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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.
• Strong tidal forces are absent in the vicinity of the event horizon. 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 galaxiesSeyfert galaxies are spiral or irregular galaxies containing an extremely bright nucleus, most likely caused by a supermassive black hole, that can sometimes outshine the surrounding galaxy. The light from the central nucleus varies in less than a year, w and quasarA quasar (from quasi-stellar radio source is an astronomical object that looks like a star in optical telescopes (i. it is a point source), and has a very high redshift. The general consensus is that this high redshift is cosmological, the result of Hubbls. Sagittarius A*Sagittarius A is a complex radio source at the centre of our galaxy, the Milky Way Galaxy. It is located in the sky in the Sagittarius constellation. It consists of three components, the supernova remnant Sagittarius A East the spiral structure Sagittariu 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

• black hole, a general survey

Classifification of black holes by type:

• Schwarzschild or static black holeA Schwarzschild black hole or static black hole is a black hole fully defined by its only parameter, the mass M''. In general black holes could have in addition angular momentum ( rotating black holes) and electric charge (see charged black holes). A Schw
• rotating or Kerr black holeA rotating black hole Kerr black hole or Kerr-Newman black hole is a black hole that possesses angular momentum. It is one of three possible types of black holes that could exist in the theory of gravitation called General Relativity. Black holes can be c
• charged black hole or Newman black hole and Kerr-Newman black holeA charged black hole is a black hole that possesses electric charge. Since the electromagnetic repulsion in compressing an electrically charged mass is dramatically greater than the gravitational attraction (by about 40 orders of magnitude), it is not exp

Classification of black holes by mass:

• micro black hole and extra-dimensional black hole
• primordial black hole, a hypothetical leftover of the Big Bang
• stellar black hole, which could either be a static black hole or a rotating black hole
• supermassive black hole, which could also either be a static black hole or a rotating black hole