Rare Type of Black Hole Found

Typically, black holes come in two varieties.  There are the stellar black holes, formed when big stars go supernova, and supermassive black holes which dwell at the center of galaxies and are theorized to have a key role in galaxy formation.  Stellar black holes are anywhere from 10-100 solar masses, while supermassive black holes are upwards of a million solar masses.  Finding a black hole whose mass is somewhere in between those two ranges is incredibly rare, so rare that scientists dispute whether or not such things even exist, let alone agree on their characteristics.

Astronomers at the University of Maryland have found one of these rare, intermediate mass black holes.  The black hole in question lies in M82, a galaxy 12 million light years away.  M82 also happens to be the closest "starburst" galaxy, meaning it has an accelerated rate of star birth.  While observing this galaxy in the past, scientists noted an unusually bright source of X-rays, imaginatively named M82 X-1.  It was suspected that this object was an intermediate-mass black hole, but accurate estimates of its mass could not be obtained.

To get a more accurate mass estimate, the scientists measured individual x-ray particles from M82 X-1, finding a distinct pattern of light pulses which formed a 3:2 ratio.  This ratio could be used to measure the mass of the black hole, which is pretty amazing, when you think about it.  They found the black hole's mass to be 428 solar masses, give or take a 100.  Doesn't sound very accurate, but it does make this black hole definitively heavier than any stellar mass black hole.  Stars don't get that massive.  So, now the challenge is figuring out how a black hole of 400 solar masses forms.