The nearest black holes might be hiding just around the corner, 150 light-years away in the Hyades star cluster.

Hyades star cluster
The Hyades star cluster, a group of thousands of stars 150 light-years away in Taurus.
Maurice Toet /

The nearest black holes might be hiding just around the corner, astronomically speaking. New research suggests that two or three stellar-mass black holes lurk in the famous open star cluster the Hyades, a mere 150 light-years away in the constellation Taurus, the Bull.

Two years ago, astronomers presented tantalizing evidence for a black hole in a binary system 1,500 light-years away in Monoceros, the Unicorn. The new result, from a team led by Stefano Torniamenti (University of Padova, Italy), would shatter that record by a factor of 10.

Simon Portegies Zwart (University of Cambridge, UK, and Leiden Observatory, The Netherlands), who studies the dynamics of star clusters and was not involved in the new study, says the claim is “not really surprising. Every open cluster older than some 5 million years and containing over 1,000 stars is expected to host a few black holes.” That’s because the most massive stars in a newborn cluster are expected to go supernova after just a few million years, leaving either a neutron star or a black hole as a remnant.

The surprising thing, according to team member Mark Gieles (University of Barcelona), is that the long-term presence of one or more black holes turns out to have a strong effect on the structure and dynamics of the host cluster, as revealed by the team’s detailed computer simulations. “There was a big difference between clusters with a small number of black holes and clusters without them,” he says.

Throughout the lifetime of a cluster (the Hyades is about 650 million years old), gravitational interactions between stars mean that more massive objects, such as black holes, slowly sink towards the center. Less massive stars gain more kinetic energy and end up in wider orbits, effectively decreasing how compact the cluster is. This compactness is expressed as the half-mass radius: the radius that contains half of the total cluster mass.

Working with the latest astrometry data from the European Space Agency’s Gaia mission, Torniamenti and his colleagues conclude that the observed half-mass radius of the Hyades is about 40% larger than you would expect if the cluster had never possessed black holes. Instead, computer simulations with two or three black holes residing in the cluster for hundreds of millions of years provide a better fit with the observations, as the team describes in a paper in Monthly Notices of the Royal Astronomical Society.

“This is an important result,” says Pavel Kroupa (University of Bonn, Germany), who was not involved in the study, “because it means that when a black hole forms, it can end up with a very small space motion.” Indeed, if a newborn black hole received a large enough “kick,” it would easily escape the cluster altogether. “This provides information on the physical processes acting during a massive star’s death,” Kroupa says.

Kroupa notes that the team’s analysis assumes the validity of our conventional ideas about gravitation. In an alternative theory of gravity, known as Modified Newtonian Dynamics (MOND), the same data might be consistent with no black holes, he says. “Independently confirming the presence of black holes in the Hyades cluster will thus not only test the physics of stellar death, but will also inform us that the Newtonian law of gravitation is probably valid.”

However, convincing evidence that the black holes are really there is very difficult to find, according to team member Zephyr Penoyre (Leiden Observatory, The Netherlands). Three-body interactions of migrating black holes with binary stars in the cluster will most likely result in one star being ejected out of the system and the other orbiting the black hole. But so far, Penoyre’s search for binary systems with one massive companion turned up empty, probably because the expected orbital periods are too long. “We may need to be more inventive to come up with ways to definitely prove the existence of the black holes,” they say.

Incidentally, binary interactions may eventually have flung the black holes out of the cluster after all. If that would have happened in the past 150 million years or so — an astronomical eyeblink — it wouldn’t show up in the present dynamics and density profile of the Hyades, says Gieles. The black holes could have traversed some 200 light-years by now. “In principle, they could have reached our solar system,” he adds, “although the chances are of course extremely low.”


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August 9, 2023 at 4:25 pm

This is almost certainly all wrong. The people in Italy sound like they've read Lyman Spitzer's 1987 book "The Dynamical Evolution of Globular Clusters" and don't realize the Hyades is an open cluster not a globular cluster. The telltale sign is their use of the half-mass radius, which Spitzer predominantly uses in his theoretical models and analysis.

Of course the environments of the two kinds of clusters are quite different, not only at the time of their formation but during their subsequent evolution, so the relevant dynamical processes are also quite different. Particularly, star-star gravitational interactions are important in globulars because of the much higher star densities and velocities, but much less so in open clusters. If this latter fact were not so then the moving cluster method of roughly determining open cluster distances wouldn't work.

