The cluster NGC 6397 is one of approximately 150 spherical clouds of ancient stars that orbit the Milky Way and possibly lie before it is formed. It’s 13 billion years old and contains roughly 250,000 stars, all old, small, and dark. The brighter, heavier stars in the cluster burned through their lives long ago and met their destiny as black holes or other products of star decay.

So the cluster is a likely candidate for a medium-mass black hole. In fact, previous studies had indicated that a black hole about 600 solar masses anchored the center of NGC 6397.

To investigate this idea, Dr. Mamon and his student made high-resolution observations of the movements of individual stars in the cluster obtained from the Hubble Space Telescope and from Gaia. The faster the stars moved, the greater the gravitational force and therefore more mass has to be to keep them in the cluster.

A total of 1,905 stars from the Gaia catalog and 7,209 stars from Hubble were rated. As it turned out, they were actually under the gravitational influence of an invisible mass. But instead of tightly orbiting a single dark point, the stars moved in all directions, suggesting that the dark mass that was affecting them was not concentrated but expanded. There was no sign of a gigantic black hole.

“Our analysis showed that the orbits of the stars throughout the globular cluster are almost random and not systematically circular or very elongated,” said Dr. Mamon in an email.

The motions of these stars showed a dark mass of 1,800 suns spread out over a region – a cloud within a cloud – that was about a third of a light year wide. Around 40,000 ordinary, luminous, but very light stars share in this space. According to models of stellar evolution, Dr. Mamon, about two-thirds of this dark material would be black holes with an average mass of 20 suns. The rest of the dark material would be remnants of dead stars like white dwarfs or neutron stars.

The mix could change, he said, if the black holes merged, causing them to lose mass in the form of gravitational waves. Even the black holes, which are much more massive than stars, would gradually lose their orbital speed through encounters with other stars, a process known as “dynamic friction,” which would cause them to congregate in the inner regions of the cluster.