1. Chapter 15: Our Galaxy
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2. 15.1 The Milky Way Revealed Our goals for learning:
What does our galaxy look like?
How do stars orbit in our galaxy?
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3. What does our galaxy look like?
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4. The Milky Way Galaxy appears in our sky as a faint band of light.
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5. Dusty gas clouds obscure our view because they absorb visible light
Dusty gas clouds obscure our view because they absorb visible light. This is the interstellar medium, the raw material for new star systems.
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6. We see our galaxy edge-on
We see our galaxy edge-on. Primary features: disk, bulge, halo, globular clusters
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7. If we could view the Milky Way from above the disk, we would see its spiral arms.
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8. How do stars orbit in our galaxy?
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9. Stars in the disk all orbit in the same direction with a little up-and-down motion.
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10. Orbits of stars in the bulge and halo have random orientations.
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11. Thought Question Why do orbits of bulge stars bob up and down?
They're stuck to the interstellar medium.
The gravity of disk stars pulls them toward the disk.
Halo stars knock them back into the disk.
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12. Thought Question Why do orbits of bulge stars bob up and down?
They're stuck to the interstellar medium.
The gravity of disk stars pulls them toward the disk.
Halo stars knock them back into the disk.
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13. Orbital Velocity Law r x v2 Mr = G
The orbital speed (v) and radius (r) of an object on a circular orbit around the galaxy tell us the mass (Mr) within that orbit.
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14. Sun's orbital motion (radius and velocity) tells us mass within Sun's orbit: 1.0 ✕ 1011MSun
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15. 15.2 Galactic Recycling Our goals for learning:
How is gas recycled in our galaxy?
Where do stars tend to form in our galaxy?
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16. How is gas recycled in our galaxy?
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17. Star–gas–star cycle Recycles gas from old stars into new star systems.
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18. Atomic hydrogen (H) gas forms as hot gas cools, allowing electrons to join with protons. Atomic hydrogen is observed by radio telescopes in the 21 cm emission line.
Molecular clouds (H2) form next, after gas cools enough to allow atoms to combine into molecules. Molecular hydrogen is difficult to observe directly, but many different molecules (e.g., CO) are observable with millimeter radio telescopes.
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19. Molecular clouds in Orion
Composition:
Mostly H2
About 28% He
About 1% CO
Many other molecules
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20. Gravity forms stars out of the gas in molecular clouds, completing the star–gas– star cycle.
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21. Radiation from newly formed stars is eroding these star-forming clouds.
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22. A Closer Look at the Eagle Nebula
This image was taken with the Hubble Space Telescope.

23. A Closer Look at the Eagle Nebula
A wide-field optical image taken from Kitt Peak.

24. A Closer Look at the Eagle Nebula
An infrared image from Europe’s VLT.

25. A Closer Look at the Eagle Nebula
Infrared image (zooms).

26. High-mass stars have strong stellar winds that blow bubbles of hot gas.
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27. Lower-mass stars return gas to interstellar space through stellar winds and planetary nebulae.
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28. X rays from hot gas in supernova remnants reveal newly made heavy elements.
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29. A supernova remnant cools and begins to emit visible light as it expands. New elements made by supernova mix into interstellar medium.
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30. Multiple supernovae create huge hot bubbles that can blow out of disk.
Gas clouds cooling in the halo can rain back down on disk.
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31. The Star–Gas–Star Cycle
Cloud cores collapse into protostars
the whole star formation process begins
the molecular cloud is eroded away by radiation from newly formed stars
This next generation of stars begins life with a greater content of heavy elements.
heavy elements which are necessary to form planets in the protostellar disks
So if there were no ISM:
supernovae would blast their matter out of the disk into intergalactic space
all generations of stars would lack the heavy elements to form planets
Eagle Nebula’s
“Pillars of Creation”

