Astronomers have discovered a black hole unlike any other previously discovered. Neither big enough to be a supermassive black hole, nor small enough to be the remnants of a collapsed star, its "average" size is a cosmic mystery.
The black hole, designated HLX-1, was discovered by a team of astronomers led by the University of Leicester's Sean Farrell. They found the black hole while searching for white dwarfs and neutron stars. HLX-1 is located just outside a galaxy some 290 million light-years away.
Previously, all black holes have fallen into one of two categories. The more commonly observed kind of black hole, the stellar-mass black hole, is the ultimate result of a massive star dying. The other kind, the supermassive black hole, can be anywhere from tens of thousands to a billion times larger than their smaller counterparts. Such black holes are generally thought to be found only at the center of galaxies.
The size of black holes can be determined using the Eddington limit. Named for the astrophysicist Sir Arthur Eddington, who devised the first version of this particular metric, the Eddington limit is the point at which the amount of material entering the black hole equals the amount of x-ray radiations it expels. This implies a direct relationship between the amount of radiation emitted and the black hole's gravitational force, which in turn can be used to determine the size of the black hole.
Farrell's team has measured the emitted radiation of HLX-1 to be ten times that of a normal stellar-mass black hole. That suggests its mass roughly 500 times bigger than our sun. Stellar-mass black holes are only about thirty times heavier than our sun. This places HLX-1 in an unprecedented intermediate stage between stellar-mass and supermassive black holes, and none of the currently theorized mechanisms for the formation of black holes can explain its existence.
There are two leading hypotheses to explain how HLX-1 came to be, and either would greatly expand and complicate our understanding of the cosmos. The preferred theory holds that HLX-1 is the result of multiple black holes being pulled together and then fusing to create one relatively large black hole. This would require HLX-1 to be located in what was once a densely packed globular cluster. Such clusters are generally extremely distant from their galaxy's center, which is where HLX-1 currently is.
Although that's strong circumstantial evidence to favor the above theory, I must admit I find the second possible explanation much more intriguing. It actually is possible that HLX-1 is the result of just one star collapsing, as long as the star in question was much, much bigger than any seen today. There is some thought that the most ancient stars were indeed considerably bigger than their modern counterparts, and so it is possible that HLX-1 is a remnant of the oldest generation of stars. Either way, HLX-1 has given astronomers yet another object to look for while scanning the skies, as it is the first of the newly dubbed intermediate-mass black holes.