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Scientists stunned to find vast collection of more than 100 black holes inside our own galaxy

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Scientists have found a vast collection of more than 100 black holes inside our own galaxy.

The black holes are sitting at the middle of a cluster of stars more than 80,000 light years from us. There are about three times as many of the black holes as scientists would usually expect to find.

Over the coming billion years, the stars themselves will be ejected, leaving behind only the black holes themselves.

The finding helps us understand the mysterious star cluster, known as Palomar 5, and how it might evolve in the future.

But it might also help us understand the fate of other similar clusters within our galaxy.

It might also help us explain the strange phenomena that have recently been found in our galaxy, of thin streams of stars that do not appear to be connected to such a cluster.

Big clusters filled with black holes, like Palomar 5, could have given birth to those unexplained streams, the researchers suggest.

“We do not know how these streams form, but one idea is that they are disrupted star clusters,” said Mark Gieles who works at the Institute of Cosmos Sciences of the University of Barcelona and is lead author on the paper.

“However, none of the recently discovered streams have a star cluster associated with them, hence we cannot be sure. So, to understand how these streams formed, we need to study one with a stellar system associated with it. Palomar 5 is the only case, making it a Rosetta Stone for understanding stream formation and that is why we studied it in detail,” he said in a statement.

Palomar 5 sits in the “Galactic Halo”, which is made up of a old stars that sit around the Milky Way. It is one of the halo’s most sparse clusters.

It is also known for its two long “tails”, made up of extended streams of ejected stars. Those tails extend across the night sky, allowing scientists to use them as a key way to understand how they are formed throughout the universe.

In the new research, scientists simulated the lives of the cluster’s stars individually. Researchers cannot look for the black holes themselves – as their name suggests, they are impossible to see – and so the new study relies on a method of learning how many there might be.

“It is believed that a large fraction of binary black hole mergers form in star clusters,” said Fabio Antonini, a co-author on the study from Cardiff University.

“A big unknown in this scenario is how many black holes there are in clusters, which is difficult to work out because we cannot see black holes.  Our new method gives us a way to learn how many black holes there are in a star cluster by simply looking at the stars they eject.’’

They showed that those two defining features – its sparse structure and those tails – could be the result of 100 black holes that together make up 20 per cent of its mass.

There are so many of those black holes, and they are together so massive, that the stars will be lost more quickly than the black holes. As such, in the simulation the cluster then inflated and its tail grew.

Eventually, the black holes will have such gravitational pull within the cluster that all of the stars will be thrown out of it, and it will be made up only of the black holes.

That could be the same story of those other streams of stars, which once would have populated clusters before being kicked out by the black holes that stick around, the scientists suggest. They do not appear to be associated with a cluster because their original one suffered the same fate that Palomar 5 will one day endure, the researchers say.

The findings are reported in a paper, titled ‘A supra-massive population of stellar-mass black holes in the globular cluster Palomar 5’, published in Nature Astronomy.

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