A team led by UC Berkeley astronomers has for the first time discovered what they believe to be a free-floating black hole located between 2,280 and 6,260 light years away from Earth.

It’s an incredibly rare discovery, since black holes are invisible. The team of researchers, led by Cal graduate student Casey Lam and astronomy professor Jessica Lu, found the object through gravitational microlensing, in which the brightening of a more distant star is the result of the strong gravitational pull of the object.

There’s a possibility the discovery is a neutron star rather than a black hole, however. Its estimated mass falls short of the 2.2 solar masses thought to collapse a leftover remnant of a dead star into a black hole, according to the researchers. While the Berkeley team pegged its mass at between 1.6 and 4.4 times the size of the sun, a team from the Space Telescope Science Institute in Baltimore that conducted the same analysis found it to be larger and definitively a black hole.

Regardless, it’s the first dark stellar remnant, which includes black holes and neutron stars, discovered in the galaxy separate from another star, the university said.

Lu, who leads Berkeley’s Moving Universe Lab, has been on the hunt for free-floating black holes since 2008.

“With microlensing, we’re able to probe these lonely, compact objects and weigh them,” she said in a release. “I think we have opened a new window onto these dark objects, which can’t be seen any other way.”

The researchers estimated that the object is in the direction of the center of the Milky Way Galaxy. If it’s indeed a black hole, it’s one of an estimated 200 million in the galaxy.

Until now, star-sized black holes have only been discovered as part of binary star systems. And to find them is rare, only through X-ray or gravitational wave detectors that sense when two or more black holes merge.

In 2020, Lu and Lam started probing the issue in 2020 after the Space Telescope Science Institute team tentatively concluded that five microlensing events might not be caused by compact objects, or stellar remnants such as black holes and neutron stars.

Only 1% of detectable microlensing events are likely to be from black holes, according to a university news release. But about 40% of microlensing events that last more than 120 days are likely to be black holes — and this one lasted nearly 300 days, according to the release.

The gravity of the object on the light of the star used to find it was also unusually long. Lam found it took about a year for the star to brighten to a peak in 2011, then another year to return to normal.

They hope to confirm whether the discovery is a black hole with further data and analysis.

Elena Kadvany is a San Francisco Chronicle staff writer. Email: elena.kadvany@sfchronicle.com

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