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Galaxy Formation • Collapse of an over-dense region of space (containing more gas and dark matter than average) under gravity • Disks are produced as the cloud of material spins faster and faster as the gravitational collapse progresses • To conserve angular momentum, the spin speed must increase inversely proportional to the decreasing size of the cloud • Spiral galaxies — contain much raw material (hydrogen gas) for star formation and a large fraction of newborn stars; ongoing star formation • Elliptical galaxies — mostly formed their stars early; little or no current star formation; very little gas or dust More Galaxy Formation • Bulges of spiral galaxies are similar to certain elliptical galaxies; formed as a result of monolithic collapse of a cloud of gas and dark matter with very little angular momentum • Other elliptical galaxies and spiral galaxy halos are thought to have formed by merging smaller galaxies; merging process results in disordered stellar orbits • Important distinction between stars and hydrogen gas: gas is dissipative (a clump of gas tends to drag surrounding material with it due to viscosity); stars are non-dissipative Large Scale Structure • Galaxy Groups: up to ~10 galaxies; typical speeds of 100-500 km/s; typical size up to 1 Mpc (e.g., Local Group) • Galaxy Clusters: a few hundred galaxies; 500-2000 km/s; 10 Mpc (e.g., Virgo & Coma clusters) • Galaxy Superclusters: a few thousand galaxies; not necessarily gravitationally bound together; over 10 Mpc in size (e.g., Perseus-Pisces supercluster) • Peculiar velocities — velocity not associated with uniform Universal expansion; “Fingers of God” effect Disk Galaxies: Structural Components • Flattened differentially-rotating disk • Dense centrally-concentrated bulge with mostly disordered orbits • Extended, not centrally concentrated, mostly dark halo • Bulge + Halo = “Spheroid” Spiral Galaxy Properties • Bulge stars are older on average than disk stars • Youngest disk stars lie in very thin plane • Older disk stars lie in a thicker disk • Disk stars, particularly young ones, are organized into spiral arms • Spiral density waves in the disk: the most successful explanation of spiral structure Globular Clusters Globular Clusters • Most galaxies, including our own, contain dense clusters of 103 – 106 stars known as globular clusters • The observed distribution of globular clusters tells us that the Sun is NOT at the center of the Milky Way galaxy