Elusive Middleweight Black Hole Found at Center of Giant Star Cluster

For decades, astronomers have tracked black holes with masses millions of times that of the sun, as well as those with tens of solar masses.

Globular Cluster 47 Tuc. Image credit: ESO/Wikipedia (CC BY 3.0)

Black holes between those two extremes have proved elusive. Now, astronomers studying a globular cluster have found just such a black hole at its center, showing that intermediate-mass black holes could be hiding out in these compact agglomerations of stars.

Astronomers found a black hole between 1,400 and 3,700 solar masses at the center of 47 Tucanae, a globular cluster in the southern sky some 16,700 light-years from Earth.

Black holes are usually found because they emit massive amounts of X-rays as matter falls in. Midsize black-hole candidates have been found in galaxies.

Lead study author Bülent Kiziltan, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA) and his team found this one by measuring motions of pulsars within the cluster. They found the telltale signs of a compact, massive object in the cluster's heart. The likeliest explanation for the motions was a black hole.

Theorists think stellar-mass black holes form from stars that are at least a few dozen times the mass of the sun. When they run out of nuclear fuel, there is no longer enough energy from radiation to hold the star's outer layers against its immense gravity.

The star collapses, and then explodes as a supernova (supernovas can outshine the galaxies in which they reside). What's left of the star then shrinks into a tiny volume. A 100-solar-mass star, as a black hole, would have a radius of about 180 miles (290 kilometers). The former star's escape velocity exceeds that of light, resulting in a black hole, from which nothing can escape.

Kiziltan's group tracked the movement of two dozen pulsars and used computer simulations to model the cluster to track down their black-hole candidate. The only reason it worked for 47 Tucanae was that there were enough pulsars in it to begin with, and they were close enough to see. (Tasnim News Agency)

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