"I would discourage people, at this stage, from hanging their hat too much on “I’m going to work on black holes when I grow up.” Black holes are something that I think are probably a passing fad. I think they may well exist, but all these Penrose diagrams and naked singularities, it’s all good clean fun, but -- you know there’s a school at Cambridge right now that’s trying to explain everything with black holes. ... we don’t know yet that a single black hole exists. Maybe. ... Maybe there are lots of black holes, but it may be possible to understand quasars, for example, without invoking black holes. Just because it’s a model that might explain some of the phenomena, it doesn’t mean it’s a unique model."
- George Abell, 1977 interview

If only he'd been right people wouldn't be thinking black holes are the explanation for everything.

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Anthony Barreiro

August 9, 2023 at 7:53 pm

Gosh, I wonder how Torniamenti et al.'s paper passed peer review at one of the most respected astronomical journals in the world?

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August 12, 2023 at 8:34 pm

All kinds of fake and junk papers are published. Several high profile high-temp superconductor papers are immediately recent examples.

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Yaron Sheffer

August 13, 2023 at 3:47 pm

So true. The number of submitted papers has grown exponentially faster than the number of referees and editors. Perhaps one solution could be publishing any breakthrough result only AFTER it's been corroborated by other teams. Thus we wouldn't be misled into believing that there is life on Venus or on Ohnomuamua.

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Monica Young

August 14, 2023 at 1:48 pm

Ah, but one wonders: If the work is never published in the first place, how will other teams hear about it and corroborate it?

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Yaron Sheffer

August 14, 2023 at 3:08 pm

Why, you think any work is ever 100% isolated from the rest of the community? I doubt it, especially in this age of electronic communications. And isn’t this the purpose of preprint distribution? At least it used to be. In addition, published papers are given two time stamps: submission and publication. Perhaps one could squeeze a corroboration during that interval. It would be a direct request by the authors AND the journal to another team. The submission date will preserve the chronological order of the papers for posterity.

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Monica Young

August 14, 2023 at 3:24 pm

I agree, preprint distribution certainly helps communicate ideas prior to publication, and during this time authors often do receive feedback from the community. They also receive feedback during conference presentations, private communication, peer review, and of course, after publication in a peer-reviewed journal.

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Andrew James

August 16, 2023 at 9:59 pm

Eh? "The people in Italy sound like they've read Lyman Spitzer's 1987 book "The Dynamical Evolution of Globular Clusters" and don't realize the Hyades is an open cluster not a globular cluster.” but their paper says, "We search for signatures of the presence of BHs in the nearest open cluster to the Sun – the Hyades – by comparing density profiles of direct N-body models to data from Gaia.” But they do know the Hyades is an open cluster!

Their premise is that the star forming region produced a massive star that went supernova and created a black hole. Dynamics of open clusters and globulars are different - but they do loose members due to interaction with component stars. Loosing stars change the dynamics and the half-mass radius subsequently evolves.

Are there any observable anomalies that affect cluster disintegration? Test. Are / were black holes present in the 56 binary systems? No. So result. "..none of which is consistent with a massive compact companion.” Hence: "Models that never possessed BHs have an half-mass radius ∼30 per cent smaller than the observed value, while those where the last BHs were ejected recently (≲150 Myr ago) can still reproduce the density profile."

Currently the age of the Hyades cluster is estimated to be about 625 million years (2002) or approximately one-eighth the age of our sun while the Hyades OB-association is about 650 to 700 million years. WEBDA presently give the age as log (8.896) or 787 million years. These were all derived from the cluster’s CMD and the stars they hold, and are mostly estimated by the rate in which the stars of various masses should evolve based on current stellar evolution theory.

As the Hyades distance is generally considered most important in determining the galactic scales near the Sun, and forms one of the first ‘stepping stones’ to find distance of galaxies, quasars and the scale of the universe - I'd think proving or disproving a tight binary star(s) [core binary] or presence of some black hole might be important somehow?
They actually say: "Models with 2–3 BHs have an elevated central velocity dispersion, but observations cannot yet discriminate." e. g. It is a method to test and not state a believe truth.

Gravitation anomalies are known to be real. Fact. Explaining was is causing them all is not.

Seriously. Are you saying if Mr. Abell were alive today would he still say black holes don't exist?

Note: Yes. George Abell was interested in gravitation anomalies 66 years ago, but the improved precision of data now make testing such ideas more feasible.The whole obsession by some that computer modelling is worthless pursuit is ignorant of the fact it examines reality against what we observe. If anything a negative result is even more valuable because it discounts avenues or new ideas that have come to mind.

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