The Milky Way, Our galaxy

Transcription

1 The Milky Way, Our galaxy Diffuse Band of light that crosses the Sky in the North Galileo: it s faint stars Early speculation and fleshing out where we are 1

2 Milky Way from Zuerich

3 Milky Way from Australia

4 Milky Way from Australia

5 Milky Way Diffuse band of light crossing the night sky. Naked eye, all cultures seen and named it: a Celestial River a Celestial Road or Path Our words "Galaxy" and "Milky Way" are derived from Greek and Latin: Greek: Galaxias kuklos = "Milky Band" Latin: Via Lactea = "Road of Milk" 5

6 "The Starry Messenger Galileo (1610) Galileo observed the Milky Way with his telescope Published the pamphlet, Siderius Nuncius (The Starry Messenger) "For the Galaxy is nothing else than a congeries of innumerable stars distributed in clusters." This was the first observation that showed that the Milky Way was simply made of many many unresolved faint stars. 6

7 Go outside on a foggy day, you are clearly at the center of the Universe! Peeking ahead Absorption absorption has an attenuation length of ~1kpc vertical structure of the disk is ~0.2kpc radial structure is ~4-5kpc we are 10kpc from the center of the galaxy, the Absorption hides radial structure, but we see that the structure is flattened 7

8 Early Models/Ideas: Thomas Wright (1750): Motivated by theology No new observations. His Milky Way was a thin spherical shell of stars. Sun is inside the shell; midway between the inner and outer edges Looking along the shell: See a broad band of stars ("Milky Way") Look out the thin part of the shell: See few stars Woodcut by Wright 8

9 Early Ideas II. Immanuel Kant (1755): Misread a newspaper account of Wright's model. Also made no observations of his own. Lens-shaped disk of stars rotating about its center. No special place for the Sun. Other "nebulae" are distant Milky Ways like ours Became known as the "Island Universe" Hypothesis, a term first used by Alexander von Humboldt, Cosmos (1845). 9

10 William & Caroline Herschel s Star Gages (1785) Heroic new observations: Counted stars along 683 lines of sight using their 20-foot long telescope with a 19-inch mirror Assumed that stars are uniformly distributed through space, out to the edges of the Milky Way. Assumed that their telescope could resolve all stars within the the Milky Way. Herschel did not assume that all stars are the same luminosity for his Star Gage method to work but he made that assumption in other work. Flattened Milky Way ("grindstone") Sun located near-ish the center. 10

11 It s always stated incorrectly that the Sun was at the center 11

12 Jacobus Kapteyn ( ) Universe Star counts from photographic plates Estimated distances statistically based on parallaxes & proper motions of nearby stars Neglected absorption!! Milky Way is a flattened disk ~15 kpc across & ~3 kpc thick The Sun is located off-center (again, it s always said nearly at the center, but ~40% of all stars were closer to the center) 12

13 Harlow Shapley (1915 thru 1921) Shapley noted two facts about Globular Clusters: Half above & half below the Milky Way. Concentrated on the sky toward Sagittarius Estimated Globular Cluster distances from observations of their RR Lyrae stars Used these distances to map the globular cluster distribution in space. Sun is at 0,0; His Galaxy center is Red X 13

14 Globular clusters Compact, spherical group of stars Up to several stars ( solar luminosities typical) All stars formed together, same age Form a halo (metal poor) and thick disk (not so metal poor) around the Milky Way

15 M15

16 M13

17 Dust Absorptions would alter the HR diagram of a Globular Cluster in very specific ways that let you determine absorption and reddening

18 Globular cluster system

19 Globular clusters in Sagittarius

20 GC distribution

21 Shapley s Final 1921 Conclusion GCs are objects centered on the Milky Way The Sun is 16 kpc from the MW center. MW is a flattened disk ~100 kpc across Right basic result, but too big Shapley ignored interstellar absorption Caused him to overestimate the distances. 21

22 Back to Absorption by Dust Interstellar space is filled with gas and dust Dust absorbs and scatters starlight Distant objects look fainter than they would be if there were no interstellar dust. When ignored, you get serious overestimates of Luminosity Distances. Absorption by Interstellar Dust affects all attempts to map the Milky Way: Shapley & Kapteyn ignored so overestimated the size of the Milky Way. Robert Trumpler (1930) showed that interstellar dust absorption was significant. 22

23 How can you tell the effect of dust? Extinction, the light is absorbed Reddening blue light is preferentially scattered (infrared provides a clear view Polarization, scattering of light leads to polarization 23

24 Reddening If particles are very small, scattering is Rayleigh scattering and attentuation would be proportional to λ -4 If absorption were by large rocks, it would be independent of wavelength λ 0 What we see is λ -1 This means that the dust particles are comparable to the wavelength of light 24

25 How can we tell something is reddened? If I have a very hot O or B star, the lines are distinctive. If I see lines that tell me that I ve got a very hot (bluish) star, but my broad band colors (define) are red, then the object must be reddened put succinctly the spectral lines would be at odds with the continuum emission 25

26 How can I tell that there is absorption? If I have objects that are extended (like a globular star cluster) and I see they are getting fainter than I expect by looking at their angular size, that s absorption Absorption is patchy, where is the most likely blue hole? Star counts betray an absorption patch and we can measure the distance to the dark cloud 26

27 Ultimately I should see absorption and reddening correlated, which is the case 27

28 Polarization Because the grains are small (e.g. slightly smaller than visual light), they are easily aligned with interstellar magnetic fields See the polarization of light passing thru grains tells us about both the grains and the B field You selectively eliminate light with polarization vectors aligned along the dust grains, the grains align with fields 28

29 Modern Model Above is 2 micron survey Sun is 8 kpc from the Galactic Center, which is in the direction of the constellation of Sagittarius The disk is ~30 kpc in diameter and ~0.5 kpc thick (0.25 each direction) at the location of the Sun. Obscuration is key AND far less in the Infrared!! 29

30 Spiral Nebulae William Parsons, 3rd Earl of Rosse (c. 1845) Built a 72-inch telescope known as the "Parsonstown Leviathan" Discovered the "Spiral Nebulae" Appeared to be disks with a spiral pattern to them Some appeared edge-on disks bisected by dark bands His telescope could not resolve them into stars. 30

31 Early big eyes Herschel s 20ft; ft; 48 Lord Rosse 72in mirror 53ft

32 Rosse drawings and photos Car Whirlpool M51 32

33 Rosse drawings and photos Car Whirlpool M51 Crab Neb. M1 33

34 Owl Neb M97 M99 galaxy 34

35 Island Universe Hypothesis Kant's idea (1755) boosted by von Humboldt (1845): Spiral Nebulae are other Milky Ways (or galaxies) made of stars. Very distant and external to our Galaxy. The Milky Way is just one of many galaxies in a vast Universe of Galaxies 35

36 Nebular Hypothesis Revival of a Solar System model of Pierre Simone Laplace (1796) and applied to other nebulae Spiral Nebulae are swirling gas clouds Nearby and internal to our Milky Way Might be forming solar systems The Milky Way is the Universe. 36

37 Stephen Alexander Synthesis of 1852 Careful study of Rosse s photographs and consideration of Nebular color Divided Green Nebulae and White Nebulae White nebulae are galaxies like ours Green nebulae are planetary nebulae Alexander really nailed it, but the history and astronomy books don t give him fair credit Had to be rediscovered years later 37