1) V, R, and I bands

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

59. Much of star formation in disk happens in spiral arms
Whirlpool Galaxy

60. Much of star formation in disk happens in spiral arms
Ionization Nebulae
Blue Stars
Gas Clouds
Whirlpool Galaxy

61. Spiral arms are waves of star formation

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

63. How is gas recycled in our galaxy?
Gas from dying stars mixes new elements into the interstellar medium which slowly cools, making the molecular clouds where stars form
Those stars will eventually return much of their matter to interstellar space
Where do stars tend to form in our galaxy?
Active star-forming regions contain molecular clouds, hot stars, and ionization nebulae
Much of the star formation in our galaxy happens in the spiral arms