Updated: Feb 17, 2021
Nearly as old as the cosmos, a dense knot of stars hides a mysterious mystery at its heart.
Recent research suggests that the globular cluster NGC 6397, a conglomeration of stars around 7,800 light-years from Earth, potentially harbors a clump of tiny black holes at its core.
Using NASA's Hubble Space Telescope and the European Space Agency's Gaia spacecraft, researchers analyzed the rotation of stars in NGC 6397. The presence of a hidden mass at the center of the cluster, a "central dark component" that makes up 0.8 to 2 percent of the total mass of NGC 6397, was discovered by these motions.
There are elusive middle black holes; only a few candidates have been identified to date. And the dark mass of NGC 6397 is not a part of those fortunate 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," writes Eduardo Vitral and Gary Mamon, both from the Paris Institute of Astrophysics in France, wrote in a new review
"should dominate the mass of this diffuse dark component, unless more than 25 percent escape from the cluster," they added.
"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, referring to a type of cluster with a very dense nucleus in a NASA statement.
The latest research may have applications that reverberate well beyond one of the closest globular clusters to Earth, NGC 6397. For instance, if closely packed black holes are a typical feature of Vitral and Mamon notice core-collapsed clusters, these star collections can be a prominent source of the gravitational waves detected by the Laser Interferometer Gravitational-Wave Observatory.