Study: Medium black holes probably rare

PASADENA, Calif., Aug. 20 (UPI) -- U.S. space agency astronomers say a new study suggests black holes, for the most part, are either very large or very small, but seldom medium in size.

National Aeronautics and Space Administration astronomers said they have long suspected the most likely place to find a medium-mass black hole would be at the core of a miniature galaxy-like object called a globular cluster. Yet nobody has been able to conclusively find one.

But now, in a new study, astronomers say they've thoroughly examined a globular cluster called RZ2109 and determined it cannot possess a medium black hole. The findings, say the scientists, suggest the elusive objects do not lurk in globular clusters, and perhaps are very rare.

"Some theories say that small black holes in globular clusters should sink down to the center and form a medium-sized one, but our discovery suggests this isn't true," said Daniel Stern of NASA's Jet Propulsion Laboratory.

Stern, Stephen Zepf of Michigan State University and their colleagues report their findings in the Astrophysical Journal.