A black hole is an object whose gravity is so strong that nothing, not even light, can escape it.
Astronomers have long thought that black holes come in just two kinds, the "stellar" and the "supermassive." The type depends on the weight of the black hole, or more specifically, its mass.
Now, thanks to research done by two teams of astronomers, including one based at NASA's Goddard Space Flight Center in Greenbelt, Md., it looks like there is a third kind. Because it has a mass somewhere between these two types, it is called an "intermediate mass black hole."
Loosely speaking, mass and weight can be used interchangeably. The only difference is that in space far removed from the influence of Earth's gravity, your weight would be zero. By contrast, your mass would be the same no matter how near or far from Earth you were. Weight takes into account Earth's gravity while mass does not. Thus, mass is a more basic measure of how much "stuff" makes up an object. (The unit of mass in the Metric system is the kilogram. The pound is the English system of weight. One kilogram equals about 2.2 pounds. A 130-pound person has a mass of just about 59 kilograms.)
The problem is that unlike the "light-weight" stellar class and the "heavy-weight" supermassive class, there is no known way to form these intermediate mass black holes. Their existence, which has only been indirectly shown, cannot be accounted for under the existing theory of how black holes form. But if they are shown conclusively to exist, the theory will have to be revised.
The mass of a black hole is usually expressed in something called a "solar mass." One solar mass is defined as the mass of our Sun. This is a very large number, about 2 x 10^30 kilograms. That's 2 with 30 zeroes after it, or written out: 2,000,000,000,000,000,000,000,000,000,000. This is about one million times more than the mass of the Earth.
A stellar black hole forms when a massive star undergoes an explosive death called a supernova. This explosion, which can outshine an entire galaxy of stars for about a week, leaves behind the small, heavy core of a star. If this core is massive enough, it will collapse on itself and form a black hole. (Our Sun is much too small, or insufficiently massive, to form a black hole when it finally runs out of fuel.) A typical stellar-class of black hole has a mass between about 3 and 10 solar masses.
Supermassive black holes exist in the center of most galaxies, including our own Milky Way Galaxy. They are astonishingly heavy, with masses ranging from millions to billions of solar masses. Why they are so incredibly massive isn't known, but astronomers are pretty sure their development is linked to their presence at the center of their galaxy. There are so many stars and so much gas and dust that the black hole can grow large very quickly. And since many galaxies collide repeatedly during their long lifetimes, supermassive black holes have a ready-made way to collide and coalesce into even heavier supermassive black holes.
The intermediate mass black hole is estimated to have a mass between 100 and 1000 solar masses. No single star could ever form such a heavy black hole. The only way astronomers think such black holes could form is for a single black hole to devour lots and lots of material to get up to the required heft, or for individual black holes to merge together. However, both of these scenarios pose problems that astronomers cannot answer right now.
"The nature of these objects is one of the most interesting conundrums in high-energy astrophysics," said Tod Strohmayer, one of the Goddard astronomers researching the mystery of these perplexing middle weight black holes.