Chapter 14: Our Galaxy, the Milky Way. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 1

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1 Chapter 14: Our Galaxy, the Milky Way 4/15/2009 Habbal Astro Chapter 14 Lecture 31 1

2 14.1 The Milky Way Revealed Learning goals What is the structure of our galaxy? How do stars orbit in our galaxy? 4/15/2009 Habbal Astro Chapter 14 Lecture 31 2

3 In the night sky, the Milky Way appears as a faint band of light. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 3

4 Dusty gas clouds obscure our view because they absorb visible light. This is the interstellar medium that makes new star systems. It comprises clouds of hydrogen gas (atomic & molecular) and dust. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 4

5 All-Sky View of the Milky Way 4/15/2009 Habbal Astro Chapter 14 Lecture 31 5

6 Size of the Milky Way (side view) Diameter ~ 100,000 light years Thickness ~ 1,000 light years (flatter than a CD!) Distance from Sun to center ~ 30,000 light years About 100 billion stars in total. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 6

7 Stellar components of the Milky Way 1. Disk: rotating, thin collection of stars, gas & dust. 2. Halo: tenuous outer sphere of stars & globular clusters, and very little gas. 3. Bulge: spherical concentration of stars near the center 4/15/2009 Habbal Astro Chapter 14 Lecture 31 7

8 Size of the Milky Way (side view) Diameter ~ 100,000 light years Thickness ~ 1,000 light years (flatter than a CD!) Distance from Sun to center ~ 30,000 light years About 100 billion stars in total. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 8

9 Stellar components of the Milky Way 1. Disk: rotating, thin collection of stars, gas & dust. 2. Halo: tenuous outer sphere of stars & globular clusters, and very little gas. 3. Bulge: spherical concentration of stars near the center 4/15/2009 Habbal Astro Chapter 14 Lecture 31 9

10 If we could view the Milky Way from above the disk, we would see its spiral arms 4/15/2009 Habbal Astro Chapter 14 Lecture 31 10

11 If we could view the Milky Way from above the disk, we would see its spiral arms 4/15/2009 Habbal Astro Chapter 14 Lecture 31 11

12 Another spiral galaxy seen edge-on 4/15/2009 Habbal Astro Chapter 14 Lecture 31 12

13 Another spiral galaxy seen face-on 4/15/2009 Habbal Astro Chapter 14 Lecture 31 13

14 Stellar Orbits: Stars in the Galactic Disk Disk stars all orbit in the same direction of rotation, with a small amount of vertical (up-and-down) motion. Rotation due to angular momentum from the galaxyʼs formation. Vertical motion due to gravitational attraction of the disk stars. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 14

15 Stellar Orbits: Galactic Halo & Bulge Stars in the halo & bulge also orbit the center of the galaxy. But their orbits have random orientations, w/o any overall sense of rotation. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 15

16 4/15/2009 Habbal Astro Chapter 14 Lecture 31 16

17 How do we measure the mass of the Galaxy? Sunʼs orbital motion (radius & velocity) tell us the mass inside Sunʼs orbit: ~1.0 x M sun. Cannot measure the mass outside of the Sunʼs orbit in this fashion. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 17

18 Orbital velocity law M r = r v 2 / G Take v = 220 km/s: orbital velocity of Sun around center of galaxy r = 28,000 ly: orbital radius M r = kg M r /M S = Similar calculations of orbits of distant stars most of galaxyʼs mass is far from center and distributed throughout halo. But since donʼt see emission dark matter (otherwise stars far away would have v decreasing with distance like planets) 4/15/2009 Habbal Astro Chapter 14 Lecture 31 18

19 Whatʼs the Milky Way got to do with us? It holds onto the gas and allows new stars to form from recycled (and enriched) material 4/15/2009 Habbal Astro Chapter 14 Lecture 31 19

20 How does our galaxy form stars? Recycles gas from old stars into new stars. With each cycle, more heavy elements are made by nuclear fusion in stars. Star-gas-star cycle 4/15/2009 Habbal Astro Chapter 14 Lecture 31 20

21 Star-gas-star cycle Recycles gas from old stars into new stars. With each cycle, more heavy elements are made by nuclear fusion in stars. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 21

22 Summary of Galactic Recycling Gas Cools Stars make new heavy elements by fusion. Dying stars expel gas and new elements, producing hot bubbles of gas (~10 6 K). These emit X-rays. This hot gas cools, allowing atomic hydrogen clouds to form (~100-10,000 K). This hydrogen emits at 21-cm wavelength emission line. Further cooling permits molecules (CO, etc) to form, making molecular clouds (~30 K). CO emits an emission line spectrum at 3 mm. Gravity forms new stars (and planets) in molecular clouds. Process starts over. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 22

23 Effect of low-mass stars on the interstellar medium Low-mass stars eject gas through their (very small) stellar winds and mass loss during the planetary nebula phase. Overall, these have much less effect on the ISM than high mass stars. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 23

24 Effect of high-mass stars on the interstellar medium During their lives, high-mass stars have strong stellar winds that blow bubbles of hot gas. 10 light-years High mass stars die as supernovae, injecting heavy elements into the interstellar medium. Have a very strong effect on the ISM. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 24

25 Supernova remnants: Xrays Supernova remnants are filled with hot gas (~10 6 K), which emit thermal radiation at mostly X-ray wavelengths. 20 light years 4/15/2009 Habbal Astro Chapter 14 Lecture 31 25

