Star cluster overrun with black holes may dissolve into space

A cluster composed of thousands of stars may dissolve to become a mob of dozens of black holes in a billion years, a new study finds.

This dark fate may arise from the actions of a few black holes that may currently lie within that cluster of stars, and the finding may shed light on the future of dozens of similar clusters in the Milky Way, researchers say.

Scientists analyzed globular clusters, which are densely packed collections of ancient stars. Roughly spherical in shape, they may each contain up to millions of stars. The Milky Way possesses more than 150 globular clusters arranged in a nearly spherical halo around the galaxy.

Related: Scientists find clump of black holes inside the heart of globular cluster (video) 

The researchers focused on Palomar 5, a globular cluster about 11.5 billion years old located in the Milky Way's halo, about 65,000 light-years from Earth in the Serpens constellation. 

Palomar 5 is one of the sparsest known globular clusters. Whereas the average globular cluster is about 200,000 times the mass of the sun and about 20 light-years in diameter, Palomar 5 is about 10,000 times the mass of the sun yet about 130 light-years across, overall making it about 3,000 times less dense than average, study lead author Mark Gieles, an astrophysicist at the University of Barcelona in Spain, told Space.com.

At the same time, Palomar 5 is known for two long tails streaming from it, composed of stars the globular cluster has shed. These spectacular tails span more than 22,800 light-years in length — more than 20 degrees across the sky, or about 40 times the apparent diameter of the full moon. Palomar 5 is one of the few known star clusters with such long tails, making it key to understanding how such tails might form.

Previous research suggested Palomar 5's tails resulted from the way in which the Milky Way was shredding apart the globular cluster. The galaxy's gravitational pull is stronger on one side of Palomar 5 than the other, tearing it apart — an extreme version of how the moon's gravitational pull causes tides on Earth. This so-called "tidal stripping" may help explain not only Palomar 5's tails but also a few dozen narrow streams of stars recently detected in the Milky Way's halo.

"I see Palomar 5 as a Rosetta Stone that allows us to understand stream formation and learn about the progenitors of streams," Gieles said. 

Scientists had suggested that Palomar 5 formed with a low density, making it easy for tidal stripping to rip it apart and form its tails. However, a number of its stars' properties suggest it was once similar to denser globular clusters.

Now Gieles and his colleagues suggest Palomar 5 may indeed have once been much denser than it is now and that its current sparse nature and its long tails may be due to more than 100 black holes lurking within it.

The researchers simulated the orbits and the evolution of each star within Palomar 5 until the globular cluster finally disintegrated. They varied the initial properties of the simulated cluster until they found good matches with actual observations of the cluster and its tails.

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The scientists discovered Palomar 5's structure and tails may have resulted from black holes making up about 20% of the mass of the globular cluster. Specifically, they suggest Palomar 5 may currently possess 124 black holes, each on average about 17.2 times the mass of the sun. Altogether, this is three times more black holes than one would currently expect of a globular cluster of that mass, Gieles said.

In this scenario, Palomar 5, like typical globular clusters, formed with black holes consisting of just a small percent of its mass. However, the gravity of the black holes slung around stars that got near them, puffing up the cluster and making it easier for the Milky Way's gravity to rip stars away. A billion years from now, they calculated Palomar 5 might have ejected all of its stars, leaving behind only black holes.

Gieles and his colleagues suggest that gravitational interactions within dense globular clusters may drive them to eject most of their black holes. As such, dense globular clusters may keep most of their stars. In contrast, the researchers found that globular clusters that start out less dense, such as Palomar 5, may eject fewer black holes and instead shed most of their stars. As such, black holes may come to completely dominate such globular clusters, making up 100% of their mass.

"I am most excited about finally understanding why some clusters are large, and others small," Gieles said. "Many people simply assumed that this was a result of different formation channels — that is, nature. We showed that the difference in appearance is due to evolution — that is, nurture." 

"Because Palomar 5 has several peculiar features that are also found in all other dense clusters, we can reconcile these findings and assume that Palomar 5 probably formed in a similar way as all the other clusters," Gieles added.

The researchers found that when it comes to globular clusters in the outer halo of the Milky Way — that is, those further from the galactic center than the sun — "about half of the clusters seems to be comparable to Palomar 5 and the other half is denser," Gieles said. The half that is similar to Palomar 5 may experience a similar black-hole-dominated fate, the researchers said.

Gieles cautioned that they were able to devise a model of Palomar 5 that had no black holes and was not dense at its formation but also matched all the details astronomers have seen of it. Still, he said there was only a 0.5% chance Palomar 5 could have formed this way.

"The 'no black hole' model is very unlikely to occur in nature, and does not resolve the issue that Palomar 5 has properties similar to other dense clusters," Gieles said.

These findings may help shed light on the 10% of the Milky Way's globular clusters that are fluffy like Palomar 5, which are less than 100,000 times the mass of the sun but more than 65 light-years in diameter. The researchers suggest these fluffy globular clusters are rich in black holes and may eventually completely dissolve, resulting in many thin stellar streams.

Future research can analyze Palomar 5 to learn more about its black holes, Gieles said.

The scientists detailed their findings online July 5 in the journal Nature Astronomy.

Originally published on Space.com.