Scientists discover an accumulation of black holes in the heart of the globular cluster.

A tight noose of stars almost as old as the universe hides a dark secret at its center.

The globular cluster NGC 6397, a conglomerate of stars located about 7800 light-years away from Earth, is probably home to a cluster of small black holes in the heart, according to a new study.

The researchers conducted research on star motion in NGC 6397 using NASA's Hubble Space Telescope and the European Space Agency's Gaia Space Probe. These movements revealed the existence of a hidden mass at the center of the cluster, a “dark central component,” which accounts for 0.8 to 2% of the total mass of NGC 6397.

That inferred mass is consistent with an intermediate black hole, a cosmic beast midway between stellar-mass black holes, which form after the collapse of big stars, and the supermassive beasts that sit at the cores of most, if not all, galaxies.

The intermediate black holes are elusive; only a few candidates have been recovered until now. And the black mass of NGC 6397 does not belong to these special ranks.

"The small, effective radius of the diffuse dark component suggests that it is composed of compact stars (white dwarfs and neutron stars) and stellar-mass black holes," authors, Eduardo Vitral and Gary Mamon, both of the Paris Institute of Astrophysics in France, wrote in the new study, which was published online Thursday (Feb. 11) in the magazine Astronomy & Astrophysics.

The black holes of the star mass "should dominate the mass of this diffuse dark component, unless more than 25% escape from the cluster," they said.

"Ours is the first study to provide both the mass and the extent of what appears to be a collection of mostly black holes in the center of a core-collapsed globular cluster," Vitral said in a NASA statement, referring to a type of cluster with an especially dense nucleus.

The new study may have applications that affect much more than NGC 6397, one of the closest globular clusters on Earth. For example, if tightly packed black holes are a common feature of core-collapsed clusters, Vitral and Mamon note, these collections of stars may be a prominent source of the gravitational waves detected by the Laser Interferometer Gravitational-Wave Observatory.