1	Astro 1050 Wed. Nov. 12, 2003 Today: Ch. 12: The Milky Way
2	Possible Origin of Spiral Arms Differential rotation smears features out into spiral patterns But can’t be whole story: Number of times Sun has orbited the galaxy: 10 billion yr/200 million yr = 50 times Spiral arms would have been wound up very tightly Something must continuously rebuild them
3	Degree of Organization of the Spiral Arms Different degrees of organization Grand Design Spirals: M51 Flocculent (“wooly”) Spirals
4	Degree of Organization of the Spiral Arms Different degrees of organization Grand Design Spirals: M51 Flocculent (“wooly”) Spirals
5	Tracing the Spiral Arms Arms NOT obvious if you look at: Old objects like the sun Arms ARE obvious if you look at: Maps of gas clouds 21 cm Hydrogen Radio maps of CO Far infrared observations of dust Young stars O, B stars “HII” ionized hydrogen regions surrounding O,B stars Clouds somehow form in arms , then dissipate between them Short lived objects only get a short distance from their places of birth O stars, Lifetime = few million years, at 250 km/s Þ500 pc
6	M51: The Whirlpool Galaxy
7	Density Wave Theory SPIRAL WAVE rotates with galaxy, but slower than individual stars Like moving traffic jam after an accident has been cleared Gas (and stars) catch up with wave, move through it, eventually reach front Just like cars catching up with moving traffic jam, eventually get through it Gas is more crowded in wave – clouds collapse to form new stars More collisions in the traffic jam There are slightly more old stars in the arm too, because they speed up slightly coming into it and slow down slightly moving out of it. But the best tracers are the things that mark recent cloud collapses: O,B stars, etc.
8	M51: The Whirlpool Galaxy
9	Self Sustaining Star Formation Cloud collapse Þ New stars New stars Þ Supernova after few million years Supernova Þ Shock Waves Shock Waves Þ Nearby clouds collapse Differential Rotation twists pattern into spiral
10	Two limiting cases of spirals Grand Design: Density Wave Flocculent: Self Sust. Star Form. + Diff. Rot. In most Galaxies you have some combination of the two
11	The Nucleus of the Galaxy Likely Black hole High velocities Large energy generation At a=275 AU P=2.8 yr Þ 2.7 million solar masses Radio image of Sgr A about 3 pc across, with model of surrounding disk
12	A movie of stars at the core www.mpe.mpg.de/www_ir/GC Very cool, brand new, and worth a look! This is the best evidence to date for a massive black hole at the Galactic core. Now essentially “proven.”
13	A movie of stars at the core