Artist's impression of a large number of black holes in a single cluster. Credit: ESA / Hubble, N. Bartmann

Astronomers Spot Swarm Of Black Holes Moving Through Our Galaxy

One hundred black holes in a single star cluster.


An international group of astrophysicists, led by specialists from the University of Barcelona, ​​discovered a huge population of black holes, which seemed to “lurk” in the globular star cluster Palomar 5 in the Milky Way. In the distant future, this cluster will completely consist of black holes.

Astrophysicists have observed the Palomar 5 star cluster in hopes of discovering previously unknown features. As a result, they discovered a huge population of black holes, with over 100 of them located in the center of this cluster.


Scientists expected to find less than a third of the number they located, which means that about 20% of the entire mass of the cluster consists of black holes. Even more, each one of these black holes has at least 20 times the mass of our Sun.

The globular cluster Palomar 5 was discovered in 1950 by astronomer Walter Baade. It is located in the constellation Serpent about 80 thousand light-years away and is one of about 150 globular clusters orbiting the Milky Way.

Palomar 5 is believed to be over 10 billion years old. This means that it, like most other globular clusters, was formed in the earliest stages of galaxy formation. Moreover, Palomar 5 is about 10 times less massive and five times longer than a typical globular cluster. This is one of the most rarefied clusters, which is in the last stages of its dissolution in space.

All-sky view of the Milky Way galaxy. The region in the higher middle is where Palomar 5 is located and where researchers discovered more than 100 black holes. Credit: Mark Gieles et al./ Gaia eDR3/ DESI DECaLS/ University of Barcelona.
An all-sky view of the Milky Way galaxy. The region in the higher middle is where Palomar 5 is located and where researchers discovered more than 100 black holes. Credit: Mark Gieles et al./ Gaia eDR3/ DESI DECaLS/ University of Barcelona.

The authors of the work also describe the results of modeling the orbits and evolution of each star in the cluster, from the early stage of the formation of this cluster to its final dissolution. Using a computer, the researchers changed the initial characteristics of the cluster until they found a match with the latest observations.

As a result, it was found that Palomar 5 was originally formed with relatively few black holes. However, the aggressive behavior of the runaway stars led to the fact that the number of black holes gradually increased. They dynamically “inflated” the cluster with their gravitational field, which led to an increase in the number of escaping stars and the formation of a tidal flow from them. Tidal streams are streams of stars that have been ejected from collapsed star clusters or dwarf galaxies. In recent years, about 30 such “jets” have been discovered in the halo of the Milky Way alone.


This phenomenon remains unexplained with the main hypothesis suggesting that they are collapsed star clusters. The problem with this theory is that it can only be confirmed if astronomers find and study a jet that is associated with a star system. Palomar is the only known example, which is what forced astronomers into studying it in depth.

In their work, astrophysicists also write that shortly before complete dissolution, somewhere in one billion years, this cluster will completely consist of black holes.

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Gieles, M., Erkal, D., Antonini, F., Balbinot, E., & Peñarrubia, J. (2021, July 5). A supra-massive population of stellar-mass black holes in the globular cluster Palomar 5. Nature News.
O’Neill, M. (2021, July 6). Astronomers Discover a Supra-Massive Black Hole Population in a “Fluffy” Star Cluster in Our Galaxy. SciTechDaily.
Starr, M. (n.d.). An Entire Swarm of Black Holes Has Been Caught Moving Through The Milky Way. ScienceAlert.
Universitat de Barcelona. (n.d.). Descoberta una població de forats negres de grans dimensions al cúmul estel·lar Palomar 5. Universitat de Barcelona.
Whitt, K. K. (2021, July 8). Astronomers find 100 black holes in Palomar 5. EarthSky.


Written by Vladislav Tchakarov

Hello, my name is Vladislav and I am glad to have you here on Curiosmos. As a history student, I have a strong passion for history and science, and the opportunity to research and write in this field on a daily basis is a dream come true.

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