Black holes can be petite, with masses only about 10 times that of our sun — or monstrous, boasting the equivalent in mass up to 10 billion suns. Do black holes also come in size medium? NASA’s Nuclear Spectroscopic Telescope Array, or NuSTAR, is busy scrutinizing a class of black holes that may fall into the proposed medium-sized category.
“Exactly how intermediate-sized black holes would form remains an open issue,” said Dominic Walton of the California Institute of Technology, Pasadena. “Some theories suggest they could form in rich, dense clusters of stars through repeated mergers, but there are a lot of questions left to be answered.”
The largest black holes, referred to as supermassive, dominate the hearts of galaxies. The immense gravity of these black holes drags material toward them, forcing the material to heat up and release powerful X-rays. Small black holes dot the rest of the galactic landscape. They form under the crush of collapsing, dying stars bigger than our sun.
Evidence for medium-sized black holes lying somewhere between these two extremes might come from objects called ultraluminous X-ray sources, or ULXs. These are pairs of objects in which a black hole ravenously feeds off a normal star. The feeding process is somewhat similar to what happens around supermassive black holes, but isn’t as big and messy. In addition, ULXs are located throughout galaxies, not at the cores.
The bright glow of X-rays coming from ULXs is too great to be the product of typical small black holes. This and other evidence indicates the objects may be intermediate in mass, with 100 to 10,000 times the mass of our sun. Alternatively, an explanation may lie in some kind of exotic phenomenon involving extreme accretion, or “feeding,” of a black hole.