The Milky Way and Other Galaxies Science A-36 12/4/2007

Outline Observing the Milky Way Galactic center Components of Milky Way –Disk –Bulge –Stellar halo –Dark matter External galaxies

Observing the MW Dust obscures our view Star counting RR Lyrae in globular clusters (Harlow Shapley, 1920) Sun 8 +/- 1 kpc from Galactic center

Dust Far IR Near IR

Gas Atomic HMolecular H

Galactic center What does this mean?

A supermassive black hole! Stars move rapidly (~1000 km/s) Kepler’s law gives mass: ~3 million solar masses in ~3000 AU sphere Focus of orbits lines up with radio source Sgr A* Radiation from accretion disk Little x-ray emission, but flares give more evidence for SMBH

Components of the Milky Way

Disk Band of stars, gas, & dust we see in sky Diameter ~50 kpc, thickness ~600 pc Disks naturally result from collapse Objects in disk rotate: Sun at ~220 km/s Use v 2 ~ GM/r to get mass interior to Sun: 9.0 x 10 9 solar masses

Disk - star formation Young, Population I (metal-rich) stars -> active star formation in GMCs See knots of SF. Star-forming regions appear blue b/c short-lived, hot O and B stars dominate radiation. Cosmic recycling in ISM

Disk - spiral arms Mapped through 21 cm Not rotation alone: winding problem Density waves -- compression triggers star formation Blue knots are star formation -- O & B stars

Bulge Flattened sphere 2 kpc in diameter Not rotating Both Pop I and II stars No O or B stars -> no recent SF

Globuar clusters Gravitationally bound star clusters ~10 5 stars in sphere of radius ~3-10 pc All Pop II -- oldest stars in Galaxy Some GCs associated with disk/bulge -- relatively metal-rich Most (~150) in halo -- metal-poor NGC 7089 (from Clay Telescope)