A globular cluster is a very dense star system in which objects are tightly bound by gravity and
Such systems are usually very old.For example, the globular cluster NGC 6397, which the study authors chose to study, is no older than Universe. In addition, this is one of the closest globular clusters to Earth - approximately 7, 8 thousand light years away. Due to its very dense core, it is classified as a compressed core cluster.
Astronomers decided to study thiscluster NGC 6397: they expected to find an intermediate mass black hole in it - this object was for scientists an intermediate link between supermassive black holes and stellar-mass black holes, which are formed after the collapse of a single massive star.
We found compelling evidence forinvisible mass in the dense core of a globular cluster, but were surprised to find that this additional mass is not "point", which would be expected from a solitary black hole, but extended - up to a few percent of the size of the cluster.
Eduard Vitral, one of the authors of the study
In order to find out how it is distributedhidden mass, scientists used the speeds and masses of visible stars. It is known that the greater the mass in a certain place, the faster the stars move around it.
Accurate measurements of the parameters of stars in the NGC cluster6397 made the Hubble for several years, after which the data was compared with well-calibrated measurements of proper motion of stars provided by the Gaia space observatory of the European Space Agency.
As a result, it turned out that the invisible componentmay consist only of the remains of massive stars - white dwarfs, neutron stars and small black holes. And in the center of the globular cluster NGC 6397 is a cluster of small black holes.
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