CLUSTER SPORTS ABUNDANCE OF BLACK HOLES
The Milky Way can be pretty disruptive to the dwarf galaxies and globular clusters that orbit it. Our galaxy’s gravity can pull and stretch these objects, creating stellar streams that arc along their orbits.
To better understand how these streams evolve, researchers looked to Palomar 5. Best known for its spectacular twin streams of stars, Palomar 5 is also one of the sparsest globular star clusters around the Milky Way. Both features are likely caused by a combination of the Milky Way’s gravity and an overabundance of stellar-mass black holes hidden within the cluster, according to new research published July 5 in Nature Astronomy.
“The number of black holes is roughly three times larger than expected from the number of stars in the cluster,” said lead author Mark Gieles of the Institute of Cosmos Sciences of the University of Barcelona in a press release. “It means that more than 20 percent of the total cluster mass is made up of black holes.”
By simulating the cluster’s evolution, the team found that Palomar 5 began with a more typical mixture of stars and black holes, but stars were lost at a higher rate than black holes. This ultimately led to an imbalance that further contributed to the loss of stars, as interactions with the black holes sent stars careening from the cluster. The researchers estimate that by the time the cluster is completely torn apart — approximately 1 billion years from now — Palomar 5 will only host black holes, with no stars left at all.