Free Astronomy Magazine July-August 2024

47 JULY-AUGUST 2024 ASTRO PUBLISHING A stronomers have identified the most massive stellar black hole yet discovered in the Milky Way galaxy. This black hole was spotted in data from the European Space Agency’s Gaia mis- sion because it imposes an odd ‘wobbling’ motion on the compan- ion star orbiting it. Data from the European Southern Observatory’s Very Large Telescope (ESO’s VLT) and other ground-based observato- ries were used to verify the mass of the black hole, putting it at an im- pressive 33 times that of the Sun. 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 our galaxy, Cygnus X-1, only reaches 21 solar masses, making this new 33-solar-mass ob- servation exceptional. Remarkably, this black hole is also extremely close to us — at a mere 2000 light-years away in the con- stellation Aquila, it is the second- closest known black hole to Earth. Dubbed Gaia BH3 or BH3 for short, it was found while the team were reviewing Gaia observations in preparation for an upcoming data release. “No one was expecting to find a high-mass black hole lurk- ing nearby, undetected so far,” says Gaia collaboration member Pasqua- le Panuzzo, an astronomer from the National Centre for Scientific Re- search (CNRS) at the Observatoire de Paris - PSL, France. “This is the kind of discovery you make once in your research life.” To confirm their discovery, the Gaia collaboration used data from ground-based observatories, includ- ing from the Ultraviolet and Visual Echelle Spectrograph (UVES) instru- ment on ESO’s VLT, located in Chile’s Atacama Desert. These observations revealed key properties of the com-