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Key takeaways

  • Palomar 5 star cluster may contain over 100 stellar-mass black holes, a surprising discovery for astronomers.
  • The cluster’s unusual structure, with stars spaced light-years apart, might be due to interactions with these black holes.
  • Globular clusters like Palomar 5 are ancient and help scientists study the early Universe and dark matter in galaxies.
  • The Gaia space observatory has revealed more star streams, suggesting these streams might form from disrupted star clusters like Palomar 5.
  • Palomar 5 is predicted to eventually dissolve into a star stream, and other globular clusters might follow the same fate, providing a rich ground for black hole studies.

A puffy cluster of stars flowing across the sky may contain a swarm of over 100 stellar-mass black holes.

If this discovery is confirmed, it will explain how the cluster got to be the way it is, with stars spaced light-years apart and spreading out into a stellar stream spanning 30,000 light-years.

Palomar 5 is a star cluster situated approximately 80,000 light-years distant. Such globular clusters are commonly regarded as ‘fossils’ of the early Universe. They’re dense and spherical, with 100,000 to 1 million very old stars; some, such as NGC 6397, are almost as old as the Universe itself.

In any globular cluster, all of the stars formed simultaneously from the same cloud of gas. The Milky Way has over 150 known globular clusters; these objects are valuable tools for investigating topics such as the history of the Universe and the dark matter composition of the galaxies they orbit.

However, another sort of star group is gaining popularity: tidal streamers, which are lengthy rivers of stars stretching across the sky. Previously, they were difficult to discern, but with the Gaia space observatory mapping the Milky Way with high precision in three dimensions, more of these streams have come to light.

“We don’t know how these streams form, but one theory is that they’re disrupted star clusters,” said astrophysicist Mark Gieles from the University of Barcelona in Spain.

“However, none of the freshly discovered streams have a star cluster linked with them, so we cannot be certain. To understand how these streams formed, we need to look at one that is related with a star system. Palomar 5 is the lone case, making it a Rosetta Stone for understanding stream formation, which is why we investigated it thoroughly.”

Gieles and his team focused on Palomar 5, which looks to be exceptional in that it contains a very wide, loose distribution of stars as well as a long tidal stream spanning more than 20 degrees of the sky.

The scientists utilized precise N-body simulations to replicate the orbits and evolutions of each star in the cluster to determine how they got to where they are now.

Recent data implies that black hole populations may occur in the center areas of globular clusters, and gravitational interactions with black holes are known to send stars careening out, so the scientists incorporated black holes in some of their simulations.

Their findings indicated that a population of stellar-mass black holes within Palomar 5 could have produced the arrangement we see today. Orbital interactions would have propelled the stars out of the cluster and into the tidal stream, but only with substantially more black holes than projected.

Stars fleeing the cluster more efficiently and readily than black holes would have increased the proportion of black holes significantly.

“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,” according to Gieles.

“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 collapse altogether in approximately a billion years, according to the team’s simulations. Just before this, what remains of the cluster will be entirely made up of black holes orbiting the galactic core. This shows that Palomar 5 is not exceptional, as it will melt fully into a star stream, exactly like others we’ve discovered.

It also predicts that more globular clusters will eventually meet the same fate. It also confirms that globular clusters may be great places to seek for black holes that may eventually clash, as well as the elusive class of middleweight black holes, which fall in between stellar mass lightweights and supermassive heavyweights.

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

“A major uncertainty in this scenario is how many black holes exist in clusters, which is difficult to constrain observationally because we cannot detect black holes. Our method allows us to determine how many black holes exist in a star cluster by looking at the stars they emit.”

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