A globular cluster is a very dense star system in which objects are closely related by gravity and
These systems are usually very old.For example, globular cluster NGC 6397, which the authors of the study chose to study, is no older than the Universe. In addition, it is one of the closest globular clusters to Earth - about 7, 8 thousand light years to it. Due to its very dense core, it was classified as a compressed core cluster.
Astronomers decided to study a similar cluster NGC6397: It was expected to find a black hole of intermediate mass - this object was for scientists an intermediate link between supermassive black holes and black holes of stellar mass, 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 study authors
In order to find out how is distributedhidden mass, scientists used the speeds and masses of visible stars. It is known that the more mass in a 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 componentcan only consist of the remnants 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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