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Supermassive black hole

A supermassive black hole is a black hole with a mass in the range of millions or billions solar masses.

A supermassive black hole has some interesting properties differing from its low-mass cousins:

Black holes of this size can only form in two ways: by slow accretion of matter (starting from a stellar size), or directly from external pressure in the first instants of Big Bang. The first method requires a long time and large amounts of matter available for the black hole growth.

Most if not all galaxies are thought to host a supermassive black hole in their center. Direct redshift measures of the matter surrounding the nucleus have revealed a very fast motion, only possible with a high concentration of matter in the center. Currently, the only known object that can pack enough matter in such a small space is a black hole.

Such supermassive black holes in the center of many galaxies are thought to be the "engine" of active objects such as seyfert galaxies and quasars. Sagittarius A* is believed to be the supermassive black hole residing at the center of the Milky Way.