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:

• The average density of a supermassive black hole can be very low, and actually can be lower than water's density. This happens because the black hole diameter increases linearly with mass, and consequently density drops much faster.
• 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 any 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 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.