1. The stars in context: the Milky Way Galaxy
Lecture 12,
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CAPSTONE Galaxy

2. The Nebulae Kant figured it out (1754)
Herschel (William) and his son John, both lords, tried to prove Kant right (1785, 1850)
1850, Sir John admitted that his father’s great quest had failed.
By 1900, virtually all astronomers agreed that the extent of the Universe was the edge of the Milky Way (~100 kpc).
By 1918, Harlow Shapley had shown that the Galaxy had a center and that the Sun was not near the center.
CAPSTONE Galaxy

3. Great Debate Great debate, Heber Curtis and Harlow Shapley. 1921.
Analogy of dust lanes to Cygnus rift, Curtis. Shapley won ( all the points were on his side)
Curtis was right, but formally lost
Lamb/Pacynski and GRB, 2nd debate.
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4. Cygnus Rift (Encyclopedia of Astronomy)
Regions with no stars are dust clouds
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5. Curtis’ argument
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6. Hubble and expansion
Cepheid variables proved that some galaxies were far way (recognized by variability and VERY luminous).
By 1929, the expansion of the Universe was known, clearly separating galaxies from Galactic nebulae.
Note: it was known that Galactic nebulae had emisson spectra and galaxies had continous (stellar) spectra, but no one figured it out.
CAPSTONE Galaxy

7. Overview of the Galaxy Scale—15kpc, 8kpc from Sun to Center.
Spiral structure—Morgan (1952), by mapping stars, others later in 21cm emission of H I.
Arms 1kpc across, 50 kpc long, we are at an inner edge.
Zone of obscuration.
Arms have : young stars, dust, forming stars, dying stars.
Rotation speed is 10’s of km/sec of pattern of spiral. Actual star motion is 250 km/sec.
This must be removed from measures of galaxies to get a true redshift, just as Earth’s motion around the Sun and the peculiar motion of the Sun must be removed to get the true motion of stars.
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8. Details Globular clusters: halo RR Lyr stars: halo
T Tau stars : near densest clouds.
White dwarfs: everywhere (but only seen locally because too faint)
Planetary nebulae: everywhere, but not many seen.
Red giants: everywhere, easiest halo stars to see.
AGB: everywhere
O stars: dust lanes
B stars: plane of galaxy, away from dust (which has moved out of the way)
A stars and cooler: everywhere, but of dramatically different ages.
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9. Details II Thick disk Thin disk Halo Galactic center
Star formation rate: 1 per year
Star death rate 1/hundred years (SNe)
Neighbors
SMC, LMC, Andromeda, local group, then nothing for 5Mpc.
Accretion of dwarf galaxies
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10. Map of the Universe http://www.astro.princeton.edu/~mjuric/universe/
CAPSTONE Galaxy