The closest black holes to Earth may be hidden in this nearby star cluster

In the constellation Taurus, there is a group of a few hundred stars known as the Hyades. The cluster is located just 150 light-years away, and could be home to a stellar-mass black hole.

The idea of ​​black holes in star clusters is not new. Clusters often contain large, bright stars that will eventually become neutron stars or black holes, so it’s possible that older stars have already taken this path. The problem is proving it. Unless a black hole is actively consuming nearby material, it will be dark and difficult to see among a group of bright stars. So astronomers must use indirect observations to detect the black hole.

To search for black holes, the team compared observations of the Hyades cluster by the Gaia spacecraft with computer simulations of the N object. The Hyades is an open cluster of stars, so it is bound only by loose gravity. Sometimes, a close encounter between two stars will knock one of them out of the cluster. Further close encounters will push the star toward the center of the cluster, making it more strongly bound to the cluster. All of this contributes to how the density of stars within a cluster varies depending on the distance from its center.

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The observed profile fits models with 2-3 black holes. Credit: Torniamenti, et al

One of the things the team compared is what is known as the radius of the semi-mass. This is the radius within which half the mass of the block falls. With some black holes, the cluster should be slightly denser, and thus the cluster radius should be smaller. Another aspect is the central density, which should be slightly higher in the case of black holes. Given all this, a comparison between N-body simulations and Gaia data finds that the best model predicts two or three stellar-mass black holes.

Unfortunately, the results are not conclusive. Although there are 2-3 black holes better Fit to observational data, a model with no black holes, or up to 5 black holes, is still a reasonable fit. The cluster’s high central density is not something Gaia’s observations were sensitive enough to observe. The team even looked at 56 stars in the cluster that were candidates for binary stars, just in case one of them was a star orbiting a black hole, but none of the candidates were compatible with a black hole companion.

So, while it is very likely that the Hyades contains a stellar-mass black hole, it will take more observations to be sure. Only then will we truly know if there is a monster lurking in our midst.

reference: Torniamenti, Stefano, et al. “Stellar-mass black holes in the Hyades star cluster?” Monthly Notices of the Royal Astronomical Society 524.2 (2023): 1965-1986.

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