Theorists estimate there may be 100 million star-size black holes lurking in the Milky Way, but astronomers have no way of knowing for sure: Only a couple dozen have been discovered, by the x-rays they emit as they swallow material from a companion star. Now, researchers say they have discovered a quiet black hole—one that isn’t putting on a show—through the wobble it exerts on a companion star. Using the technique to get a better handle on the true population of hidden black holes will help researchers understand the lives and deaths of the stars that produce them.

The researchers involved have a reputation for demolishing black hole discovery claims. Now, they have one of their own and they think it’s watertight. “It depends on who you ask,” says Tomer Shenar of KU Leuven, who led the study. “But I think this is the first unambiguous [quiet black hole].”

When a visible star is tugged by the gravity of an unseen companion, the motion to and from Earth stretches and scrunches the light. By looking for these periodic shifts in the star’s spectrum, astronomers can determine the mass of whatever is causing the wobble. This “radial velocity” technique has already been used to detect dozens of exoplanets with hundreds more awaiting confirmation. “It’s not necessarily difficult to find radial velocities,” Shenar says. “But it is difficult to prove it is a black hole.”

If the detected companion is 2.2 times more massive than the Sun, it must be either a star or a black hole. Distinguishing between the two is not so simple; the light of a binary pair of stars blurs together at galactic distances. So Shenar and his colleagues developed a technique called spectral disentanglement, an algorithm that iteratively adjusts the spectra of two hypothetical orbiting stars until their combined light matches the observations. If, at the end of the process, the algorithm finds that one of the stars is producing no light, it must be a black hole.

Over the past several years, the team has used the technique to refute other recent black hole discovery claims. Both LB-1, a supposed monster black hole of 70 solar masses, and HR 6819, a claimed

black hole in a triple star system

, are probably not real, Shenar says. He says all the proposed black hole radial velocity sightings of recent years now have papers casting doubt on them, debunked by his team or others.

In a case of gamekeeper-turned-poacher, Shenar’s team now has a detection of its own. Using the European Southern Observatory’s Very Large Telescope in Chile, the researchers looked over the course of 6 years for radial velocities in nearly 1000 massive stars in the Tarantula nebula, which is part of the Large Magellanic Cloud, a satellite galaxy of the Milky Way. They zoomed in on VFTS 243, a star weighing 25 Suns that seems to orbit something every 10.4 days. The unseen companion

weighs 9 solar masses

, the team reports today in

Nature Astronomy

, and, based on spectral disentangling, all of the light seems to come from the single, larger star

, making the companion massive enough and dark enough to be a black hole


To put that claim to the test, Shenar sent the results to Kareem El-Badry of the Harvard-Smithsonian Center for Astrophysics, another renowned black hole debunker, but he could not find a better explanation of the data. “This is really a thorough scientific work,” says Benjamin Giesers, who was also a black hole hunter while at the University of Göttingen but has since left astronomy.

Stellar black holes form when a dying giant star explodes and much of its material—minus the blast debris—collapses. Interestingly, there was no evidence of any blast surrounding VFTS 243. That raises the possibility that the star’s onetime companion may have collapsed directly into a black hole without exploding, an idea some theorists have suggested.

That’s an important result for researchers working with gravitational wave observatories: Supernova explosions can blow binaries apart, leaving fewer black hole mergers for their detectors to detect. But if stars can form black holes without exploding, astronomers may look forward to more merger events, Shenar says. Up to now, there have been few clues that stars can collapse without exploding first. “This is a unique empirical piece of evidence,” Shenar says.

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