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Next: The Universe Up: Stars, Galaxies, and the Previous: Black holes


Stars are grouped into what are called galaxies, classified into four main types according to their shape and size. Our own galaxy, the Milky Way, is an example of a spiral galaxy.

Usually galaxies are rather tame and benign, but a few, perhaps about 10,000 of the billions, are called active galaxies. The most spectacular of these are quasars (quasi-stellar radio sources), which are violent explosive galaxies emitting vast amounts of radiation into the universe. It is unknown at present what processes lead to the formation of quasars; some believe they occur when an enormous black hole forms at the center of a galaxy and starts pulling in large amounts of matter.