spacetoday.net: Middleweight black holes discovered

Posted: Thu, Sep 19, 2002, 10:14 AM ET (1414 GMT)
Astronomers have found evidence for medium-size black holes in the centers of globular clusters. Hubble Space Telescope observations confirmed that a black hole with a mass 4,000 times that of the Sun is in the center of the globular cluster M15, while another 20,000 times as massive as the Sun is in the center of another cluster, G1. Astronomers measured the motions of individual stars and groups of stars around the centers of these clusters to measure the mass of these objects. These black holes are more massive than the stellar-mass black holes left over from supernovae, but much smaller than the supermassive black holes, billions of times as massive as the Sun, found in larger galaxies. Astronomers did note that the ratio of the mass of the black hole to the overall mass of the galaxy is the same for both globular clusters and larger galaxies — 0.5 percent — suggesting that some unknown physical principle may be at work.