Dense swarms. The globular clusters M15 (left) in our Milky Way and G1 in the nearby Andromeda galaxy both harbor medium-size black holes at their cores.

Even Globular Clusters Have Black Holes

Peer deeply into a star system of any size, and you'll probably find a black hole. That's the lesson from new observations by the Hubble Space Telescope, which has spotted the signs of midsize black holes at the hearts of ancient stellar swarms called globular clusters. The masses of these black holes suggest that some precise but unknown cosmic recipe dictates how large a given black hole will become.

Discoveries during the last 2 decades have unveiled holes of many sizes. The deaths of giant stars in supernova explosions can create black holes with several times the mass of our sun. At the other extreme, galaxies harbor supermassive black holes millions or even billions of times more massive. Last year, x-ray astronomers also found hints of "intermediate" black holes with hundreds to thousands of times our sun's mass in other galaxies (ScienceNOW, 7 June 2001), but they hadn't measured the gravitational pulls of such holes--the best way to confirm their presence and gauge their masses.

Now, Hubble has done just that for two globular clusters: M15, in our Milky Way, and G1, in the nearby Andromeda galaxy. The clusters are tight knots of hundreds of thousands to millions of stars that orbit around galactic centers like moths around a streetlamp. Two research teams used Hubble's sharp vision to spy stars moving at the cores of the clusters. The rapid motions could arise only from the strong gravity of hidden objects: black holes with 4,000 solar masses in M15 and 20,000 solar masses in G1. Astronomers announced the results on 17 September at NASA Headquarters in Washington, D.C.

Astronomers had long debated whether globular clusters were massive enough for black holes to form, either when the clusters condensed in the early universe or when gas and stars accumulated at their cores. Curiously, the fraction of each cluster's mass that resides in the black hole--about 0.5%--is the same ratio seen for supermassive black holes in the central bulges of giant galaxies. "Whenever you see such a perfect relationship in astronomy, there's almost always an underlying cause," says astronomer Karl Gebhardt of the University of Texas, Austin, a member of both research teams. However, it's not yet clear whether all black holes and their host star systems are born with that half-percent ratio, or whether they grow at the same rate over time. Nor do astronomers know whether all globular clusters house black holes today, or whether many lost theirs when gravitational jostling at their crowded hearts flung the holes into space.

Related sites
News release and background from Space Telescope Science Institute
A primer on globular clusters

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