<P> Some galaxies, such as the galaxy 4C + 37.11, appear to have two supermassive black holes at their centers, forming a binary system . If they collided, the event would create strong gravitational waves . Binary supermassive black holes are believed to be a common consequence of galactic mergers . The binary pair in OJ 287, 3.5 billion light - years away, contains the most massive black hole in a pair, with a mass estimated at 18 billion M . A supermassive black hole was recently discovered in the dwarf galaxy Henize 2 - 10, which has no bulge . The precise implications for this discovery on black hole formation are unknown, but may indicate that black holes formed before bulges . </P> <P> On March 28, 2011, a supermassive black hole was seen tearing a mid-size star apart . That is the only likely explanation of the observations that day of sudden X-ray radiation and the follow - up broad - band observations . The source was previously an inactive galactic nucleus, and from study of the outburst the galactic nucleus is estimated to be a SMBH with mass of the order of a million solar masses . This rare event is assumed to be a relativistic outflow (material being emitted in a jet at a significant fraction of the speed of light) from a star tidally disrupted by the SMBH . A significant fraction of a solar mass of material is expected to have accreted onto the SMBH . Subsequent long - term observation will allow this assumption to be confirmed if the emission from the jet decays at the expected rate for mass accretion onto a SMBH . </P> <P> In 2012, astronomers reported an unusually large mass of approximately 17 billion M for the black hole in the compact, lenticular galaxy NGC 1277, which lies 220 million light - years away in the constellation Perseus . The putative black hole has approximately 59 percent of the mass of the bulge of this lenticular galaxy (14 percent of the total stellar mass of the galaxy). Another study reached a very different conclusion: this black hole is not particularly overmassive, estimated at between 2 and 5 billion M with 5 billion M being the most likely value . On 28 February 2013 astronomers reported on the use of the NuSTAR satellite to accurately measure the spin of a supermassive black hole for the first time, in NGC 1365, reporting that the event horizon was spinning at almost the speed of light . </P> <P> In September 2014, data from different X-ray telescopes has shown that the extremely small, dense, ultracompact dwarf galaxy M60 - UCD1 hosts a 20 million solar mass black hole at its center, accounting for more than 10% of the total mass of the galaxy . The discovery is quite surprising, since the black hole is five times more massive than the Milky Way's black hole despite the galaxy being less than five - thousandths the mass of the Milky Way . </P>

How big is black hole at center of milky way