The closest black holes to Earth are likely hidden in this nearby star cluster
In the constellation Taurus there is a cluster of a few hundred stars known as the Hyades.
The cluster is just 150 light-years away and could host a stellar-mass black hole.
The idea that black holes lurk in star clusters is not new.
Clusters often contain large, bright stars that eventually become neutron stars or black holes, so it is likely that earlier stars have already taken this path.
The problem is proving it.
Unless a black hole is actively consuming nearby material, it is dark and difficult to see among a cluster of bright stars. Therefore, astronomers must use indirect observations to reveal the black hole.
To search for black holes, the team compared observations of the Hyades cluster by the Gaia spacecraft with N-body computer simulations.
The Hyades are an open star cluster and therefore only weakly gravitationally bound. Occasionally, a close encounter between two stars results in one of them being ejected from the cluster.
Other close encounters push a star more toward the center of the cluster, making it more tightly bound to the cluster. All of this contributes to the density of stars within the cluster varying depending on the distance from its center.
The team compared, among other things, the so-called half-mass radius.
This is the radius in which half the mass of the cluster is contained. If there are some black holes, the cluster should be slightly denser and therefore the half-mass radius should be smaller.
Another consideration is the central density, which should increase somewhat when black holes are present. With this in mind, a comparison between N-body simulations and Gaia data finds that the best model predicts the presence of two or three stellar-mass black holes.
Unfortunately, the results are inconclusive. Although 2-3 black holes are best for observational data, a model without black holes or with up to 5 black holes is still a reasonable solution.
And the cluster’s central density peak is not something the Gaia observations made sensitive enough to observe.
The team We even looked at 56 stars in the cluster that were binary candidates, just in case one of them was a star orbiting a black hole, but none of the candidates matched a black hole companion.
Although it is entirely possible that the Hyades contains a stellar-mass black hole, more observations are needed to be sure.
Only then will we truly know if there is a monster lurking in our midst.
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Written by Brian Koberlein/Universe today.
