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Supermassive Black HoleA supermassive black hole is a black hole with a mass in the range of millions or billions solar masses. The Milky Way is believed to have one supermassive black hole, at the Galactic Center. Most if not all galaxies are thought to host a supermassive black hole in their center. A supermassive black hole has some interesting properties differing from its low-mass cousins: - The average density of a supermassive black hole can be very low, and actually can be lower than the density of water. This happens because the black hole radius increases linearly with mass, and consequently density drops with the square of the mass.
- The tidal forces in the vicinity of the event horizon are significantly weaker. Since the central singularity is so far away from the horizon, a hypothetical astronaut travelling towards the black hole center would not experience significant tidal force until very deep into the black hole.
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 Doppler measures of the matter surrounding the nucleus of nearby galaxies 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. For active galaxies farther away, the width of broad spectral lines is thought to be related to the mass of the black hole that powers the active galaxy's "engine". 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. In May 2004, Paolo Padovani and other leading astronomers announced their discovery of 30 previously hidden supermassive black holes outside the Milky Way. Their discovery also suggests there are at least twice as many of these black holes as previously thought. See also Classification by type: A classification by mass:
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