Scientists spot startlingly close black holes in Hyades star cluster

Not to alarm anyone, but the nearest black holes to Earth are closer than we previously thought. 

Way, way closer, in fact. 

Like, more-than-1,400-light-years-closer-to-Earth-than-the-last-one close.

These new, uncomfortably close black holes were discovered by an international group of EU scientists looking at the Hyades star cluster, which sits in the Taurus constellation around 150 light years from Earth. According to the team's research, the data only points to one conclusion: The presence of two or three black holes in the cluster.

"Our simulations can only simultaneously match the mass and size of the Hyades if some black holes are present at the center of the cluster today (or until recently)," explained Stefano Torniamenti, first author on the paper and postdoctoral researcher at the University of Padua in Italy. 

A black hole 150 light years away is still a ways off locally speaking, but not in the grand scheme of the Milky Way, which is some 100,000 light years across. In that context, it's practically right on top of us. The previously discovered nearest black hole, Gaia BH1, is located 1,560 light years from Earth. 

If, as the ESA described it, Gaia BH1 is "in our cosmic backyard," then the Hyades black holes are in your fridge rummaging for leftovers.

Tracked, but not seen

Given black holes are, well, black, it's tough to get a read on one. Torniamenti's team didn't directly see the Hyades black holes – but they did see evidence for them.

The goal of the study was to see if they could detect the presence of black holes in star clusters like Hyades, which are groups of relatively nearby stars that form as massive clouds of stellar gas collapse, maintaining some gravitational pull between objects. Some clusters hang around, like Hyades. Others, like the cluster that birthed the Sun, have long since dissipated. The presence of black holes and their immense gravitational pull could be one reason why such clusters still exist. 

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To determine if Hyades contained a black hole, the team modeled the positions and mass of Hyades' stars and tracked their motion with a variable number of black holes included in the group. A single, central black hole "quenches the segregation of visible stars," causing reduced distribution, the team said, while simulations without a black hole end up 30 percent smaller than the Hyades we see today.

Add two to three black holes, however, and the models start to line up with the present-day position of stars in Hyades, which the team said is further confirmed by the actual distribution of high-mass stars in the cluster. Even if one of the black holes were ejected from the cluster "recently" – that is, less than 150 million years ago – the team said the same density profile is still found. 

Note: For the sake of modeling a black hole ejection, the team estimated a stray black hole would be around 60 parsecs from the Hyades – around 195 light years. So don't worry: if a stray black hole was ejected in our direction we'd probably already be dead!

The results of the model and study of Hyades, the team said, suggest that studying the radial mass distribution of stars in a cluster is the most promising method for finding the presence of black holes in star clusters. Since it's the closest cluster to Earth, Hyades was an ideal candidate for observation. 

The team isn't sure whether Hyades is unique in its black hole richness, or whether such objects are common in other star clusters, and hopes to use the same method to find out. ®