A100 Exploring the Universe: The Milky Way as a Galaxy. Martin D. Weinberg UMass Astronomy

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1 A100 Exploring the Universe: The Milky Way as a Galaxy Martin D. Weinberg UMass Astronomy November 12, 2014 Read: Chap 19 11/12/14 slide 1

2 Exam #2 Returned and posted tomorrow (Thursday) Read: Chap 19 11/12/14 slide 2

3 Exam #2 Returned and posted tomorrow (Thursday) Mean 72.2 Std Dev 15.1 Read: Chap 19 11/12/14 slide 2

4 Exam #2 Returned and posted tomorrow (Thursday) Remainder of the semester: Chaps Read: Chap 19 11/12/14 slide 2

5 Exam #2 Returned and posted tomorrow (Thursday) Remainder of the semester: Chaps Today: More on the Milky Way Galaxy How we understand the structure of a galaxy How we use the Milky Way to study galaxies Clues to galaxy formation of galaxy formation Read: Chap 19 11/12/14 slide 2

6 Exam #2 Returned and posted tomorrow (Thursday) Remainder of the semester: Chaps Today: More on the Milky Way Galaxy How we understand the structure of a galaxy How we use the Milky Way to study galaxies Clues to galaxy formation of galaxy formation Your questions? Read: Chap 19 11/12/14 slide 2

7 How is gas recycled in the Galaxy? Star-gas-star cycle: Recycles gas from old stars into new star systems Read: Chap 19 11/12/14 slide 3

8 We observe star-gas-star cycle operating in Milky Ways disk using many different wavelengths of light Read: Chap 19 11/12/14 slide 4

9 Radio (Atomic hydrogen) Visible 21-cm radio waves emitted by atomic hydrogen show where gas has cooled and settled into disk Read: Chap 19 11/12/14 slide 5

10 Radio (CO) Visible Radio waves from carbon monoxide (CO) show locations of molecular clouds Read: Chap 19 11/12/14 slide 6

11 IR (dust) Visible Long-wavelength infrared emission shows where young stars are heating dust grains Read: Chap 19 11/12/14 slide 7

12 Infrared Visible Infrared light reveals stars whose visible light is blocked by gas clouds Read: Chap 19 11/12/14 slide 8

13 X-rays Visible X-rays are observed from hot gas above and below the Milky Ways disk Read: Chap 19 11/12/14 slide 9

14 Gamma rays Visible Gamma rays show where cosmic rays from supernovae collide with atomic nuclei in gas clouds Read: Chap 19 11/12/14 slide 10

15 Interstellar extinction! Dust particles in interstellar space (approx. 0.1 micron in size) Absorb and scatter light strongly in the UV and visible Stars appear dimmer We infer larger distances Dust In the disk, we can only see stars to about 1 to 2 kpc away (visible) Dust Read: Chap 19 11/12/14 slide 11

16 Where do stars form in the Galaxy? Ionization nebulae are found around short-lived high-mass stars, signifying active star formation Read: Chap 19 11/12/14 slide 12

17 Where do stars form in the Galaxy? Ionization nebulae are found around short-lived high-mass stars, signifying active star formation Read: Chap 19 11/12/14 slide 12

18 Where do stars form in the Galaxy? Reflection nebulae scatter the light from stars Read: Chap 19 11/12/14 slide 13

19 Where do stars form in the Galaxy? Reflection nebulae scatter the light from stars Thought question: Why do reflection nebulae look bluer than the nearby stars? Read: Chap 19 11/12/14 slide 13

20 Where do stars form in the Galaxy? Reflection nebulae scatter the light from stars Thought question: Why do reflection nebulae look bluer than the nearby stars? For the same reason that the sky is blue... scattering of shorter wavelength light! Read: Chap 19 11/12/14 slide 13

21 Where do stars form in the Galaxy? How many kinds of nebulae in this image? Read: Chap 19 11/12/14 slide 14

22 Where do stars form in the Galaxy? Halo: No ionization nebulae, no blue stars no star formation Disk: Ionization nebulae, blue stars star formation Read: Chap 19 11/12/14 slide 15

23 Where do stars form in the Galaxy? Halo: No ionization nebulae, no blue stars no star formation Disk: Ionization nebulae, blue stars star formation Read: Chap 19 11/12/14 slide 15

