1. III. Cycle of Birth and Death of Stars: Interstellar Medium
Ch. 14
End of Ch. 13
III. Cycle of Birth and Death of Stars: Interstellar Medium

2. III. Cycle of Birth and Death of Stars: Interstellar Medium
A. Interstellar Matter: Gas (mostly hydrogen) and dust
•Nebulae •Extinction and reddening
•Interstellar absorption lines •Radio observations
B. Nebulae
• Emission • Reflection • Dark
C. Cycle of Birth and Death of Stars

3. IIIA. Interstellar Matter: Gas (mostly hydrogen) and dust
Interstellar Medium
IIIA. Interstellar Matter: Gas (mostly hydrogen) and dust
How do we know that Interstellar Matter is there:
•Nebulae
•Extinction and reddening
•Interstellar absorption lines
•Radio observations

4. Extinction and Reddening: interstellar dust will make stars look fainter and redder

5. Review: Absorption Spectrum

6. Interstellar Absorption Lines

7. Radio Observations: some molecules can be detected with radiotelescopes

8. IIIB. Nebulae
• Emission Nebulae
• Reflection Nebulae
• Dark Nebulae

9. Question 1 Dark nebulae are Regions of space without any stars
Dense clouds of gas and dust that obscure the light from stars
Black holes
All the answers are correct

10. Question 1 Dark nebulae are Regions of space without any stars
Dense clouds of gas and dust that obscure the light from stars
Black holes
All the answers are correct

11. Question 2 Emission nebulae are: Regions of space without any stars
Low density gas near hot stars that show emission line spectra
Light from stars reflected by nearby dust
None of the answers are correct

12. Question 2 Emission nebulae are: Regions of space without any stars
Low density gas near hot stars that show emission line spectra
Light from stars reflected by nearby dust

13. Question 3 Reflection nebulae are: Regions of space without any stars
Low density gas near hot stars that show emission line spectra
Light from stars reflected by nearby dust
None of the answers are correct

14. Question 3 Reflection nebulae are: Regions of space without any stars
Low density gas near hot stars that show emission line spectra
Light from stars reflected by nearby dust
None of the answers are correct

15. Emission Spectrum

16. Emission Nebula (Eagle Nebula)
Hubble Space Telescope Image

17. Reflection Nebula

18. Dark Nebulae

19. Question 4
What happens after an interstellar cloud of gas and dust is compressed and collapses:
It will heat and contract
If it gets hot enough (10 million K) it can produce energy through hydrogen fusion
It can produce main sequence stars

20. Question 4
What happens after an interstellar cloud of gas and dust is compressed and collapses:
It will heat and contract
If it gets hot enough (10 million K) it can produce energy through hydrogen fusion
It can produce main sequence stars
All of the above

21. How does our galaxy recycle gas into stars?

22. IIIC. Cycle of Birth and Deaths of Stars
Interstellar cloud of gas and dust is compressed and collapses to form stars
After leaving the main sequence red giants eject their outer layers back to the interstellar medium
Supernovae explode and eject their outer layers back to the interstellar medium
Supernova explosions and other events can compress an interstellar cloud of gas and dust that collapses to form stars ………..

23. Remember Sun’s Evolutionary Process
Remember Sun’s Evolutionary Process* *This is an artist conception, not an HR diagram or a real motion of the Sun

24. Remember mass loss in Intermediate Mass Stars

25. Remember Supernova explosions

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

27. Ch. 14
Ch. 14.
The Milky Way

28. Ch. 14 OUTLINE Shorter than book
14.1 The Milky Way Revealed
14.2 Galactic Recycling (closely related to Ch. 13)
14.3 The History of the Milky Way
14.4 The Mysterious Galactic Center

29. 14.1 The Milky Way Revealed
Our Goals for Learning (not exactly like book)
• What does our galaxy look like?
• Where do stars form in our galaxy?

30. What does our galaxy look like?

31. The Milky Way galaxy appears in our sky as a faint band of light

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

33. All-Sky View at visible wavelengths
plotted in galactic coordinates
What if we could see through most of that dust?

34. All-Sky View at visible wavelengths
All-Sky View at infrared wavelengths

35. Remember Extinction and Reddening: interstellar dust will make stars look fainter and redder. Dust will affect more the shorter (bluer) wavelengths and less the longer (redder) wavelengths. By looking at infrared wavelengths we can see through most of the dust.

