A cluster of stars covered with black holes may dissolve into space, according to research.

A new study reveals that a cluster of thousands of stars could dissolve and become a host of dozens of black holes in a billion years.

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.

The scientists analyzed globular clusters that constitute dense collections of ancient stars. They are spherical and can contain up to millions of stars. The Milky Way has more than 150 globose clusters arranged in a quasi-spherical halo around the galaxy.

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

Palomar 5 is one of the most sparse globular clusters in the world. 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 famous for its two long tails, composed of stars that the globular cluster has lost. These spectacular tails are more than 22,800 light-years long, more than 20 degrees in the sky, or approximately 40 times the apparent diameter of the full moon. Palomar 5 is one of the few known star clusters with such long tails, which is the key to understanding how such tails could form.

Earlier research suggested that the tails of Palomar 5 were the result of the way the Milky Way shredded 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" can help explain not only the tails of Palomar 5 but also some dozens of narrow streams of newly detected stars in the halo of the Milky Way.

"I see Palomar 5 as a Rosetta Stone that helps us understand the formation of streams and learn more about their ancestors," Giles said.

The scientists had suggested that Palomar 5 formed with a low density, which facilitated the stripping of the tide to pull it up and form its tails. However, several of the properties of its stars suggest that it was once similar to denser globose 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 evolution of each star in Palomar 5 until the globular cluster eventually decayed. They varied the initial properties of the simulated clump until they found good correlations with the actual observations of the clump and its tails.

The scientists found that the structure and tail of Palomar 5 may have been caused by black holes representing around 20% of the mass of the globular cluster. More precisely, they suggest that Palomar 5 currently has 124 black holes, each representing an average of 17.2 times the weight of the sun. Altogether, these are three times as many black holes as could currently be expected from a globular cluster of this mass, Gieles says.

In this scenario, Palomar 5, as typical globular clusters, formed with black holes composed of 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. In a billion years, they calculated that Palomar 5 could have ejected all its stars, leaving behind nothing but black holes.

Gieles and his colleagues suggest that gravitational interactions within dense globular clusters can drive them to eject most of their black holes. For example, dense globular clusters can conserve most of their stars. On the other hand, the researchers found that globular clusters that begin less dense, like Palomar 5, can eject fewer black holes and instead throw most of their stars. As such, black holes can gain complete control over these globular clusters, constituting 100% of their mass.

"I am very excited to finally understand why some clusters are large and some are small," said Dr. Gieles. "Many people simply thought that it was the result of different training channels, that is, of nature. We showed that the difference in appearance is due to evolution, in other words, to care."

"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 similarly 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. According to the researchers, half that is similar to Palomar 5 may experience a similar spell dominated by black holes.

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. Nevertheless, he said that there was only a 0.5% chance that Palomar 5 could have formed in this manner.

"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 that these fluffy globular clusters are rich in black holes and can ultimately dissolve completely, resulting in numerous thin stellar fluxes.

Future research may analyze Palomar 5 for more information on its black holes, Gieles says.

The scientists set out their results in the journal Nature Astronomy.