24 Where do stars form in the Galaxy? Halo: No ionization nebulae, no blue stars no star formation Disk: Ionization nebulae, blue stars star formation Read: Chap 19 11/12/14 slide 15

25 Where do stars form in the Galaxy? Most of star formation in disk happens in spiral arms Read: Chap 19 11/12/14 slide 16

26 Where do stars form in the Galaxy? Most of star formation in disk happens in spiral arms Ionization nebulae Blue stars Gas clouds and dust Read: Chap 19 11/12/14 slide 16

27 Where do stars form in the Galaxy? are waves of star formation! Read: Chap 19 11/12/14 slide 16

28 Wave pattern occur in the disk of the Milky Way and other galaxies Earliest thought: material waves Read: Chap 19 11/12/14 slide 17

29 Wave pattern occur in the disk of the Milky Way and other galaxies Earliest thought: material waves Read: Chap 19 11/12/14 slide 17

30 Not material waves: density waves Example: waves in traffic flow Cars bunch together and spread out as a density wave passes through traffic Slow vehicle, line painting Read: Chap 19 11/12/14 slide 18

31 Solution to the winding problem The orbits of stars are not quite circles but ellipses Where orbits bunch, gravity is enhanced Extra gravity correlates the orbits and prevents the wave from wrapping quickly Read: Chap 19 11/12/14 slide 19

32 Young stars in the 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 Read: Chap 19 11/12/14 slide 20

33 Young stars in the arms The spiral density waves cause a density enhancement which triggers star formation In a galaxy like the Milky Way there will be a progression of star formation across the spiral arm. Gas falls into arm, compresses Forms molecular clouds Forms stars Asssociations and clusters trail arm Read: Chap 19 11/12/14 slide 21

34 Clues to Galaxy formation Halo Stars: % heavy elements (O, Fe,... ), only old stars Disk Stars: 2% heavy elements, stars of all ages Read: Chap 19 11/12/14 slide 22

35 Clues to Galaxy formation Halo Stars: % heavy elements (O, Fe,... ), only old stars Disk Stars: 2% heavy elements, stars of all ages Read: Chap 19 11/12/14 slide 22

36 Clues to Galaxy formation Halo Stars: % heavy elements (O, Fe,... ), only old stars Disk Stars: 2% heavy elements, stars of all ages Read: Chap 19 11/12/14 slide 22

37 Stellar Populations Astronomers (since mid 20th century) divide stars into two Populations: Population I: Disk and Open Cluster stars Population II: Spheroid and Globular Cluster stars Distinguished by: Location, Age, and Chemical Composition Read: Chap 19 11/12/14 slide 23

38 Population I Stellar Populations Location: Disk and Open Clusters Age: Mix of young and old stars Composition: Metal rich (roughly solar composition) Environment: Often gas rich, especially for the young stars 70% Hydrogen 28% Helium 2% metals Read: Chap 19 11/12/14 slide 24

39 Population II Stellar Populations Location: Spheroid and Globular Clusters Ages: Oldest stars, > 10 Gyr Composition: Metal Poor (0.1-1% solar) Environment: gas poor, no star formation 75% Hydrogen 24.99% Helium 0.01% metals Read: Chap 19 11/12/14 slide 25

40 of Galaxy Formation Read: Chap 19 11/12/14 slide 26

41 of Galaxy Formation Our galaxy probably formed from a giant gas cloud Read: Chap 19 11/12/14 slide 27

42 of Galaxy Formation Halo stars formed first as gravity caused cloud to contract Read: Chap 19 11/12/14 slide 28

43 of Galaxy Formation Remaining gas settled into spinning disk Read: Chap 19 11/12/14 slide 29

44 of Galaxy Formation Stars continuously form in disk as galaxy grows older Read: Chap 19 11/12/14 slide 30

45 of Galaxy Formation Stars continuously form in disk as galaxy grows older VERY oversimplified Read: Chap 19 11/12/14 slide 30

46 of Galaxy Formation Detailed studies: Halo stars formed in clumps that later merged Read: Chap 19 11/12/14 slide 31

47 Galaxy Formation: summary What clues to our galaxys history do halo stars hold? Halo stars are all old, with a smaller proportion of heavy elements than disk stars, indicating that the halo formed first How did our galaxy form? Our galaxy formed from a huge cloud of gas, with the halo stars forming first and the disk stars forming later, after the gas settled into a spinning disk Read: Chap 19 11/12/14 slide 32

