Astronomers May Have Found the Closest Black Hole to Earth

Artist’s impression of the star system, with the black hole in red.
Artist’s impression of the star system, with the black hole in red.
Image: ESO/L. Calçada

This black hole—if that’s really what it is—is so close to Earth that you can see its star system without a telescope.

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Observations demonstrate that a star system 1,000 light-years away, called HR 6819, contains an invisible source of mass influencing a companion star—in other words, a black hole. The detection of one of these “quiet” black holes, meaning one that isn’t spewing telltale x-rays toward Earth, gives astronomers hope that they may be able to spot more in the future. 

“We think that the vast majority of black holes in the Milky Way are quiet black holes, maybe a hundred million in total, and we’ve only found about 100,” Marianne Heida, postdoctoral fellow at the European Southern Observatory, told Gizmodo. “This one is really the tip of the iceberg in that sense.”

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Scientists first made observations of this star system back in 2004, figuring that it was a pretty typical pair of stars orbited by a third star. But when they tried to disentangle the light of the three stars, they realized one of the inner stars wasn’t there. Their calculations revealed that a normal star wouldn’t be able to hide in the light of the second star and that, with a mass of at least 4.2 times the Sun, this unseen object would be too huge to be a neutron star. They concluded that they must have spotted a black hole.

The excitement around this object is twofold: First, 1,000 light-years away is really close, cosmically speaking. “On the scale of the Milky Way, it’s in our backyard, almost at our doorstep,” study author Dietrich Baade, emeritus astronomer at European Southern Observatory, told Gizmodo. Second, astronomers have detected nearly every other black hole based on the way they emit x-rays after sucking in and tearing up matter from a companion. This black hole doesn’t emit x-rays that we can see. Stumbling upon a quiet black hole this close suggests that there could be many more systems like this one.

Some of the astronomers Gizmodo spoke with found the analysis robust. “It’s impressive that they managed to find a new black hole candidate hiding in plain sight around such a bright and well-studied star,” Kareem El-Badry, Ph.D. student in the UC Berkeley Department of Astronomy who was not involved in the study, told Gizmodo in an email. But he pointed out that the authors need to rely on an assumption in order to calculate the visible star’s mass, and if that assumption is wrong, then the invisible object might be less massive—and potentially a neutron star or even a very light regular star, not a black hole at all.

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University of Auckland associate professor JJ Eldridge told Gizmodo in an email that she didn’t feel the analysis was enough to claim the detection of a black hole. “This system is probably more ordinary,” she said. “The ordinary explanation is they might just involve two stars... one of them, having a disk around them (the Be star) rather than three stars, one of which is a black hole,” where the Be star refers to the third star orbiting the other two.

This all ties into a conversation around another triple system that was recently in the news, called LB-1, thought to feature a huge black hole with a mass 70 times that of the Sun. Subsequent analyses have cast doubt on whether that system had such a large mass—and recent papers doubt whether LB-1 has a black hole at all. The authors behind the newest work contend that LB-1 has a black hole that’s much less massive than original estimates, while Eldridge thinks that neither system contains a black hole.

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Regardless, this would just be one black hole in the Milky Way, with millions or billions still undetected. These researchers are going to look for more systems like HR 6819 in the hopes that they can uncover more quiet black holes. If you’re in the southern hemisphere, look for HR 6819 in the constellation Telescopium. You might just be looking at a black hole.

Science Writer, Founder of Birdmodo

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DISCUSSION

Fascinating, good read Ryan!

Just as a heads up, the way the title hit me was seeming to imply that they also have reason to believe that no black hole could be closer. Maybe a slight change would be beneficial - “Astronomers may have found the closest known black hole to earth yet” - or something similar. Or it’s very possible I just jumped to a weird conclusion, so I trust your judgement on it.