A starry group of stars known as Palomar 5 might hold an exciting secret: it could be home to over 100 black holes. This cluster spans across 30,000 light-years and is around 80,000 light-years away from us.
Globular clusters like Palomar 5 act as windows into the early Universe. They are dense collections of old stars, usually numbering between 100,000 and a million. Some, such as NGC 6397, are nearly as ancient as the Universe itself. Each star in a globular cluster forms from the same gas cloud simultaneously. The Milky Way hosts over 150 of these clusters, making them valuable for studying cosmic history and dark matter.
Recently, researchers have turned their attention to tidal streams—long, thin trails of stars that stretch across the sky. Identifying these streams became easier thanks to the Gaia space observatory, which has provided detailed three-dimensional mappings of our galaxy.
Astrophysicist Mark Gieles from the University of Barcelona commented on the mystery of these streams. “We don’t know how they form. They might come from disrupted star clusters, but currently, we don’t have enough evidence to confirm that,” he said. Palomar 5 is particularly special because it holds the only known association with a star cluster.
Gieles and his team used advanced simulations to recreate how the stars in Palomar 5 might have evolved. Their findings indicated that black holes lurking within the cluster could lead to the unique arrangement we observe. In fact, they discovered the number of black holes—about three times the expected amount—suggesting that over 20% of Palomar 5’s mass is made up of black holes, each roughly twenty times the mass of our Sun.
In a billion years, Palomar 5 will likely dissolve completely, leaving behind a collection of black holes orbiting the Milky Way. This pattern could be typical for other globular clusters, too.
Studies by experts like Fabio Antonini from Cardiff University also reinforce this idea. “A significant number of black hole mergers happen in star clusters, but we struggle to measure the number of black holes since they are invisible,” Antonini noted. “By examining the stars they eject, we can estimate how many black holes exist in a cluster.”
Research published in Nature Astronomy highlights the importance of studying clusters like Palomar 5. They can serve as hotspots for future black hole discoveries and might help in understanding mysterious “middleweight” black holes, which are between stellar and supermassive varieties.
As scientists continue to analyze these cosmic structures, they may unravel more secrets about black holes and the destiny of star clusters in our galaxy.
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