Astronomers spot a massive 'sleeping giant' black hole less than 2,000 light-years from Earth

Astronomers have spotted the most massive known stellar black hole in the Milky Way galaxy after detecting an unusual wobble in space.

The so-called "sleeping giant," named Gaia BH3, has a mass that is nearly 33 times that of our sun, and it's located 1,926 light-years away in the Aquila constellation, making it the second-closest known black hole to Earth. The closest black hole is Gaia BH1, which is located about 1,500 light-years away and has a mass that is nearly 10 times that of our sun.

Astronomers discovered the black hole while combing through observations taken by European Space Agency's Gaia space telescope for an upcoming data release to the scientific community. The researchers weren't expecting to find anything, but a peculiar motion - caused by Gaia BH3's gravitational influence on a nearby companion - caught their eye.

Many "dormant" black holes don't have a companion close enough to munch on, so they are much more difficult to spot and don't generate any light. But other stellar black holes siphon material from companion stars, and this exchange of matter releases bright X-rays that can be spotted through telescopes.

The wobbling movement of an old giant star in the Aquila constellation revealed that it was in an orbital dance with a dormant black hole, and it's the third such dormant black hole spotted by Gaia.

The researchers used the European Southern Observatory's Very Large Telescope in Chile's Atacama Desert and other ground-based observatories to confirm the mass of Gaia BH3, and their study has also offered new clues to how such huge black holes came to be. The findings appeared Tuesday in the journal Astronomy & Astrophysics.

"No one was expecting to find a high-mass black hole lurking nearby, undetected so far," said lead study author Pasquale Panuzzo, an astronomer at the Observatoire de Paris, part of France's National Centre for Scientific Research, and a Gaia collaboration member, in a statement. "This is the kind of discovery you make once in your research life."

The secrets of ancient stars

The title for the most massive black hole in our galaxy will always belong to Sagittarius A*, the supermassive black hole located at the center of the Milky Way, which has about 4 million times the mass of the sun, but that is because it's a supermassive black hole, rather than a stellar black hole.

The process by which supermassive black holes form is poorly understood, but one theory suggests it happens when massive cosmic clouds collapse. Stellar black holes form when massive stars die. So Gaia BH3 is the most massive black hole in our galaxy that formed from the death of a massive star.

Stellar black holes observed across the Milky Way galaxy are about 10 times as massive as the sun on average. Until the discovery of Gaia BH3, the largest known stellar black hole in our galaxy was Cygnus X-1, which is 21 times the mass of the sun. While Gaia BH3 is an exceptional find within our galaxy by astronomers' standards, it is similar in mass to objects found in very distant galaxies.

Scientists believe stellar black holes with masses such as Gaia BH3's formed when metal-poor stars collapsed. These stars, which include hydrogen and helium as their heaviest elements, are thought to lose less mass over their lifetimes, so they have more material at the end that can result in a high-mass black hole.

But astronomers hadn't been able to find evidence directly linking high-mass black holes and metal-poor stars until they found Gaia BH3.

The study authors said that paired stars tend to be similar in composition. True to expectations, the researchers found that the star orbiting Gaia BH3 was metal-poor, which means that the star that formed Gaia BH3 was likely the same.

"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," said study coauthor Elisabetta Caffau, a Gaia collaboration member at the Observatoire de Paris, in a statement.

The star orbiting Gaia BH3 likely formed in the first 2 billion years after the big bang created the universe 13.8 billion years ago. The star's trajectory, which moves in the opposite direction of many stars in the galactic disk of the Milky Way, suggests it was part of a small galaxy that merged with the Milky Way more than 8 billion years ago.

Now, the team hopes the research can allow other astronomers to study the colossal black hole and uncover more of its secrets without having to wait for the rest of the Gaia data release, slated for late 2025.

"It's impressive to see the transformational impact Gaia is having on astronomy and astrophysics," said Carole Mundell, the European Space Agency's director of science, in a statement. "Its discoveries are reaching far beyond the original purpose of the mission, which is to create an extraordinarily precise multi-dimensional map of more than a billion stars throughout our Milky Way."

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