Hubble finds many black holes in a nearby star cluster

Hubble finds many black holes in a nearby star cluster
Hubble finds many black holes in a nearby star cluster

Hubble has discovered several dozen black holes in the globular cluster NGC 6397. Some of them can be paired and generate gravitational waves. An article with the research results was published by the scientific journal Astronomy & Astrophysics.

Globular star cluster NGC 6397 is located at a distance of 7, 8 thousand light years from Earth. Observing it, astronomers from the Paris Institute of Astrophysics discovered that there is a large accumulation of invisible matter.

A detailed study showed that this cluster is not a point object, but a set of separate structures. The authors of the work suggest that their find confirms one of the theories that explains the merging of black holes.

The fact that such events occur relatively often in the Universe, scientists learned thanks to the LIGO and VIRGO observatories. They track the so-called gravitational waves - oscillations of space-time, the source of which are massive objects moving with varying acceleration. The existence of such waves more than a hundred years ago was predicted by Albert Einstein, but scientists were able to record them only in September 2015.

Over the past five years, scientists have recorded many bursts of gravitational waves. More than 50 of these were caused by mergers of stellar mass black hole pairs. Scientists do not yet know where and how such couples originated and what made them get closer to each other.

Scientists explain these events using two theories, both of which involve tight globular clusters of stars on the outskirts of galaxies. Some astronomers believe that pairs of black holes arise in binary star systems, which are composed of very large stars. Others speculate that black holes form alone, but then pair up due to gravitational interactions with their neighbors within the clusters.

Cluster NGC 6397 contains some of the oldest stars in the Milky Way, and its center is filled with extremely dense matter. Such globular clusters have long attracted the attention of astronomers, since their centers may contain intermediate-mass black holes - objects whose mass lies between stellar mass black holes and supermassive black holes.

Tracking the motion of stars on the outskirts of this cluster, the authors of the article discovered that in the central regions of NGC 6397 there is an invisible cluster of matter with a huge mass. At first, astronomers assumed they had found a black hole of intermediate mass.

However, later it turned out that invisible matter, the mass of which is about 1-2 thousand times greater than the sun, was dispersed throughout the central part of this cluster. This means that the cluster contains not one, but many objects of stellar mass. Some of them may be black holes, while others may be pulsars and white dwarfs.

After simulating the typical evolution of stars located in the center of NGC 6397, the researchers concluded that most of these invisible objects are still black holes. This means that in the center of this cluster there are several tens or even hundreds of black holes. Moreover, they are located relatively next to each other.

Astronomers believe that this number and density of black holes create ideal conditions for the formation of pairs of similar objects that emit gravitational waves. They hope that further observations of NGC 6397 and other similar globular clusters will confirm their hypothesis and help more accurately localize new sources of space-time oscillations.

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