A stellar cluster known as Palomar 5, located around 80,000 light-years away from Earth, may harbour a hidden swarm of over 100 stellar-mass black holes, according to research by astrophysicists. This star cluster, which extends across 30,000 light-years, has sparked interest due to its unusual configuration and long tidal stream of stars.
Palomar 5 is classified as a globular cluster, dense and spherical, typically containing between 100,000 and 1 million ancient stars. These clusters are often regarded as ‘fossils’ from the early Universe. The Milky Way hosts around 150 of these, which help researchers study various cosmic phenomena, including dark matter and the history of galaxies.
However, a newer class of star formations, known as tidal streams, is gaining more attention. These are long stretches of stars scattered across the sky, and their discovery has been enhanced by the Gaia space observatory’s precise three-dimensional mapping of the Milky Way.
Astrophysicist Mark Gieles from the University of Barcelona explained in 2021, “We do not know how these streams form, but one idea is that they are disrupted star clusters. Palomar 5 is the only case where a stellar system is associated with a stream, making it a Rosetta Stone for understanding stream formation.”
What makes Palomar 5 unique is its loosely distributed stars alongside its vast tidal stream, which spans over 20 degrees of the sky. Gieles and his team conducted detailed N-body simulations to recreate the paths and evolution of each star in the cluster, exploring how they ended up in their current locations.
The simulations included the presence of black holes, as recent evidence suggests that black holes within globular clusters can gravitationally interact with stars, ejecting them into the surrounding space. The team’s findings revealed that Palomar 5 likely contains a much higher number of black holes than previously predicted, which could explain the cluster’s present structure.
“The number of black holes is roughly three times larger than expected,” Gieles said, “with more than 20 percent of the cluster’s total mass made up of black holes.” These black holes, each about 20 times the mass of the Sun, were formed by supernova explosions in the early stages of the cluster’s existence.
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A groundbreaking study reveals that Palomar 5, a globular cluster, will dissolve in approximately 1 billion years, leaving behind a trail of black holes orbiting the Milky Way’s center. This discovery suggests other clusters may face a similar fate, underscoring the importance of globular clusters in detecting black hole collisions and intermediate-mass black holes.
According to Fabio Antonini of Cardiff University, “The number of black holes in clusters remains a mystery, but our innovative method provides a solution by analyzing the stars ejected from these clusters”, according to a report by Science alert.
The study has been published in Nature Astronomy. An earlier version of this article was published in July 2021.
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