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)
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