Black Hole Found in Our Neighboring Galaxy Could Help Uncover Milky Way's Hidden Population

Astronomers have discovered a small black hole in the Milky Way's galactic neighbor, the Large Magellanic Cloud, by observing the effect it has on a star that orbits it.

Using the Very Large Telescope (VLT) located in the Atacama Desert of northern Chile, a team of researchers spotted a star in the NGC 1850 star-cluster whose shape is being warped by the intense gravitational influence of a black hole that has about 11 times the mass of our sun.

The method used to find this black hole in a cluster 160,000 light-years away could help astronomers hunt for hidden black holes much closer to home, hidden within our own galaxy.

"Similar to Sherlock Holmes tracking down a criminal gang from their missteps, we are looking at every single star in this cluster with a magnifying glass in one hand trying to find some evidence for the presence of black holes but without seeing them directly," said Sara Saracino from the Astrophysics Research Institute of Liverpool John Moores University in the U.K.

Saracino led the research which has been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society and tells the tale of the first "criminal" tracked by the team, lurking in a young star cluster orbited by a star with five times the mass of the sun, which it is squashing and squeezing.

The team's discovery is a stellar-mass black hole. This isn't the first time we've spotted one of these relatively diminutive black holes, but astronomers usually find them because of the X-rays emitted from around them as they swallow matter, or from the gravitational waves they emit when they collide with a neutron star or another black hole.

Black holes are difficult to spot because they trap light behind a point called the event horizon. If they aren't consuming material and causing the emission of electromagnetic radiation in this way, black holes are pretty much invisible.

The problem with these methods is many stellar-mass black holes don't give away these tell-tale signals. That means that astronomers have to find more ingenious ways of spotting them.

"Most of the black holes detected so far are very bright in X-rays, produced by the energy released by matter falling into the black hole. In this case, the black hole seems to be dormant and is not accreting much matter," Sebastian Kamann of the Astrophysics Research Institute of Liverpool John Moores University told Newsweek.

The team instead used a dynamical model that considers instead the influence black holes have on stars that orbit them and the role they have played in developing the systems they are a part of.

"The vast majority can only be unveiled dynamically," team member and researcher from the University of Göttingen, Germany, Stefan Dreizler, said. "When they form a system with a star, they will affect its motion in a subtle but detectable way, so we can find them with sophisticated instruments."

Kamann continued: "So we searched for the gravitational pull that black holes exert on stars around them.

"Such stars are forced on almost circular orbits around the black holes, resulting in periodic variations of the stars' velocities that we can measure with spectrographs. In this case, the star is swung back and forth at a velocity of 300 kilometers [186 miles] per second."

The successful detection of the black hole in NGC 1850, home to thousands of stars, marks the first time a black hole has been found in a young star cluster.

The cluster in the Large Magellanic Cloud is just 100 million years old, barely an infant in cosmic terms, and finding a black hole in such a young collection of stars could help reveal how such systems evolve and grow.

Researchers can now compare this black hole to more mature examples and attempt to trace how they swallow matter, including stars and planets, and merge with other black holes to achieve truly massive sizes.

A better understanding of stellar-mass black holes and the processes that they are involved in could also help us understand gravitational wave sources in distant galaxies better.

In our own galaxy, the Milky Way, researchers believe a hidden population of stellar-mass black holes exists, albeit effectively hidden. As many as 20,0000 of these stellar remnants are expected to dwell around the center of the Milky Way near its central supermassive black hole, Sagittarius A*. But until now researchers have had difficulty uncovering them.

This new method could play an important role in that search.

"The result shown here represents just one of the wanted criminals, but when you have found one, you are well on your way to discovering many others, in different clusters," Saracino added.

But, the researchers aren't quite done with this cluster yet. Kamann concluded: "As a next step, we will look into all the other systems in the cluster, in the hopes of finding more good candidates for black holes or other exotic objects."

UPDATE 11/18/21 9:28 a.m. ET: This article was updatedto include comments from Sebastian Kamann.

Hidden Black hole
An illustration of a compact black hole orbiting a stars in the NGC 1850 star cluster 160,000 light-years away in the Large Magellanic Cloud. The star's shape is distorted by the intense gravitational influence of the black hole. /M. Kornmesser/ESO

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