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http://en.wikipedia.org/wiki/Supermassive_black_hole
Milky Way galactic center black hole
Further information: Sagittarius_A*#Supermassive_black_hole_hypothesis
Astronomers are confident that our own Milky Way galaxy has a supermassive black hole at its center, in a region called Sagittarius A*[8] because:
The star S2 follows an elliptical orbit with a period of 15.2 years and a pericenter (closest distance) of 17 light hours from the central object.[9]
Early estimates indicated that the central object contains 2.6 million solar masses and has a radius of less than 17 light hours. Only a black hole can contain such a vast mass in such a small volume.
Further observations[10] strengthened the case for a black hole, by showing that the central object's mass is about 3.7 million solar masses and its radius no more than 6.25 light-hours.
The Max Planck Institute for Extraterrestrial Physics and UCLA Galactic Center Group[11] have provided the strongest evidence to date that Sagittarius A* is the site of a supermassive black hole,[8] based on data from the ESO[12] and the Keck telescope.[13] Our galactic central black hole is calculated to have a mass of approximately 4.1 million solar masses,[14] or about 8.2 ?- 1036 kg.
[edit] Supermassive black holes outside the Milky Way
It is now widely accepted that the center of nearly every galaxy contains a supermassive black hole.[15][16] The close observational correlation between the mass of this hole and the velocity dispersion of the host galaxy's bulge, known as the M-sigma relation, strongly suggests a connection between the formation of the black hole and the galaxy itself.[15]
The explanation for this correlation remains an unsolved problem in astrophysics. It is believed that black holes and their host galaxies coevolved between 300-800 million years after the Big Bang, passing through a quasar phase and developing correlated characteristics, but models differ on the causality of whether black holes triggered galaxy formation or vice versa, and sequential formation cannot be excluded. The unknown nature of dark matter is a crucial variable in these models.[17][18]
At least one galaxy, Galaxy 0402+379, appears to have two supermassive black holes at its center, forming a binary system. Should these collide, the event would create strong gravitational waves. Binary supermassive black holes are believed to be a common consequence of galaxy mergers [19]. As of November 2008[update], another binary pair, in OJ 287, contains the most massive black hole known, with a mass estimated at 18 billion solar masses.[20]
Currently, there is no compelling evidence for massive black holes at the centers of globular clusters, or smaller stellar systems.[citation needed]
[edit] See also