48 of the Milky Way Spectral types to estimate luminosity L Herschel ( 1800) and Kapteyn ( 1920) counted stars to infer the shape of the Galaxy Brightness-distance relation, b = L/(4πR 2 ), to map the locations of the stars His answer: we are near the center of an elliptical distribution of stars! Read: Chap 19 11/12/14 slide 33

49 of the Milky Way Spectral types to estimate luminosity L Herschel ( 1800) and Kapteyn ( 1920) counted stars to infer the shape of the Galaxy Brightness-distance relation, b = L/(4πR 2 ), to map the locations of the stars His answer: we are near the center of an elliptical distribution of stars! What happened? Read: Chap 19 11/12/14 slide 33

50 of the Milky Way Spectral types to estimate luminosity L Herschel ( 1800) and Kapteyn ( 1920) counted stars to infer the shape of the Galaxy Brightness-distance relation, b = L/(4πR 2 ), to map the locations of the stars His answer: we are near the center of an elliptical distribution of stars! What happened? Interstellar extinction! Read: Chap 19 11/12/14 slide 33

51 How did astronomers figure this out? 1920s: Harlow Shapley measured distance to globular clusters using RR Lyrae variables Deduced the correct size and shape of the Milky Way Read: Chap 19 11/12/14 slide 34

52 How did astronomers figure this out? 1920s: Harlow Shapley measured distance to globular clusters using RR Lyrae variables Deduced the correct size and shape of the Milky Way Read: Chap 19 11/12/14 slide 34

53 How did astronomers figure this out? 1920s: Harlow Shapley measured distance to globular clusters using RR Lyrae variables Deduced the correct size and shape of the Milky Way Read: Chap 19 11/12/14 slide 34

54 How did astronomers figure this out? 1920s: Harlow Shapley measured distance to globular clusters using RR Lyrae variables Deduced the correct size and shape of the Milky Way Read: Chap 19 11/12/14 slide 34

55 with studying Milky Way Like being inside a forest, and seeing only the trees Because our view is obscured, it looks as if there are about as many stars in all directions Need to use radio waves or other forms of radiation to see through interstellar matter What we now know: The Milky Way is a spiral galaxy containing about 100 billion stars Read: Chap 19 11/12/14 slide 35

56 from neutral hydrogen The n = 1 level (ground state) of H is split into 2 levels separated by a very small energy This splitting is due intrinsic spin of electron and proton Behave like small magnets When the North poles are aligned, the energy is higher and vice versa Read: Chap 19 11/12/14 slide 36

57 from neutral hydrogen The n = 1 level (ground state) of H is split into 2 levels separated by a very small energy This splitting is due intrinsic spin of electron and proton Behave like small magnets When the North poles are aligned, the energy is higher and vice versa Read: Chap 19 11/12/14 slide 36

58 from neutral hydrogen HI emission is rare (per atom) but hydrogen is plentiful in the Galaxy Read: Chap 19 11/12/14 slide 37

59 Rotation of the Disk Measure using the Doppler Effect Read: Chap 19 11/12/14 slide 38

60 Rotation of the Disk Measure using the Doppler Effect Stars: Doppler shifts of stellar absorption lines Ionized Gas: emission lines from HII regions Atomic Hydrogen (HI) Gas: Cold H clouds emit a radio emission line at a wavelength of 21-cm Can trace nearly the entire disk beyond where the stars have begun to thin out Read: Chap 19 11/12/14 slide 38

61 the Milky Way Distribution of gas in velocity along the line of sight distance map Read: Chap 19 11/12/14 slide 39

62 the Milky Way Read: Chap 19 11/12/14 slide 40

63 the Milky Way NGC 1232 (similar to the Milky Way, face on... ) Read: Chap 19 11/12/14 slide 41

64 the Milky Way NGC 4565 (similar to the Milky Way, edge on... ) Read: Chap 19 11/12/14 slide 42

65 of the Milky Way from above Milky Way: four-armed barred spiral Read: Chap 19 11/12/14 slide 43

66 of the Milky Way from above Milky Way: four-armed barred spiral Read: Chap 19 11/12/14 slide 44

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