Astro 1050     Wed. Nov. 12, 2003

	Today: Ch. 12: The Milky Way

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

Degree of Organization
 of the Spiral Arms

	Different degrees of organization
		Grand Design Spirals: M51
		Flocculent (“wooly”) Spirals

Degree of Organization
 of the Spiral Arms

	Different degrees of organization
		Grand Design Spirals: M51
		Flocculent (“wooly”) Spirals

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

M51:  The Whirlpool Galaxy

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.

M51:  The Whirlpool Galaxy

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

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

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

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.”

A movie of stars at the core