1

Diffuse Band of light that crosses the Sky in the North Galileo: it’s faint stars Early speculation and fleshing out where we are

The Milky Way, Our galaxy

Milky Way from

Zuerich

Milky Way from Australia

Milky Way from Australia

5

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"

Milky Way

6

• 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.

"The Starry Messenger” Galileo (1610)

7

Go outside on a foggy day, you are clearly at the center of the Universe! Peeking ahead

• 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”

Absorption

8

• 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

Early Models/Ideas: Thomas Wright (1750):

Woodcut by Wright

9

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).

Early Ideas II. Immanuel Kant (1755):

10

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.

William & Caroline Herschel’s Star Gages (1785)

11

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

12

• 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)

Jacobus Kapteyn (1901-22) Universe

13

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.

Harlow Shapley (1915 thru 1921)

Sun is at 0,0; His Galaxy center

is Red X

Globular clusters

• Compact, spherical group of stars• Up to several 105-6 stars (105-6 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

M15

M13

Dust Absorptions would alterthe HR diagram of a Globular

Cluster in very specificways that let you determine absorption and reddening

Globular cluster system

Globular clusters in Sagittarius

GC distribution

21

• 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.

Shapley’s Final 1921 Conclusion

22

• 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.

Back to Absorption by Dust

23

• Extinction, the light is absorbed • “Reddening” blue light is preferentially scattered

(infrared provides a clear view • Polarization, scattering of light leads to

polarization

How can you tell the effect of dust?

24

• 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 λ0

• What we see is λ-1 This means that the dust particles are comparable to the wavelength of light

Reddening

25

• 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

How can we tell something is reddened?

26

• 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”

How can I tell that there is absorption?

27

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

Ultimately

28

• 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

Polarization

29

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!!

Modern Model

30

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.

Spiral Nebulae

31

Early big eyes

Herschel’s ☚20ft; 18.5” 40ft; 48”☛

Lord Rosse 72in mirror

53ft 1845

32

Car

Rosse drawings and photos

Whirlpool M51

33

Car

Rosse drawings and photos

Crab Neb. M1

Whirlpool M51

34

Owl Neb M97

M99 galaxy

35

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

Island Universe Hypothesis

36

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.

Nebular Hypothesis

37

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 70-80 years later

Stephen Alexander Synthesis of 1852