the milky way and other galaxies science a-36 12/4/2007

The Milky Wayand 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 H Molecular 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 v2 ~ GM/r to get mass interior to Sun: 9.0 x 109 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• ~105 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)