Free Astronomy Magazine July-August 2024

JULY-AUGUST 2024 T his artist’s impression compares side-by-side three stellar black holes in our galaxy: Gaia BH1, Cygnus X-1 and Gaia BH3, whose masses are 10, 21 and 33 times that of the Sun respectively. Gaia BH3 is the most massive stellar black hole found to date in the Milky Way. The radii of the black holes are directly proportional to their masses, but note that the black holes themselves have not been directly imaged. [ESO/M. Kornmesser] than hydrogen and helium in their chemical composition. These so- called metal-poor stars are thought to lose less mass over their lifetimes 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 lack- ing until now. Stars in pairs tend to have simi- lar compositions, mean- ing that BH3’s compan- ion holds important clues about the star that collapsed to form this exceptional black hole. UVES data showed that the companion was a very metal-poor star, in- dicating that the star that collapsed to form BH3 was also metal- poor — just as pre- dicted. The research study, led by Panuzzo, is published in Astron- omy & Astrophysics . “We took the exceptional step of publishing this paper based on prelimi- nary data ahead of the forthcoming Gaia re- A stronomers have found the most massive stel- lar black hole in our galaxy, thanks to the wob- bling motion it induces on a companion star. This wobbling was measured over several years with the European Space Agency’s Gaia mission. Additional data from other telescopes, including ESO’s Very Large Telescope in Chile, confirmed that the mass of this black hole, dubbed Gaia BH3, is 33 times that of our Sun. The chemical composition of the compan- ion star suggests that the black hole was formed after the collapse of a massive star with very few heavy elements, or metals, as predicted by theory. This video summarises the discovery. [ESO] panion star, which, together with Gaia data, allowed astronomers to precisely measure the mass of BH3. Astronomers have found similarly massive black holes outside our galaxy (using a different detection method), and have theorised that they may form from the collapse of stars with very few elements heavier lease because of the unique nature of the discovery,” says co-author Elisabetta Caffau, also a Gaia collab- oration member and CNRS scientist from the Observatoire de Paris - PSL. Making the data available early will let other astronomers start studying this black hole right now, without waiting for the full data release, planned for late 2025 at the earliest. 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 un- derstand this exciting object. !