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SIZE OF MILKY WAY Kapteyn all visible stars – 30,000 parsecs sun close to center Shapley globular clusters – 100,000 parsecs sun 2/3 toward rim Population I – blue, young, found in disk particularly in arms, circular orbits, high concentration (3%) heavy elements Population II – red, old, found in bulge and halo, elliptical orbits, low concentration of heavy elements Probably smooth transition between end members (i.e. the sun). ? Population III ? – pure H and He TWO MODELS OF STAR FORMATION Density wave model Self-propagating star formation model Galaxy Shapes spiral (S) barred spiral (BS) spiral with no arms (S0) elliptical (E) irregular (Irr) Rotation of Galaxies all stars follow velocity/gravity relationships all galaxies have too little visible mass for rotation speeds Any model for the formation and evolution of galaxies must account for collisions! Other types of galaxies Radio Galaxies Seyfert Galaxies Quasars probably all black holes with accretion disks Stars are grouped in clusters and galaxies Galaxies are grouped in clusters. Even clusters of galaxies are clumped together.