26 Recap: Learning from Light How does light tell us what things are made of? Every kind of atom, ion, and molecule produces a unique set of spectral lines, seen in emission or absorption spectra. How does light tell us the temperatures of dense objects? We can determine temperature from the (continuous) spectrum of thermal radiation. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 26

27 Supernova remnants The gas of the supernova remnant expands and cools. Begins to emit visible light, mostly emission line spectra. 130 light years 4/15/2009 Habbal Astro Chapter 14 Lecture 31 27

28 Supernova remnants The gas of the supernova remnant expands and cools. Begins to emit visible light as emission line spectra. These spectra show heavy elements (O, Ne, N, S) made by the star, which are distributed back into the ISM. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 28

29 SN superbubbles Multiple supernovae can create huge bubbles of hot gas, which blow out of the galactic disk. Gas clouds cooling in the halo can rain back down onto the disk. These collisions may trigger future star formation. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 29

30 Atomic hydrogen in the ISM As the hot gas cools, electrons combine with protons to form clouds of atomic hydrogen (H). Hydrogen produces an emission line at 21cm wavelength (in the radio). Can use this to map the spatial distribution. Radio (21 cm) 4/15/2009 Habbal Astro Chapter 14 Lecture 31 30

31 Molecular hydrogen in the ISM Atomic hydrogen clouds slowly contract & cool further. Once they get cold & dense enough, the single H atoms combine to form molecular hydrogen (H 2 ) clouds. Optical image 4/15/2009 Habbal Astro Chapter 14 Lecture 31 31

32 Molecular clouds Composition: Mostly H 2 About 28% He About 1% CO Many other molecules. Unlike atomic hydrogen (H), molecular hydrogen (H 2 ) is very hard to detect, as it emits very weak radiation. Detect molecular clouds from 3-mm emission line of CO (a trace constituent by mass). 4/15/2009 Habbal Astro Chapter 14 Lecture 31 32

33 Molecular clouds collapse due to gravity to form new stars, thereby completing the star-gas-star cycle. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 33

34 Star formation in molecular clouds Young massive stars can erode the birth clouds, preventing further star formation. Only a small fraction of gas in molecular clouds forms into stars. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 34

35 Summary of Galactic Recycling Gas Cools Stars make new heavy elements by fusion. Dying stars expel gas and new elements, producing hot bubbles of gas (~10 6 K). These emit Xrays. This hot gas cools, allowing atomic hydrogen clouds to form (~100-10,000 K). This hydrogen emits at 21-cm wavelength emission line. Further cooling permits molecules (CO, etc) to form, making molecular clouds (~30 K). CO emits an emission line spectrum at 3 mm. Gravity forms new stars (and planets) in molecular clouds. Process starts over. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 35

36 QUESTION: Where will our Galaxyʼs gas be in 1 trillion years from now? A. Blown out of galaxy B. Still recycling just like now C. Locked into white dwarfs and lowmass stars 4/15/2009 Habbal Astro Chapter 14 Lecture 31 36

37 QUESTION: Where will our Galaxyʼs gas be in 1 trillion years from now? A. Blown out of galaxy B. Still recycling just like now C. Locked into white dwarfs and lowmass stars Galactic recycling is an imperfect process. More and more gas gets locked up into low-mass stars and white dwarfs, which never return their material to the interstellar medium. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 37

38 We observe star-gas-star cycle operating in the Milky Wayʼs disk using many different wavelengths of light. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 38

39 Infrared Visible Infrared light reveals stars whose visible light is blocked by clouds of gas & dust. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 39

40 X-rays X-rays are observed from hot gas above and below the Milky Wayʼs disk. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 40

41 Radio (21cm) 21-cm radio waves emitted by atomic hydrogen show where gas has cooled and settled into disk. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 41

42 Radio (3 mm) 3-mm radio waves from carbon monoxide (CO) show locations of molecular clouds. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 42

43 Far-IR (dust) Long-wavelength infrared emission shows where young stars have heated dust grains. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 43

44 Gamma rays show where cosmic rays from supernovae collide with atomic nuclei in gas clouds 4/15/2009 Habbal Astro Chapter 14 Lecture 31 44

45 4/15/2009 Habbal Astro Chapter 14 Lecture 31 45

46 Where do stars tend to form in our galaxy? 4/15/2009 Habbal Astro Chapter 14 Lecture 31 46

47 Much of the star formation in disk galaxies happens in the spiral arms. Ionization Nebulae Blue (massive) stars Dusty Gas Clouds Whirlpool Galaxy 4/15/2009 Habbal Astro Chapter 14 Lecture 31 47

48 Ionization nebulae Regions of ionized gas Found around short-lived high-mass stars and signify active star formation. The blue light of the massive stars is scattered by nearby dust clouds. The nebulae tend to appear reddish, b/c of strong emission lines at these wavelengths. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 48

49 Reflection nebulae are dusty gas clouds which scatter the light from stars. Why do reflection nebulae look bluer than the nearby stars? For the same reason our sky is blue, and sunsets are red. Blue light is preferentially scattered by gas molecules and small dust particle. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 49

50 Spiral arms are waves of star formation 1. Gas clouds get squeezed as they move into spiral arms 2. Squeezing of clouds triggers star formation. 3. Young stars flow out of spiral arms. 4/15/2009 Habbal Astro Chapter 14 Lecture 31 50

51 4/15/2009 Habbal Astro Chapter 14 Lecture 31 51

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