1. 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?

2. How does our galaxy recycle gas into stars?

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

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

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

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

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

8. Multiple supernovae create huge hot bubbles that can blow out of disk; gas cooling in the halo can rain back down on disk.

9. 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

10. Molecular clouds in Orion
Composition:
Mostly H2
About 28% He
About 1% CO
Many other
molecules

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

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

13. 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

14. Where will most of the mass now in our galaxy’s gas be in a hundred billion years?
Blown out of the galaxy
Still recycling just like it is now
Locked into white dwarfs and low-mass stars
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15. We observe star-gas-star cycle operating in Milky Way’s disk using many different wavelengths of light

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

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

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

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

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

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

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

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

24. Reflection nebulae scatter the light from stars.
Why do reflection nebulae look blue, even when the stars near them aren't blue?
Chamaeleon 1 complex (VLT UT1+FORS1) V, R, and I bands

25. 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

26. What kinds of nebulae do you see?

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

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

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

30. Spiral arms are waves of star formation

31. 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

32. What have we learned? • How does our galaxy recycle gas into stars?
Stacie Kent:
Insert thumbnail of 14.8
What have we learned?
• How does our galaxy recycle gas into stars?
Stars are born from the gravitational collapse of gas clumps in molecular clouds. Near the ends of their lives, stars more massive than our Sun create elements heavier than hydrogen and helium and expel them into space via supernovae and stellar winds. The supernovae and winds create hot bubbles in the interstellar medium, but the gas within these bubbles gradually slows and cools as they expand. Eventually, this gas cools enough to collect into clouds of atomic hydrogen. Further cooling allows atoms of hydrogen and other elements to collect into molecules, producing molecular clouds.These molecular clouds then form stars, completing the star–gas–star cycle.

33. What have we learned? • Where do stars tend to form in our galaxy?
Active star-forming regions, marked by the presence of hot, massive stars and ionization nebulae, are found preferentially in spiral arms. The spiral arms represent regions where a spiral density wave has caused gas clouds to crash into each other, thereby compressing them and making star formation more likely.

34. Activity 19, pages 67-70

35. 1B: The hotter a star’s temperature, the ____ the wavelength at which its peak emission occurs.
Smaller
Larger
(stays the same)
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36. 1C: A star whose spectrum peaks at 300 nm is ___ than the stars in Fig
Hotter
Cooler
It depends; need more info
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37. 2. Who’s right?
Felix
Isidor
Neither
Both
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38. 3D: Two stars are of equal size. Which radiates more ultraviolet energy?
T=5000 K star
T=8000 K star
Both radiate the same
It depends; more info needed
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39. 3E: If two visible stars are identical in distance and size, the hotter one will appear to be the ____ of the two in the night sky.
Brighter
Dimmer
It depends; need more info
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40. 6A: Which of the 3 stars in Figure 2 is the hottest?B C
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41. 6B: Which star is coolest?
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42. 6C: Which star is most luminous?B C
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43. 6D:Which is larger, star A or star B?
Same size
More info needed
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44. 6E: Which star(s) will appear red?B C
None of them
All of them
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