Huge Swarm Of Blackholes Spotted Moving Through The Milky Way

A mesmerizing cluster of stars scattered all over the sky holds something we did not know about. A little secret that is yet to be unfolded, a swarm of over 100 stellar-mass black holes. 

This discovery has the possibility of explaining how the cluster came into existence with its stars scattered light-years away from each other, looking like a stellar stream stretching across 30,000 light-years.

Such star clusters are known as fossils of the early Universe, and the one we are particularly speaking of is the Palomar 5, located around 80,000 light-years. These clusters often contain old and dense stars spherical in shape like NGC 6397, typically as old as the existence of the mighty Universe. Each cluster holds approximately 100,000 to 1 million stars.

The Milky Way has about 150 globular clusters formed at once from the same gas cloud, which is the case with every globular cluster. These clusters can help in a great deal studying the history of the Universe or the dark matter content of the galaxies they orbit.

Tidal streams, another star cluster recently in the spotlight, are long rivers of stars spreading across the sky. Further streams like these are brought to people’s knowledge by Gaia space observatory that map the Milky way with great accuracy.

“We do not know how these streams form, but one idea is that they are disrupted star clusters,” astrophysicist Mark Gieles at the University of Barcelona in Spain explained.

“However, none of the recently discovered streams has 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.”

Palomar 5 has an extensive, loose distribution of stars and a long tidal stream, covering more than 20 degrees of the sky; thus, Gieles and his team felt the urge to explore this unique cluster.

Since recent evidence suggests that populations of black holes could exist in the central regions of globular clusters, and since gravitational interactions with black holes are known to send stars careening away, the scientists included black holes in some of their simulations.

Recent studies suggested the possibility of a black hole in the globular cluster of stars. Because gravitational interactions with black holes sway stars away, the researchers included black holes in some of their simulations.

Their results showed that black holes within Palomar 5 could have resulted in the arrangement we observe now. Orbital contacts would have tossed the stars out of the cluster into the tidal stream but with a substantially greater number of black holes than expected.

The stars evading the cluster more quickly than black holes would have altered the symmetry of black holes, damaging it a little.

“The number of black holes is roughly three times larger than expected from the number of stars in the cluster, and it means that more than 20 percent of the total cluster mass is made up of black holes,” Gieles said.

“They each have a mass of about 20 times the mass of the Sun, and they formed in supernova explosions at the end of the lives of massive stars when the cluster was still very young.”

The simulation demonstrated that the cluster would melt away completely in over a billion years. Still, before it occurs, the cluster will only depend on black holes orbiting the galactic centre. This means Palomar 5 will also melt away, making it an ordinary cluster like others.  

“A big unknown in this scenario is how many black holes there are in clusters, which is hard to constrain observationally because we cannot see black holes. Our method gives us a way to learn how many black holes there are in a star cluster by looking at the stars they eject.”

The research has been published in Nature Astronomy.

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