A swarm of over 100 stellar-mass black holes may be hidden in the core of a fluffy cluster of stars flowing across the sky.

If this finding can be validated, it will explain how the cluster came to be the way it is – with its stars spaced light-years apart, smearing out into a stellar stream stretching across 30,000 light-years.

The star cluster in question is called Palomar 5, located around 80,000 light-years away. Such globular clusters are commonly considered as ‘fossils’ of the early Universe. They’re highly dense and spherical, with 100,000 to 1 million very old stars; some, like NGC 6397, are nearly as old as the Universe itself.

All of the stars in a globular cluster formed at the same time, from the same cloud of gas. The Milky Way has around 150 known globular clusters; these objects are excellent tools for studying, for example, the history of the Universe, or the dark matter content of the galaxies they orbit.

But there’s another type of star group that is gaining more attention – tidal streams, long rivers of stars that stretch across the sky. Previously, they were difficult to identify, but with the Gaia space observatory aiming to map the Milky Way in three dimensions with great precision, more of these streams have come to light.

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

“However, none of the recently discovered streams have a star cluster associated with them, hence we can not 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 appears unique in that it has both a very wide, loose distribution of stars and a long tidal stream, spanning more than 20 degrees of the sky, so Gieles and his

The researchers used detailed N-body simulations to recreate the orbits and evolutions of each star in the cluster to determine how they got to where they are today.

Because recent evidence implies that black hole populations may exist in the center regions of globular clusters, and because gravitational interactions with black holes are known to send stars careening away, the scientists included black holes in some of their simulations.

Their results suggested that a population of stellar-mass black holes within Palomar 5 could have resulted in the current structure. Orbital interactions would have slingshot the stars out of the cluster and into the tidal stream, but only if there were much more black holes than predicted.

The stars escaping the cluster more efficiently and quickly than black holes would have changed the proportion of black holes considerably.

“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 cluster will dissolve fully in approximately a billion years, according to the team’s simulations. Just before this, what is left of the cluster will be entirely made up of black holes orbiting the galactic center. This suggests that Palomar 5 is not special after all; it will dissolve totally into a stellar stream, exactly like the others we’ve discovered.

It also implies that other globular clusters will likely experience the same fate in the future. It also indicates that globular clusters may be great places to look for merging black holes, as well as the elusive class of middleweight black holes, which exist between stellar mass lightweights and supermassive heavyweights.

“It is believed that a large fraction of binary black hole mergers form in star clusters,” said astrophysicist Fabio Antonini of Cardiff University in the UK.

“A big unknown in this scenario is how many black holes there are in clusters, which is hard to constrain observationally because we can not 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.”