Astronomers find biggest stellar black hole in Milky Way galaxy ‘by chance’

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Astronomers at the European Space Agency (ESA) have identified the biggest stellar black hole, named Gaia BH3, discovered in the Milky Way, with a mass 33 times that of the Sun. This black hole was detected “by chance” during data collection from ESA’s Gaia mission and it imposes an odd ‘wobbling’ motion on the companion star orbiting it.
“Remarkably, this black hole is also extremely close to us — at a mere 2000 light-years away in the constellation Aquila, it is the second-closest known black hole to Earth,” a statement from ESA said.
“No one was expecting to find a high-mass black hole lurking nearby, undetected so far,” says Gaia collaboration member Pasquale Panuzzo, an astronomer from the National Centre for Scientific Research (CNRS) at the Observatoire de Paris – PSL, France. “This is the kind of discovery you make once in your research life,” he said.
Data from the European Southern Observatory’s Very Large Telescope (ESO’s VLT) and other ground-based observatories were used to verify the mass of the black hole. The research study was published in the peer-reviewed scientific journal Astronomy & Astrophysics on Tuesday.
Stellar black holes are formed from the collapse of massive stars and the ones previously identified in the Milky Way are on average about 10 times as massive as the Sun. Even the next most massive stellar black hole known in the Milky Way galaxy, Cygnus X-1, only reaches 21 solar masses, making this new 33-solar-mass observation exceptional, the statement said.

Comparison of several stellar black holes in our galaxy. Credit: ESA

To confirm its discovery, the Gaia collaboration used data from ground-based observatories, including from the ultraviolet and visual echelle spectrograph instrument on ESO’s VLT, located in Chile’s Atacama desert. These observations revealed key properties of the companion star, which, together with Gaia data, allowed astronomers to precisely measure the mass of BH3.
Astronomers have found similarly massive black holes outside the Milky Way galaxy, and have theorized that they may form from the collapse of stars with very few elements heavier than hydrogen and helium in their chemical composition. These so-called metal-poor stars are thought to lose less mass over their lifetime and hence have more material left over to produce high-mass black holes after their death. But evidence directly linking metal-poor stars to high-mass black holes has been lacking until now.
“We took the exceptional step of publishing this paper based on preliminary data ahead of the forthcoming Gaia release because of the unique nature of the discovery,” said co-author Elisabetta Caffau.
Further observations of this system could reveal more about its history and about the black hole itself. The GRAVITY instrument on ESO’s VLT Interferometer, for example, could help astronomers find out whether this black hole is pulling in matter from its surroundings and better understand this exciting object.

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