Astronomers Theorize about Black Hole Sizes

A new study by Stephen Zepf of Michigan State University suggests that black holes may only occur in small and large sizes. The most likely place to find a medium-sized black hole is at the center of a globular cluster--a dense round clump of millions of stars. Zepf found a black hole, which was recognizable by its telltale X-ray signature, in a globular cluster called RZ 2109.  Using the M.W. Keck Observatory in Hawaii, he obtained an optical spectrum and found that the spectral line is broadened by the Doppler Effect. In other words, the black hole is spewing gas out of the cluster at 2000 kilometers per second-a sign that this black hole is actively swallowing up material.

However, it would be difficult for a small black hole to accrete material, and so would eject it in the form of high winds. According to theory, a medium black hole would either swallow a small black hole or eject it out of the cluster. The small black hole in RZ 2109 then, rules out the possibility of a medium hole. If this cluster is typical, then medium-sized black holes are probably very rare. Studies like this one help expand our knowledge of deep-space phenomena.