A fluffy cluster of stars, known as Palomar 5, may hold a hidden secret at its core: a swarm of over 100 stellar-mass black holes. This stellar stream, stretching across 30,000 light-years and located about 80,000 light-years from Earth, is providing new insights into the evolution of star clusters.
Palomar 5 is part of a class of dense, spherical star clusters called globular clusters. These ancient cosmic formations, sometimes containing up to a million stars, are considered “fossils” from the early Universe. The Milky Way hosts around 150 known globular clusters, which serve as key tools for studying the Universe’s history and dark matter.
What makes Palomar 5 unique, however, is its loose distribution of stars and a tidal stream that spans over 20 degrees of the sky. Tidal streams are long rivers of stars stretching across space, and until recently, were difficult to detect. The Gaia space observatory has now brought more of these structures to light.
A 2021 study led by astrophysicist Mark Gieles of the University of Barcelona found that Palomar 5’s peculiar configuration could be explained by a large population of black holes within the cluster. Simulating the orbits and evolutions of each star, the team concluded that interactions with black holes likely slingshot stars out of the cluster, forming the tidal stream.
“The number of black holes is roughly three times larger than expected,” said Gieles. “More than 20% of the total cluster mass is made up of black holes.”
These stellar-mass black holes, each about 20 times the mass of the Sun, formed during supernova explosions early in the cluster’s life. In roughly a billion years, the cluster is expected to dissolve entirely, leaving only black holes orbiting the galactic center. This suggests that other globular clusters may meet a similar fate, eventually turning into stellar streams themselves.
The discovery could also help uncover black holes in other star clusters, offering valuable clues about binary black hole mergers and the elusive middleweight black holes.
The research was published in Nature Astronomy, offering a glimpse into the hidden dynamics of Palomar 5 and the future of star clusters throughout the Universe.
