They found 100 black holes inside a star cluster – science – life

Palomar 5 It is a unique stellar mass, first because it is one of the Milky Way’s most “spongy” halos, with an average distance of a few light years between stars, comparable to the distance from the Sun to the nearest star; And secondly, to associate it with a specular stellar stream that covers more than 20 degrees in the sky.

In an article published in natural astronomy, an international team of astronomers and astrophysicists led by University of Barcelona It shows that the two distinguishing features of Palomar 5 are likely the result of a population of over a hundred black holes in the center of the cluster.

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“The number of black holes is approximately three times greater than would be expected for the number of stars in the cluster, which means that more than 20% of its total mass is made up of black holes,” explains Mark Gillies, Professor University of Barcelona Institute of Cosmology (ICCUB) and lead author of the work.

“Each one has a mass twenty times the mass of the Sun, and was formed in supernova explosions at the end of the life of massive stars, when the cluster was still very young,” says the expert.

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Tidal currents are collections of stars that have been ejected from star clusters or dwarf galaxies. In recent years, about 30 narrow currents have been discovered in the Milky Way’s halo.

“We don’t know how these currents form, but one idea is that they are star clusters that have experienced some turbulence,” explains Giles. However, none of the newly discovered streams has an associated star cluster, so researchers cannot be sure of this theory.

To understand how these currents are formed, it is necessary to study one of them with a related star system: “Palomar 5 is the only case, and this makes it a kind of Rosetta Stone, which will allow us to understand the formation of stellar currents,” says Gillis. “That’s why we studied it in detail,” he explains.

The authors simulated the orbits and evolution of each star from mass formation to eventual melting. They altered the initial properties until they found that the current and cluster observations coincided. The team believes that Palomar 5 formed from a smaller portion of black holes, but stars were able to escape more efficiently from black holes, so the black hole portion gradually increased.

futuristic black hole group

Black holes have dynamically amplified mass through gravitational-assisted interactions with stars, causing more stars to escape and form a stream. Before it melts completely – about a billion years from now – the entire cluster will be made up of black holes.

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“This work has helped us understand that although Palomar 5 has the smartest and longest tails of any other cluster in the Milky Way, it is not unique. Instead, we believe that many black hole groups similarly dominate it. They have already broken up in tides in the Milky Way to form the first recently discovered stellar streams, says Dennis Erkall, co-author and study at University of Surrey (United kingdom).

As Gilles points out, the study shows that the presence of “a large number of black holes may be common to all groups in which streams formed.” This is important for understanding the formation of globular clusters and protostellar masses, as well as the evolution of massive stars. In addition, the action may have important effects on gravitational waves.

Many binary black hole mergers are thought to form in star clusters. An unknown in this scenario is the number of black holes in the clusters, which is difficult to determine by observation, because we cannot observe black holes,” says Fabio Antonini, Professor Cardiff University (Wales, UK), who is also a co-author of the work, adds: “Our method provides a way to find out how many black holes there are in a stellar mass by checking the stars emanating from it.”

Mass in the constellation Serpent

Palomar 5 is a globular cluster discovered by Walter Bade in 1950. It is located in the constellation Serpens, at a distance of about 80,000 light-years, and is one of about 150 globular clusters orbiting around Milky Way.

It is more than ten billion years old, like most other globular clusters, which means that it formed in the early stages of the formation of galaxies. It is about ten times less massive and five times more massive than a typical spherical mass and in the final stages of dissolution.