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Published byEustace Wilson Modified over 9 years ago
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Chapter 16 The Milky Way
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Herschel “discovered” that we live in a disk of stars sun
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Doppler-velocity measurements tell us we live in a spiral galaxy
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Disk Nuclear bulge Halo Components of a spiral galaxy:
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Spiral Galaxy disk bulge halo
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Disk Component: stars of all ages, many gas clouds Spheroidal Component: bulge & halo: old stars, Very little gas & dust
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Halo Stars: Population II 0.02-0.2% heavy elements (O, Fe, …), only old stars Disk Stars: Pop. I 2% heavy elements, stars of all ages Halo stars formed first, then stopped Disk stars formed later, kept forming
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Stars in the disk all orbit in the same direction with a little up-and-down motion
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Orbits of stars in the bulge and halo have random orientations
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Multiple supernovae create huge hot bubbles that can blow out of disk Gas clouds cooling in the halo can rain back down on disk
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Star-gas-star cycle Recycles gas from old stars into new star systems
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The orbital speed (v) and radius (r) of a star on a circular orbit around the galaxy tells us the total mass (M r ) contained within that orbit Gravitational force = centripetal force
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Fritz Zwicky discovered dark matter in 1933 (Coma cluster)
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Sun’s orbital motion (radius and velocity) tells us mass within Sun’s orbit: 1.0 x 10 11 M Sun The total amount of light suggests ~ few x 10 9 M sun Dark matter!
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Swirling gas near centerOrbiting star near center Galactic Center
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Stars appear to be orbiting something massive but invisible … a black hole?
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X-ray flares from galactic center suggest that a black hole occasionally tears apart chunks of matter as it falls in
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Galactic Center (Chandra, Hubble, Spitzer)
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Best hypothesis for how galaxies form: 1. A giant cloud of H and He condensed after the Big Bang
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2. halo stars (oldest) formed first, in clumps, cloud contracted due to gravity. These clumps later merged.
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3. Bulge stars form next 4. Remaining gas settled into spinning disk 5. Star formation continues as long as there is material in the ISM
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