Black Holes Galore

If you like black holes, you might want to check out the core of NGC 6397, a giant ball of stars almost 8,000 light-years away. A recent study says the core could house dozens of black holes.

NGC 6397 is a globular cluster. It contains hundreds of thousands of stars. But the stars are ancient — as old as the Milky Way Galaxy.

All of the cluster’s big, heavy stars expired billions of years ago. When those stars died, they left small but heavy remnants — white dwarfs, neutron stars, or black holes. And in a cluster, such massive objects sink to the middle.

Earlier studies had suggested that there’s a small, dark mass at the center of NGC 6397 — perhaps a black hole a few hundred times the mass of the Sun.

The more recent study used the Hubble and Gaia space telescopes to plot the orbits of stars in the cluster. The way the stars move suggests they’re orbiting a large group of stellar remnants, not a heavier single black hole.

They total up to about 2,000 times the mass of the Sun. Most of that mass should consist of black holes. They range up to about 40 times the mass of the Sun. Black holes ought to occasionally kick out a normal star, or even merge with each other — stirring things up in this ancient cluster.

NGC 6397 is in Ara, the altar. From the U.S., it’s visible only from Hawaii and southern Texas and Florida. It’s in the south around midnight.
 

Script by Damond Benningfield