Astronomers Discover a ‘Sleeping Giant’ Black Hole in Our Galaxy—the Second-Closest Known to Earth

Called Gaia BH3, the dormant black hole is 33 times more massive than the sun, making it the largest recorded stellar black hole in the Milky Way

An artist's illustration of Gaia BH3 and its companion star's "wobbling" orbit
An artist's illustration of Gaia BH3 and its companion star's "wobbling" orbit. L. Calçada / ESO

Less than 2,000 light-years away from Earth, scientists have discovered a “sleeping giant”—the Milky Way’s largest stellar black hole, with a mass 33 times greater than that of our sun. Though the black hole, named Gaia BH3, is the second-closest known to our home planet, it is “dormant” and had therefore gone undetected.

“It’s a complete surprise,” Pasquale Panuzzo, the study’s lead author and an astronomer at the Observatoire de Paris, tells the Guardian’s Ian Sample.

Located in the constellation Aquila the eagle, Gaia BH3 was uncovered by chance in a mass of data and observations collected by European Space Agency (ESA) astronomers. As the team reviewed findings from the agency’s Gaia Mission, which is building a 3D map of our galaxy, something unusual caught their attention: a wobbling star some 1,926 light-years away. The object’s curious motion indicated that a large black hole nearby had locked it into an orbit.

To verify their finding, the team of astronomers and ground operators used the European Southern Observatory’s Very Large Telescope, located in Chile, to confirm Gaia BH3’s mass. In findings published in the journal Astronomy & Astrophysics, they shared that the wobbling star orbits the black hole every 11.6 years.

“No one was expecting to find a high-mass black hole lurking nearby, undetected so far,” Panuzzo says in a statement. “This is the kind of discovery you make once in your research life.”

Gaia's discovery of a massive black hole in our Milky Way: Gaia BH3 (long version - voice - music)

Gaia BH3 is a type of black hole caused by the collapse of a star, known as a stellar black hole—and it’s the largest of this type discovered in our galaxy. As a “dormant” black hole, it isn’t actively siphoning energy and materials from a companion star, making it inconspicuous in surveys of the sky.

The average size of stellar black holes in the Milky Way is about ten times the mass of the sun, with the previous record-holder, called Cygnus X-1, being 21 times the mass of the sun. Still, on the grand scale of all black holes, the 33-solar-mass Gaia BH3 is relatively small. For instance, Sagittarius A*, the supermassive black hole in the center of the Milky Way, has a mass four million times larger than that of the sun.

Gaia BH3 is the second-closest black hole to Earth, behind only Gaia BH1, which is 1,500 light-years away and ten times the mass of the sun.

“The relatively small distance to Gaia BH3 can be illustrated by noting that the light observed by Gaia was emitted by the star at the time when classical Rome was governed by Emperor Nero,” Tomaž Zwitter, an astronomer at the University of Ljubljana in Slovenia and a member of the Gaia Collaboration, says in an ESA statement.

A diagram illustrating the relative size and masses of the three stellar black holes in the Milky Way
Three stellar black holes in the Milky Way. The newly discovered Gaia BH3, on the right, is the most massive. M. Kornmesser / ESO

The discovery is significant, as it provides astronomers with clues to better understand how the deaths of ancient stars gave rise to stellar black holes.

Astronomers had previously posited that more massive stellar black holes are formed from stars that do not contain many heavy metals. Instead composed largely of hydrogen and helium, these metal-poor stars are thought to lose less mass over time, resulting in more material left over at the end of their lives with which to form more massive black holes.

But until finding Gaia BH3 and its companion star—which was indeed metal-poor, suggesting the one that formed the black hole was also low in metal—scientists hadn’t had an example to draw from. This finding adds substance to the idea.

“What strikes me is that the chemical composition of the companion is similar to what we find in old metal-poor stars in the galaxy,” Elisabetta Caffau, an astronomer with CNRS, Observatoire de Paris and a member of the Gaia collaboration, says in a statement.

Gaia BH3’s companion star likely formed within two billion years of the Big Bang, which scientists estimate occurred 13.8 billion years ago. Because the star orbits in the opposite direction as the majority of Milky Way stars, it may have originally been part of another galaxy that then merged with ours more than eight billion years ago. It doesn’t seem affected from the stellar explosion that would have created Gaia BH3, so ESA astronomers suggest the black hole captured this star after it was born.

As the Gaia project continues to develop its map of the Milky Way, the next installment of its data is slated to be published no sooner than 2025. But the team chose to release the news of Gaia BH3 early, in hopes their initial observations can serve as bread crumbs for other astronomers’ future study.

“It’s impressive to see the transformational impact Gaia is having on astronomy and astrophysics,” Carole Mundell, the European Space Agency’s director of science, says in the statement. “Its discoveries are reaching far beyond the original purpose of the mission.”

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