<P> Donald Lynden - Bell and Martin Rees hypothesized in 1971 that the center of the Milky Way galaxy would contain a supermassive black hole . Sagittarius A * was discovered and named on February 13 and 15, 1974, by astronomers Bruce Balick and Robert Brown using the Green Bank Interferometer of the National Radio Astronomy Observatory . They discovered a radio source that emits synchrotron radiation; it was found to be dense and immobile because of its gravitation . This was, therefore, the first indication that a supermassive black hole exists in the center of the Milky Way . </P> <P> The origin of supermassive black holes remains an open field of research . Astrophysicists agree that once a black hole is in place in the center of a galaxy, it can grow by accretion of matter and by merging with other black holes . There are, however, several hypotheses for the formation mechanisms and initial masses of the progenitors, or "seeds", of supermassive black holes . </P> <P> One hypothesis is that the seeds are black holes of tens or perhaps hundreds of solar masses that are left behind by the explosions of massive stars and grow by accretion of matter . Another model hypothesizes that before the first stars, large gas clouds could collapse into a "quasi-star", which would in turn collapse into a black hole of around 20 M . The "quasi-star" becomes unstable to radial perturbations because of electron - positron pair production in its core and could collapse directly into a black hole without a supernova explosion (which would eject most of its mass, preventing the black hole from growing as fast). Given sufficient mass nearby, the black hole could accrete to become intermediate - mass black hole and possibly a SMBH if the accretion rate persists . </P> <P> 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 could have been produced directly from external pressure in the first moments after the Big Bang . These primordial black holes would then have more time than any of the above models to accrete, allowing them sufficient time to reach supermassive sizes . Formation of black holes from the deaths of the first stars has been extensively studied and corroborated by observations . The other models for black hole formation listed above are theoretical . </P>

Black hole at the centre of the milky way galaxy