A surprising discovery has been made in the Milky Way, a star cluster called Palomar 5 is home to more than 100 stellar-mass black holes. This finding was published in Nature Astronomy.
Palomar 5
Palomar 5 is a unique star cluster that stretches across 30,000 light-years and is located about 80,000 light-years from Earth. While globular clusters like Palomar 5 are typically dense, spherical groups of stars, Palomar 5 stands out because of its loose, spread-out formation and long tidal stream—a vast river of stars trailing behind it. These stellar streams were hard to detect in the past, but with the help of Gaia, a space observatory mapping the Milky Way with incredible precision, astronomers are now discovering more of them.
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Researchers led by Gieles used advanced simulations to study how the stars in Palomar 5 might have spread out to form this tidal stream. They found that the cluster's stars were likely pushed into the stream by the gravitational pull of black holes. These black holes, which are created when massive stars collapse, interact with the cluster’s stars and send them careening away. The simulations also showed that the number of black holes in Palomar 5 is higher than expected, helping explain the current structure of the cluster.
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The discovery that black holes are common in globular clusters is exciting for astronomers. These regions are thought to be the birthplace of many black hole mergers, which occur when black holes collide. These mergers are important because they can help scientists learn about the elusive "middleweight" black holes—those that are bigger than stellar-mass black holes but smaller than the supermassive ones at the centres of galaxies.
Astrophysicist Fabio Antonini, of Cardiff University, says, "It is believed that a large fraction of binary black hole mergers form in star clusters."