An Entire Swarm of Black Holes Has Been Caught Moving Through The Milky Way

Of all the things you may discover buried in your own cosmic backyard, a black hole is perhaps the last thing you'd anticipate. Something of such magnitude and effect, you'd think, would be rather visible. As it happens, you would be incorrect.

Over 100 stellar-mass black holes have been detected concealed inside a cluster of stars traveling through the Milky Way, according to research published in the journal Nature. While that may seem scary on the surface, they are so far away from Earth that scientists are not regarding them as a direct danger. Rather than that, they're seizing the chance to learn more about our newly discovered neighbors.

Palomar 5 is located roughly 80,000 light-years from Earth, with stars inside the cluster separated up to 30,000 light-years apart, generating a stellar stream across the galaxy. However, it is black holes that have drawn scientists' attention.

Mark Gieles, an astrophysicist from the University of Barcelona and lead author of the study, said, "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% of the total cluster mass is made up of black holes."

"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."

Palomar 5 is now classified as a tidal cluster, not a globular cluster, as a result of this finding. The distinction is in the distribution of stars — globular clusters include stars that were all produced about the same time, while tidal clusters have stars of varying ages that are loosely scattered in a stream.

Prior to this study, the origins of tidal streams were only speculated. Now, with further investigation of Palomar 5 – a unique example – Gieles' team was able to reproduce the orbits and evolutions of the cluster's stars using N-body simulations.

The findings indicate that the cluster's tidal stream was most likely produced as a consequence of a large number of black holes slingshotting stars out from the cluster. This effect is dependent on orbital interactions inside the cluster and would be impossible to reproduce with fewer black holes.

Not only might this finding provide light on how these tidal streams originate, but it could also aid in narrowing down the approximate age and number of black holes contained inside clusters such as Palomar 5.

Reference(s): Nature Astronomy

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