The cluster NGC 6397 is one of about 150 spherical clouds of ancient stars that orbit the Milky Way and perhaps predate its formation. It is 13 billion years old and contains about 250,000 stars, all of them old, small and dim. The brighter, heavier stars in the cluster long ago burned through their lives and met their fate as black holes or other products of stellar decay.
So the cluster is a likely candidate to harbor an intermediate-mass black hole. Indeed, earlier studies had suggested that a black hole of about 600 solar masses anchored the center of NGC 6397.
To investigate that idea, Dr. Mamon and his student turned to high-resolution observations of the motions of individual stars in the cluster, obtained by the Hubble Space Telescope and Gaia. The faster the stars were moving, the greater the gravitational force, and hence more mass, must be present to hold them in the cluster.
In all, 1,905 stars from the Gaia catalog and 7,209 stars from Hubble were appraised. As it turned out, they were indeed under the gravitational influence of an invisible mass. But rather than tightly circling a single dark point, the stars were moving every which way, suggesting that whatever dark mass was influencing them was not concentrated, but extended. There was no sign of a gargantuan black hole.
“Our analysis indicated that the orbits of the stars are close to random throughout the globular cluster, rather than systematically circular or very elongated,” Dr. Mamon said in an email.
The motions of those stars offered evidence of a dark mass equal to 1,800 suns spread across a region — a cloud within the cloud — about one-third of a light-year wide. Sharing that space are some 40,000 ordinary, luminous but very lightweight stars. According to models of stellar evolution, Dr. Mamon said, about two-thirds of this dark stuff would be black holes, with an average mass of 20 suns. The rest of the dark material would be remnants of dead stars, such as white dwarfs or neutron stars.
The mix could change, he said, if the black holes merged, which would cause them to lose mass in the form of gravitational waves. Also the black holes, being much more massive than stars, would gradually lose their orbital speeds through encounters with other stars, a process called “dynamical friction,” which would lead them to congregate in the inner regions of the cluster.