A supermassive black hole is a black hole with a mass of the order of between 105 and 1010 solar masses. Most, if not all, galaxies, including the Milky Way, are believed to contain supermassive black holes at their centers. Supermassive black holes have properties which distinguish them from their relatively low-mass cousins: There are several models for the formation of black holes of this size.

The most obvious is by slow accretion of matter starting from a black hole of stellar size. Another model of supermassive black hole formation involves a large gas cloud collapsing into a relativistic star of perhaps a hundred thousand solar masses or larger. The star would then become unstable to radial perturbations due to electron-positron pair production in its core, and may collapse directly into a black hole without a supernova explosion, which would eject most of its mass and prevent it from leaving a supermassive black hole as a remnant.

Yet another model involves a dense stellar cluster undergoing core-collapse as the negative heat capacity of the system drives the velocity dispersion in the core to relativistic speeds. Finally, primordial black holes may have been produced directly from external pressure in the first instants after the Big Bang.

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