Chapter 16 The Milky Way

Herschel “discovered” that we live in a disk of stars sun

Doppler-velocity measurements tell us we live in a spiral galaxy

Disk Nuclear bulge Halo Components of a spiral galaxy:

Spiral Galaxy disk bulge halo

Disk Component: stars of all ages, many gas clouds Spheroidal Component: bulge & halo: old stars, Very little gas & dust

Halo Stars: Population II % 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

Stars in the disk all orbit in the same direction with a little up-and-down motion

Orbits of stars in the bulge and halo have random orientations

Multiple supernovae create huge hot bubbles that can blow out of disk Gas clouds cooling in the halo can rain back down on disk

Star-gas-star cycle Recycles gas from old stars into new star systems

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

Fritz Zwicky discovered dark matter in 1933 (Coma cluster)

Sun’s orbital motion (radius and velocity) tells us mass within Sun’s orbit: 1.0 x M Sun The total amount of light suggests ~ few x 10 9 M sun Dark matter!

Swirling gas near centerOrbiting star near center Galactic Center

Stars appear to be orbiting something massive but invisible … a black hole?

X-ray flares from galactic center suggest that a black hole occasionally tears apart chunks of matter as it falls in

Galactic Center (Chandra, Hubble, Spitzer)

Best hypothesis for how galaxies form: 1. A giant cloud of H and He condensed after the Big Bang

2. halo stars (oldest) formed first, in clumps, cloud contracted due to gravity. These clumps later merged.

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