Before LIGO, astronomers found black holes mostly by searching for the X-rays they produce as they suck in matter from a nearby star. Astronomers could also detect the gravitational effect a black hole would have on another star in a binary system. The unicorn researchers used this latter method, focusing on a system called V723 Monoceros, located about 1,000 light-years away. They studied the motion of a red-giant star with a variety of telescopes—including the European Space Agency’s Gaia satellite, which is mapping the position of billions of stars in our galaxy, and NASA’s exoplanet-hunting Transiting Exoplanet Survey Satellite, or TESS.
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The researchers concluded that the red giant appears to be dancing with an unseen partner. The team wrote: “The simplest explanation for the dark companion is a single compact object, most likely a black hole, in the ‘mass gap.’”
The discovery, if confirmed, would help illuminate the fine distinction that nature makes at the end of a massive star’s life. When a giant star exhausts its fuel, the star’s mass flows inward and its core collapses. If the incoming mass can explode and overcome the star’s gravitational force, it bursts into a supernova. But if not—if there’s just too much mass—the star collapses in on itself and forms a black hole.
“It’s a race between the explosion happening and black-hole formation,” says Todd Thompson, a theoretical astrophysicist at Ohio State University and a co-author of the recent paper. “This race has to be won within about a second. If it doesn’t explode in that one second, then it forms a black hole. If it does explode, it leaves behind a neutron star.”
Exactly what determines whether a star explodes as a supernova or collapses into a black hole isn’t clear. “The actual physics of the supernova explosion is a huge unknown,” Thompson says. The black-hole mass gap “could be a vital clue to that process.”
There have been a few tentative discoveries in the mass gap so far. Benjamin Giesers from the University of Göttingen in Germany and colleagues discovered a possible 4.4-solar-mass black hole in 2018, while Thompson and his colleagues found a 3.3-solar-mass candidate in 2019.
Then last year, scientists from LIGO announced the detection of an object 2.6 times the mass of our sun—a very tantalizing candidate with a mass comfortably in the mass gap. “The best case for a mass-gap black hole is from LIGO,” Thompson says.
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Yet at these lower masses, it’s hard to tell the difference between a black hole and a neutron star, since the latter can bulk up to a theoretical maximum of 3 solar masses. And depending on the conditions, neutron stars can appear dark as well. “If a neutron star is a pulsar with a beam pointed at you, it does some very obvious things,” says Tom Maccarone, an expert in black holes and neutron stars at Texas Tech University. “But if it’s not,” he adds, “it can be very difficult to separate” from a black hole.