The star cluster in question is called Palomar 5. It is a stream of stars that stretches over 30,000 km. light years and located approximately 80 thousand light years. Such globular clusters are often considered “fossils” of the early Universe. They are very dense and spherical, usually containing from 100 thousand. up to 1 million very old stars; some, like NGC 6397, are nearly as old as the Universe itself.
The focus is on tidal currents, long rivers of stars
In any globular cluster, all its stars formed at the same time, from the same cloud of gas. The Milky Way has about 150 known globular clusters; these objects are an excellent tool for studying, for example, the history of the Universe or the dark matter content of the galaxies they orbit.
But another type of star cluster is also gaining more and more attention – tidal streams, long rivers of stars that stretch across the sky.
They were previously difficult to identify, but the Gaia space observatory has worked to map the Milky Way in three dimensions with great precision, revealing more of these streams.
“We don’t know how these streams form, but one idea is that they are disrupted star clusters,” the 2021 explained astrophysicist Mark Gieles of the University of Barcelona in Spain when scientists first announced the discovery.
“However, none of the recently detected streams are related to the star cluster, so we cannot be sure.” So, to understand how these streams formed, we need to study the kind of flow that the star system is associated with. Palomar 5 is the only such case, so it is like the Rosetta Stone (a 760 kg brick with inscriptions in Egyptian and Greek, – op. cit.) that helps us understand how the flows formed, so we studied it in detail.”
Palomar 5 is unique in that it has a very wide, loose distribution of stars and a long tidal flow, which is why Gieles and his team focused on it.
Because recent evidence suggests that populations of black holes may exist in the central regions of globular clusters, and because gravitational interactions with black holes are known to eject stars, the researchers included black holes in some of their simulations.
Their results suggested that a population of stellar-mass black holes in Palomar 5 could have led to the configuration we see today.
“The number of black holes is about three times higher than expected based on the number of stars in the cluster, which means that black holes account for more than 20 percent of the total mass of the cluster,” Gieles said.
Will dissolve in the stream of stars
Each has 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.
In about a billion years, the team’s simulations showed, the cluster will completely dissolve. Just before that, what’s left of the cluster will consist of nothing but black holes orbiting the galactic center. This suggests that Palomar 5 is not unique – it will completely dissolve in the stellar flow, like others that have been discovered.
“The big unknown in this scenario is how many black holes are in the clusters, which is difficult to determine observationally because we can’t see the black holes. Our method makes it possible to find out how many black holes are present in a star cluster by observing the stars they eject,” – sciencealert.com said astrophysicist Fabio Antonini of Cardiff University in the United Kingdom.
The study was published in the journal Nature Astronomy.
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2024-09-25 23:53:54
