A cluster of thousands of stars could disintegrate and become a mob with dozens of black holes in a billion years, a new study finds.
This dark fate may arise due to the actions of a few black holes that 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 that are tightly packed collections of ancient stars. Approximately spherical, they can each contain up to millions of stars. The Milky Way possesses more than 150 globular clusters arranged in an almost spherical halo around the galaxy.
Related: Scientists find lump of black holes inside the heart of the globular cluster (video)
The researchers focused on Palomar 5, a globular cluster about 1
Palomar 5 is one of the thinnest known globular clusters. While the average spherical cluster is approx. 200,000 times the mass of the sun and approx. 20 light years in diameter, Palomar 5 is approx. 10,000 times the mass of the sun, however approx. 130 light years across, which generally makes it approx. 3,000 times less than average, 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 flowing from it, composed of stars that the spherical cluster has tossed. These spectacular tails extend over more than 22,800 light-years in length – more than 20 degrees above the sky or approx. 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 the key to understanding how such tails can be formed.
Previous studies suggested that Palomar 5’s tails were due to the way the Milky Way shredded the spherical cluster. The galaxy’s gravity is stronger on one side of Palomar 5 than the other, tearing it apart – an extreme version of how the moon’s gravity causes tides on Earth. This so-called “tidal stripping” may help explain not only the tails of the Palomar 5, but also a few dozen narrow streams of stars recently discovered in the halo of the Milky Way.
“I see Palomar 5 as a Rosetta stone that allows us to understand power generation and learn about the ancestors,” Gieles said.
Researchers had suggested that Palomar 5 was formed at a low density, making it easy for tidal stripes to tear it apart and form tails. However, a number of the properties of its star suggest that it once resembled denser globular clusters.
Now, Gieles and his colleagues suggest that the Palomar 5 may once have 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 inside it.
The researchers simulated the orbits and evolution of each star within Palomar 5 until the spherical cluster finally dissolved. They varied the original properties of the simulated cluster until they found good match with actual observations of the cluster and its tails.
The researchers discovered the structure and tails of Palomar 5 may have been the result of black holes that make up approx. 20% of the mass of the spherical cluster. Specifically, they suggest that the Palomar 5 may currently have 124 black holes, each averaging approx. 17.2 times the mass of the sun. All in all, this is three times more black holes than one could currently expect from a ball pile of this mass, Gieles said.
In this scenario, Palomar 5 formed, like typical ball clusters, with black holes consisting of only a small percentage of its mass. However, the severity of the black holes threw around stars that came close to them, inflating the cluster and making it easier for the gravity of the Milky Way to tear stars away. In a billion years, they calculate that Palomar 5 may have pushed all its stars out, leaving only black holes.
Gieles and his colleagues suggest that gravitational interactions within dense globular clusters can cause them to push most of their black holes out. As such, dense globular clusters can retain most of their stars. In contrast, researchers found that spherical clusters that start less densely, such as Palomar 5, may push fewer black holes out and instead shed most of their stars. As such, black holes can completely dominate such ball piles, making up 100% of their mass.
“I’m most excited to finally understand why some clusters are large and others small,” Gieles said. “Many people simply assumed that this was a result of different channels of formation – that is, nature. We showed that the difference in appearance is due to evolution – that is, 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 was probably formed in the same way as all the other clusters,” Gieles added.
The researchers found that when it comes to spherical clusters in the outer halo of the Milky Way – that is, those further away from the galactic center than the sun – “half of the clusters appear to be comparable to Palomar 5, and the other half is closer, “Gieles said. Half equivalent to Palomar 5 may experience a similar black hole-dominated fate, the researchers said.
Gieles warned that they were able to devise a model of the Palomar 5 that did not have black holes and was not close to its formation, but also matched all the details astronomers have seen about it. Still, he said there was only a 0.5% chance that Palomar 5 could have formed this way.
“The ‘no black hole’ model is very unlikely to occur in the wild and does not solve the problem that the Palomar 5 has features 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 as soft as Palomar 5, which are less than 100,000 times the mass of the sun, but more than 65 light-years in diameter. Scientists suggest that these soft globular clusters are rich in black holes and may eventually dissolve completely, resulting in many thin star currents.
Future research could analyze Palomar 5 to learn more about its black holes, Gieles said.
The researchers detailed their findings online on July 5 in the journal Nature Astronomy.
Originally published on Space.com.