Midsized Black Holes

First, it is important to determine just how massive a black hole must be to qualify as a giant. The standard stellar black holes that scientists believe exist in some binary star systems are mostly in the range of about eight to twenty, and occasionally up to about fifty, solar masses. By earthly and human standards, these are very massive objects to be sure. But in the last few years, evidence has been found for the existence of much more substantial black holes.

These larger black holes fall into two broad categories intermediate, or midsized, holes, and supermassive, or giant, holes. Since the early 1970s, astronomers had speculated about the possibility of midsized black holes, which they theorized would contain from a few hundred to several tens of thousands of solar masses. It was clear that such objects would most likely form in regions of densely packed stars and gas clouds; after all, the holes would have to have a lot of matter to feed on to grow so large. One such crowded region is a globular cluster, of which the Milky Way contains several hundred. Isaac Asimov describes globular clusters as stellar groups in which

some tens of thousands or even hundreds of thousands of stars are clustered together in a well-packed sphere. Here in our own neighborhood of the universe, stars are separated by an average distance of about 5 light-years. At the center of a globular cluster, they may be separated by an average distance of 1⁄2 light-year. A given volume of space in a globular cluster might include 1,000 times as many stars as that same volume in our own neighborhood.

Astronomers examined several globular clusters in the 1970s and found that they did emit high doses of X rays, as the likely black hole candidate Cyg X-1 did. However, no concrete evidence for midsized black holes in these star groups surfaced until 2002. Late that year, a team led by Roeland Van Der Marel at the Space Telescope Institute found two midsized black holes. One, possessing about four thousand solar masses, is in M15, a globular cluster in the Milky Way. The other resides in G1, a globular cluster in the neighboring Andromeda galaxy, and has roughly twenty thousand solar masses. In an interview following the

The globular cluster M15, located about thirty-three thousand light-years away in the constellation Pegasus, appears to have a large black hole in its center.

discovery, Luis Ho, one of the team members, exclaimed: “It’s very exciting to finally find compelling evidence that nature knows how to make these strange beasts.”

Early in 2003, another research team, this one led by Jon Miller at the Harvard-Smithsonian Center for Astrophysics, discovered two more midsized black holes. Situated in a spiral galaxy designated NGC 1313, lying at a distance of 10 million light-years from Earth, they each contain several hundred solar masses.