About 80,000 light-years from Earth in the Milky Way’s inner halo, a sparse collection of stars called Palomar 5 has long baffled astronomers. Its total mass is somewhat smaller than expected, and the star cluster is trailed by very thin streams of stars stretching some 22,800 light-years behind it.
Now, in a new study published July 5 in the journal Nature Astronomy, researchers suggest more than 100 black holes may be tucked within Palomar 5, which could explain its ability to spit out trails of stars.
A team of researchers led by astrophysicist Mark Gieles of the University of Barcelona in Spain, set out to model the evolution of star clusters to find out how Palomar 5 formed. They used computer simulations to calculate how stars in a cluster interact gravitationally as they orbit the center of the Milky Way, reports John Timmer of Ars Technica.
The most important factor in Palomar 5’s formation turned out to be the presence of black holes. At first, scientists expected to find around 30 black holes in Palomar 5. They were surprised to learn that the cluster holds as many as 124 black holes.
“The number of black holes is roughly three times larger than expected from the number of stars in the cluster, and it means that more than 20 percent of the total cluster mass is made up of black holes,” Gieles tells Steph Panecasio of CNET. “They each have a mass of about 20 times the mass of the Sun and they formed in supernova explosions at the end of the lives of massive stars, when the cluster was still very young.”
It’s possible Palomar 5 was doomed from the start—initial density is a key factor in predicting whether star clusters will become overrun by black holes, Ars Technica reports. Palomar 5 is considered quite “fluffy” because it is ten times less massive and five times more spread out than most clusters of its kind, according to a University of Barcelona statement. High-density star clusters have enough gravitational might to boot black holes out of their area before they take over, Ars Technica reports.
“This work has helped us understand that even though the fluffy Palomar 5 cluster has the brightest and longest tails of any cluster in the Milky Way, it is not unique,” says study author Denis Erkal, an astrophysicist at the University of Surrey, in a statement. “Instead, we believe that many similarly puffed up, black hole-dominated clusters have already disintegrated in the Milky Way tides to form the recently discovered thin stellar streams.”
But at 11.5 billion years old, Palomar 5 is well on its way toward dissolution. According to the team's calculations, Palomar 5 will likely become a solely black hole cluster in about one billion years as it continues to slingshot stars off into the cosmos.
The researchers suspect other star clusters may have operated similarly in the past, reports Enrico de Lazaro of Sci-News. Using Palomar 5 as a template, astronomers should be able to better calculate the number of black holes in different parts of the galaxy in the future.
“It is believed that a large fraction of binary black hole mergers form in star clusters,” study author Fabio Antonini, an astrophysicist at Cardiff University, tells Michelle Starr of Science Alert. “A big unknown in this scenario is how many black holes there are in clusters, which is hard to constrain observationally because we cannot see black holes. Our method gives us a way to learn how many black holes there are in a star cluster by looking at the stars they eject.”