Astronomers find a needle in a haystack 18,000 light-years away

Most of the black holes that make headlines are absolute space monsters.

They outweigh millions — or even billions — of suns, anchor entire galaxies, and reveal themselves by blasting out powerful jets or guzzling huge amounts of gas.

This one is different.

Astronomers say they've found a tiny black hole — just 4.5 times the mass of the sun — hidden inside Omega Centauri, a crowded cluster of millions of stars about 18,000 light-years away from Earth. 

The discovery, using observations from NASA's James Webb Space Telescope and Hubble, doesn't merely add another black hole to the census. It's a demonstration of just how precise modern astronomy has become.

The black hole, dubbed oMEGACat BH-2, isn't in the middle of a feast. It isn't glowing or flashing. 

"The precision of these measurements is incredible, down to a fraction of a pixel on Hubble and Webb's detectors," said Matthew Whitaker, a University of Utah researcher who led the study, in a statement. "It would not have been possible to find this black hole without these two space telescopes."

Black holes like Sagittarius A*, the supermassive vortex defining the center of the Milky Way, are thought to lurk at the core of virtually all galaxies. Prior Hubble observations have bolstered the theory that these behemoths get their start in the dusty cores of starburst galaxies.

Only recently have scientists succeeded in photographing the gargantuan, along with another supermassive black hole known as M87*. The radio images reveal the invisible objects in silhouette: What we see in the pictures are the rings of bright gas and debris surrounding them; the "holes" in the center are the objects' shadows. 

But the most ubiquitous black holes, like the one just discovered, are not supermassive. They're known as stellar black holes, the aftermath of enormous stars dying in supernovas. During those explosions, a star's material collapses onto itself, getting crushed into a compact area. 

How scientists spotted oMEGACat BH-2 from 18,000 light-years away was not by a ring of light outlining it. They instead inferred the invisible object by sifting through 23 years of archival data and tracking one ordinary star's almost imperceptible wiggle. The star, about three-quarters the mass of the sun, has orbited the black hole from a great distance, taking nearly a century to make a loop. 

The only explanation that fit was that a stellar black hole's gravity was tugging on it, the authors said. The results are published in The Astrophysical Journal Letters. 

This is the first time a black hole of such a small scale has been found using that technique, known as astrometry, according to NASA. Other past attempts involving other methods have been unsuccessful. 

Some scientists have long suspected many more black holes are hiding in Omega Centauri, but they have been difficult to detect. Computer models have estimated this star cluster should contain roughly 10,000 of them.   

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Whether it actually does remains unclear. Other simulations have predicted that rather than holding onto generations of black holes over billions of years, the congested environment of a globular cluster leads to little black holes getting kicked out into the galactic void. 

Now astronomers want to know whether finding a tiny black hole in a globular cluster is the exception or the rule. If it's the latter, do these dark objects have a meaningful impact on what's happening inside clusters? 

The black hole also carries a surprise: oMEGACat BH-2 is not as heavy as computer models predicted, based on how ancient the stars are in the cluster. 

Early stars, formed before the prevalence of much heavier elements in the universe, are thought to produce heavier stellar black holes. Though that might seem counterintuitive, the idea is that stars made from lighter elements retain more of their material over their life, leaving behind more mass to collapse into a black hole when they eventually die. 

"We now know that a metal-poor star is able to form a black hole like this," said Anil Seth, a coauthor of the paper, also based at the University of Utah, "and we need to figure out how that happens."