Our Galaxy, the Milky Way

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1 Our Galaxy, the Milky Way

2 In the night sky, the Milky Way appears as a faint band of light.

3 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 All-Sky View of the Milky Way

5 MILKY WAY TOP (artist s conception) SIDE

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.

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

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

9 Another spiral galaxy seen edge-on

10 The Sombrero Galaxy in optical and IR light

11 Closest large spiral galaxy M31 the Andromeda Galaxy 8X the diameter of the Full Moon!

12 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 galaxy s formation. Vertical motion due to gravitational attraction of the disk stars.

13 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.

14

15 How do we measure the mass of the Galaxy? Sun s orbital motion (radius & velocity) tell us the mass inside Sun s orbit: ~1.0 x M sun. (i.e. 100 billion stars ) Cannot measure the mass outside of the Sun s orbit in this fashion (we use other methods).

16 Galactic Recycling How does our galaxy recycle gas into stars? Where do stars tend to form in our galaxy?

17 What s the Milky Way got to do with us? It holds onto the gas and allows new stars to form from recycled (and enriched) material

18 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

19 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.

20 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.

21 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.

22 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.

23 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

24 Review: 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.

25 Supernova remnants The gas of the supernova remnant expands and cools. Begins to emit visible light, mostly emission line spectra. 130 light years

26 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.

27 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.

28 Atomic hydrogen in the ISM As the hot gas cools, electrons combine with protons to form clouds of atomic hydrogen (H). Atomic hydrogen produces an emission line at 21cm wavelength (in the radio) called Hydrogen hyperfine structure or the spin flip transition. Can use this to map the spatial distribution.

29 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

30 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).

31 Molecular clouds collapse due to gravity to form new stars, thereby completing the star-gas-star cycle.

32 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.

33 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.

34 QUESTION: Where will our Galaxy s 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

35 QUESTION: Where will our Galaxy s 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.

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

37 Infrared Visible Infrared light reveals stars whose visible light is blocked by clouds of gas & dust.

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

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

40 Radio (3 mm) 3-mm radio waves from carbon monoxide (CO) show locations of molecular clouds.

41 Long-wavelength infrared emission shows where young stars have heated dust grains. Far-IR (dust)

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

43

44 Where do stars tend to form in our galaxy?

45 Much of the star formation in disk galaxies happens in the spiral arms. Ionization Nebulae Blue (massive) stars Dusty Gas Clouds Whirlpool Galaxy

46

47 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.

48 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 particles.

49 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.

50

51 How did our galaxy form?

52 Halo: No ionization nebulae, no blue stars no star formation (and hence no recycling) Disk: Ionization nebulae, blue stars star formation Milky Way s star formation rate is about 1 M Sun /yr.

53 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, & keep on forming.

54 Our galaxy probably formed from a giant gas cloud

55 Halo stars formed first as gravity caused cloud to contract

56 Remaining gas settled into spinning disk due to conservation of angular momentum

57 Stars continuously form in disk as galaxy grows older

58 The collapsing cloud model explains the age, chemical, and orbital differences between halo and disk stars.

59 More detailed studies: Halo stars formed in clumps that later merged ( galactic cannibalism ).

60 What do we learn about the galaxy s history from its halo stars? The halo generally contains only old, lowmass 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.

61 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.

62 The black hole at the center of the Galaxy

63 Review: What is a black hole? A black hole is a place where gravity is so powerful that nothing can ever escape from it, not even light. (Therefore, out of contact with the rest of the Universe.) The event horizon the surface of the BH, where escape velocity is the speed of light (c) Escape speed > c Escape speed < c

64 Size of event horizon = Schwarzschild radius = 2GM/c 2 Schwarzschild radius of a 1M Sun black hole ~ 3 km

65 Galactic center in IR light

66 Galactic center in IR light Galactic center in radio

67 Strange radio sources in galactic center Galactic center in radio

68 Strange radio sources in galactic center Stars at galactic center

69 Stellar orbits size and mass black hole

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