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Astronomers find 100 black holes in Palomar 5

100 black holes

The spherical star cluster Palomar 5 has always been strange. The cluster is one of the “softest” known, it is less massive than other spheres and has a greater distance between its stars. It also has two long streamers of stars floating away from the cluster. And now astronomers have found a new quirk of Palomar 5: it hides more than 100 black holes in the center. A team of astronomers announced on July 1, 2021, that this unusual globular cluster has approx. three times as many black holes as expected. About 20% of the mass of the cluster comes from black holes. These astronomers said that one day the stars will be gone and nothing will be left but the black holes.

The researchers published their study on July 5, 2021

in the peer-reviewed journal Natural astronomy.

Palomar 5 is a spherical cluster, a giant sphere of stars orbiting the halo of the Milky Way. Discovered by Walter Baade in 1950, Palomar is 5 65,000 light-years away from us in the direction of the constellation Serpens. This globular cluster is old – more than 10 billion years old – because it formed at the same time as the galaxy. The spherical cluster is spreading, and long streamers of stars are floating away from it into space.

Wide, light strip with dark bands along it and lighter rectangles on dark background above it.
See larger. | This sky view of the Milky Way shows brighter regions with concentrated stars and dark, dusty spots. The small lighted patch floating at the top is a region where deeper data is available, allowing us to see the Palomar 5 and its tide tails. The cluster and streamers span more than 20 degrees of sky. Compare that to a full moon that is only 1/2 degree above. Image via Mark Gieles et al./ Gaia eDR3 / DESI DECaLS / University of Barcelona.

A spherical star cluster with a black hole

Palomar 5 has an unusual number of black holes for a spherical cluster. Mark Gieles from the University of Barcelona and lead author of the paper said:

The number of black holes is about three times greater than expected by the number of stars in the cluster, which means that more than 20% of the total cluster mass consists of black holes. They each have a mass of about 20 times the mass of the sun, and they are formed in supernova explosions at the end of life for massive stars, when the cluster was still very young.

Short-haired man in light blue polo shirt sitting at a desk with a computer.
Mark Gieles of the Institute of Cosmos Sciences at the University of Barcelona led the study. Image via University of Barcelona.

A Rosetta Stone of tidal currents

The heavy cluster with black hole loses stars as they float away from the cluster into space, leaving long tails visible in deep sky surveys. Tidal currents, as they are called, consist of stars thrown out by disturbing star clusters or dwarf galaxies. Palomar 5 is the only spherical star cluster showing tidal currents. But in the last few years, astronomers have discovered nearly 30 thin tidal currents not connected to clusters in the halo of the Milky Way. As Gieles said:

We do not know how these currents are formed, but one idea is that they are disturbed star clusters. However, none of the newly discovered streams are associated with a star cluster, which is why we cannot be sure. So to understand how these currents were formed, we need to study one with a star system associated with it. Palomar 5 is the only case that makes it a Rosetta stone for understanding power generation, which is why we studied it in detail.

The higher black hole concentration can play a key in the formation of tidal currents. He continued:

We have shown that the presence of a large black ape population may have been common in all the clusters that formed the streams.

From spherical shape to black hollow cluster

Using computer modeling, the researchers ran various simulations until they found one that matched the properties of Palomar 5. They found that Palomar 5 was likely formed with a lower concentration of black holes. Then stars escaped more easily than black holes, so the fraction of black holes for stars in the cluster increased. The interaction with the black hole with the stars created gravity slingshots, which helped create its “fluffy” appearance and led to more stars escaping and nurturing the exciting tidal current. The researchers said that a billion years from now, just before the cluster completely dissolves, it will consist solely of black holes.

The researchers’ study gives us a more complete picture for understanding ball formation, the initial masses of stars and the evolution of massive stars and has important implications for gravitational waves.

Bottom line: Scientists have discovered that the spherical cluster known as Palomar 5 has a large proportion of black holes and will one day lose all its stars and be made entirely of black holes.

Source: A supra-massive population of black holes in star mass in the spherical cluster Palomar 5

Via the University of Barcelona

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