32. 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
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33. Thought Question Where will the gas be in 1 trillion years?
Blown out of galaxy
Still recycling just like now
Locked into white dwarfs and low-mass stars
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34. Thought Question Where will the gas be in 1 trillion years?
Blown out of galaxy
Still recycling just like now
Locked into white dwarfs and low-mass stars
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35. We observe the star–gas–star cycle operating in Milky Way's disk using many different wavelengths of light.
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36. Infrared
Visible
Infrared light reveals stars whose visible light is blocked by gas clouds.
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37. X rays are observed from hot gas above and below the Milky Way's disk.
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38. Radio (21cm)
21-cm radio waves emitted by atomic hydrogen show where gas has cooled and settled into disk.
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39. Radio (CO)
Radio waves from carbon monoxide (CO) show locations of molecular clouds.
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40. IR (dust)
Long-wavelength infrared emission shows where young stars are heating dust grains.
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41. Gamma rays show where cosmic rays from supernovae collide with atomic nuclei in gas clouds.
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42. Where do stars tend to form in our galaxy?
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43. Ionization nebulae are found around short-lived high-mass stars, signifying active star formation.
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44. Reflection nebulae scatter the light from stars
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
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45. 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
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46. What kinds of nebulae do you see?
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47. Halo: No ionization nebulae, no blue stars  no star formation
Disk: Ionization nebulae, blue stars  star formation
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48. Much of star formation in disk happens in spiral arms.
Whirlpool Galaxy
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49. Much of star formation in disk happens in spiral arms.
Ionization nebulae
Blue stars
Gas clouds
Whirlpool Galaxy
Spiral Arms
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50. Spiral arms are waves of star formation.
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51. Spiral arms are waves of star formation:
Gas clouds get squeezed as they move into spiral arms.
The squeezing of clouds triggers star formation.
Young stars flow out of spiral arms.
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52. 15.3 The History of the Milky Way
Our goals for learning:
What do halo stars tell us about our galaxy's history?
How did our galaxy form?
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53. What do halo stars tell us about our galaxy's history?
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54. 0.02–0.2% heavy elements (O, Fe, …), only old stars
Halo Stars:
0.02–0.2% heavy elements (O, Fe, …),
only old stars
Disk Stars:
2% heavy elements,
stars of all ages
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55. 0.02–0.2% heavy elements (O, Fe, …), only old stars
Halo Stars:
0.02–0.2% heavy elements (O, Fe, …),
only old stars
Halo stars formed first, then stopped.
Disk Stars:
2% heavy elements,
stars of all ages
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56. 0.02–0.2% heavy elements (O, Fe, …), only old stars
Halo Stars:
0.02–0.2% heavy elements (O, Fe, …),
only old stars
Halo stars formed first, then stopped.
Disk stars formed later, and kept forming.
Disk Stars:
2% heavy elements,
stars of all ages
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57. 0.02–0.2% heavy elements (O, Fe, …), only old stars
Halo Stars:
0.02–0.2% heavy elements (O, Fe, …),
only old stars
Halo stars formed first, then stopped.
Spheroidal Population
Disk Population:
Disk stars formed later, and kept forming.
Disk Stars:
2% heavy elements,
stars of all ages
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58. How did our galaxy form?
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59. Our galaxy probably formed from a giant gas cloud.
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60. Halo stars formed first as gravity caused the cloud to contract.
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61. The remaining gas settled into a spinning disk.
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62. Stars continuously form in the disk as the galaxy grows older.
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63. Stars continuously form in the disk as the galaxy grows older.
Warning: This model is oversimplified.
Stars continuously form in the disk as the galaxy grows older.
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64. Detailed studies: Halo stars formed in clumps that later merged.
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65. 15.4 The Galactic Center Our goals for learning:
What is the evidence for a black hole at our galaxy's center?
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66. What is the evidence for a black hole at our galaxy's center?
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67. Infrared light from Galactic center
Radio emission from Galactic center
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68. Radio emission from Galactic center Infrared light from
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69. Swirling gas near Galactic center Orbiting stars near Galactic center
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70. 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:

71. Stars appear to be orbiting something massive but invisible …
a black hole?
Orbits of stars indicate a mass of about 4 million Msun.
Insert ECP7e, figure here
Download a great movie of star motions from
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72. X-ray flares from galactic center suggest that tidal forces of suspected black hole occasionally tear apart chunks of matter about to fall in. The mystery of G2!
Download a great movie of star motions from
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