36. The Shape of our Galaxy: a flattened disk
We see our galaxy edge-on
Primary features: disk, bulge, halo, globular clusters

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

38. Andromeda Galaxy: our twin galaxy
Our Galaxy looks like Andromeda

39. How do we know what our galaxy would look like if viewed from the top
How do we know what our galaxy would look like if viewed from the top? Infrared and Radio observations penetrate dark interstellar clouds

41. Stellar Populations
Turns out that there are two types of stars in the Galaxy
Population I: Relatively young. Similar to the Sun. Tend to be in the galactic disk. Richer in heavy elements
Population II: Few heavy elements, very old (12-14 billion years), tend to be in the center of the galaxy or in globular clusters

42. Remember CH 11 Part III Star Clusters:
Two types of clusters: Open and Globular

43. Two types of star clusters
Open clusters: young, contain up to several thousand stars and are found in the disk of the galaxy (Population I).
Globular clusters: old, contain hundreds of thousands of stars, all closely packed together. They are found mainly in the halo of the galaxy (Population II).

44. 14.2 Galactic Recycling Our Goals for Learning
• How does our galaxy recycle gas into stars?
• Where do stars tend to form in our galaxy?

45. How does our galaxy recycle gas into stars?

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

47. 14.2 Galactic Recycling
• Where do stars tend to form in our galaxy?

48. 14.2 Galactic Recycling
• Where do stars tend to form in our galaxy? In the Disk

49. Question 4
What happens after an interstellar cloud of gas and dust is compressed and collapses:
It will heat and contract
If it gets hot enough (10 million K) it can produce energy through hydrogen fusion
It can produce main sequence stars

50. Question 4
What happens after an interstellar cloud of gas and dust is compressed and collapses:
It will heat and contract
If it gets hot enough (10 million K) it can produce energy through hydrogen fusion
It can produce main sequence stars
D. All of the above

51. How does our galaxy recycle gas into stars?

52. Cycle of Birth and Deaths of Stars
Interstellar cloud of gas and dust is compressed and collapses to form stars
After leaving the main sequence red giants eject their outer layers back to the interstellar medium
Supernovas explode and eject their outer layers back to the interstellar medium
Supernova explosions and other events can compress an interstellar cloud of gas and dust that collapses to form stars ………..

53. Remember the Sun’s Evolutionary Process

54. Remember mass loss in Intermediate Mass Stars

55. Remember Supernova explosions

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

57. Halo: no blue stars  no star formation
Disk: blue stars  star formation

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

59. Much of star formation in disk happens in spiral arms
Emission Nebulae
Blue Stars
Gas Clouds
Spiral arms are waves of star formation
The Whirlpool Galaxy

60. 14.3 The History of the Milky Way
Our Goals for Learning
• What clues to our galaxy’s history do halo stars hold?
How did our galaxy form?

61. What clues to our galaxy’s history do halo stars hold?

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

63. Halo Stars:
% heavy elements (O, Fe, …),
only old stars
Halo stars formed first, then stopped
Disk Stars:
2% heavy elements,
stars of all ages

64. Halo Stars:
% heavy elements (O, Fe, …),
only old stars
Halo stars formed first, then stopped
Disk Stars:
2% heavy elements,
stars of all ages
Disk stars formed later, kept forming

65. How did our galaxy form?

66. Our galaxy probably formed from a giant gas cloud

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

68. Remaining gas settled into spinning disk

69. Stars continuously form in disk as galaxy grows older
Note: This model is oversimplified
Stars continuously form in disk as galaxy grows older

70. What have we learned?
• What clues to our galaxy’s history do halo stars hold?
The halo generally contains only old, low-mass stars with a much smaller proportion of heavy elements than stars in the disk. Thus, halo stars must have formed early in the galaxy’s history, before the gas settled into a disk.

71. What have we learned? • How did our galaxy form?
The galaxy probably began as a huge blob of gas called a protogalactic cloud. Gravity caused the cloud to shrink in size, and conservation of angular momentum caused the gas to form the spinning disk of our galaxy. Stars in the halo formed before the gas finished collapsing into the disk.

72. 14.4 The Mysterious Galactic Center
Our Goals for Learning
• What lies in the center of our galaxy?

73. What lies in the center of our galaxy?

74. Strange radio sources in galactic center
Stars at galactic center

75. Orbits of stars indicate a mass of about 4 million MSun
Stars appear to be orbiting something massive but invisible … a black hole!
Orbits of stars indicate a mass of about 4 million MSun
Download a great movie of star motions from:

76. What have we learned? • What lies in the center of our galaxy?
Motions of stars near the center of our galaxy suggest that it contains a black hole about 4 million times as massive as the Sun. The black hole appears to be powering a bright source of radio emission.