Astronomers have found the largest stellar mass black hole in the Milky Way so far. At 33 solar masses, it dwarfs the previous record-holder, Cygnus X-1, which has only 21 solar masses. Most stellar mass black holes have about 10 solar masses, making the new one\u2014Gaia BH3\u2014a true giant. <\/p>\n
<\/p>\n Supermassive black holes (SMBH) like Sagittarius A Star at the heart of the Milky Way capture most of our black hole attention. Those behemoths can have billions of solar masses and have enormous influence on their host galaxies. <\/p>\n But stellar-mass holes are different. Unlike SMBHs that grow massive through mergers with other black holes, stellar black holes result from massive stars exploding as supernovae. SMBHs are always found in the center of a massive galaxy, but stellar black holes can be hidden anywhere. <\/p>\n \u201cThis is the kind of discovery you make once in your research life.\u201d<\/p>\n Pasquale Panuzzo, National Centre for Scientific Research (CNRS) at the Observatoire de Paris<\/cite><\/p><\/blockquote>\n<\/figure>\n Astronomers found BH3 in data from the ESA\u2019s Gaia spacecraft. It\u2019s Gaia\u2019s third stellar black hole. BH3 has a stellar companion, and the black hole\u2019s 33 combined solar masses tugged on its aged, metal-poor companion. The star\u2019s tell-tale wobbling betrayed BH3\u2019s presence. At only 2,000 light-years away, BH3 is awfully close in cosmic terms. <\/p>\n A new research letter in Astronomy and Astrophysics presented the discovery. Its title is \u201cDiscovery of a dormant 33 solar-mass black hole in pre-release Gaia astrometry.\u201d The lead author is Pasquale Panuzzo, an astronomer from the National Centre for Scientific Research (CNRS) at the Observatoire de Paris.<\/p>\n \u201cNo one was expecting to find a high-mass black hole lurking nearby, undetected so far,\u201d said Panuzzo. \u201cThis is the kind of discovery you make once in your research life.\u201d<\/p>\n This black hole is remarkable for its considerable mass. Researchers have found stellar black holes with similar masses, but always in other galaxies. The size is confounding, but astrophysicists have pieced together how they may become so massive.<\/p>\n They could result from the collapse of metal-poor stars. These stars are composed almost entirely of hydrogen and helium, the primordial elements. Scientists think these stars lose less mass over their lifetimes of fusion than other stars. They retain more mass, so they collapse into more massive black holes. This idea is based on theory; there\u2019s no direct evidence.<\/p>\n But BH3 could change that.<\/p>\n Binary stars tend to form together and have the same metallicity. Follow-up observations showed that BH3\u2019s companion star is likely a remnant of a globular cluster that the Milky Way absorbed more than eight billion years ago. Since binary stars tend to have the same metallicity, this metal-poor companion bolsters the idea that low-metallicity stars can retain more mass and form larger stellar black holes. This is the first evidence supporting the idea that ancient and metal-poor massive stars collapse into massive black holes. It also supports the idea that these early stars may have evolved differently than modern stars of similar masses.<\/p>\n But there\u2019s another interpretation. <\/p>\n When stars explode as supernovae, they forge heavier elements that are blown out into space. Shouldn\u2019t the companion show evidence of contamination by the metals from BH3\u2019s supernova?<\/p>\n \u201cWhat strikes me is that the chemical composition of the companion is similar to what we find in old metal-poor stars in the galaxy,\u201d explains Elisabetta Caffau of CNRS, Observatoire de Paris, also a member of the Gaia collaboration. \u201cThere is no evidence that this star was contaminated by the material flung out by the supernova explosion of the massive star that became BH3.\u201d From this perspective, the pair may not have formed together. Instead, the black hole could\u2019ve acquired its companion only after its birth, capturing it from another system.<\/p>\n BH3 and the two other black holes found by Gaia are dormant. That means there\u2019s nothing close enough for them to \u201cfeed\u201d on. Even though BH3 has a companion, it\u2019s about 16 AU away. If BH3 was actively accreting matter, it would release energy that would betray its presence. Its dormancy enabled it to remain undetected.<\/p>\n At only 2,000 light years away, astronomers are bound to keep studying BH3. <\/p>\n \u201cFinally, the bright magnitude of the system and its relatively small distance makes it an easy target for further observations and detailed analyses by the astronomical community,\u201d the discoverers write in their research letter. <\/p>\n This discovery may have been serendipitous, but it was no accident. A dedicated team of researchers scours Gaia data for stars with odd companions. This includes light and heavy exoplanets, other stars, and black holes. Gaia can\u2019t spot planets or dormant black holes but can spot their effect on their stellar companions.<\/p>\n The researchers behind the discovery released their findings before Gaia\u2019s next official data release. They felt it was too important to sit on. \u201cWe 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,\u201d said co-author Elisabetta Caffau, also a Gaia collaboration member and CNRS scientist from the Observatoire de Paris \u2013 PSL.<\/p>\n \u201cWe have been working extremely hard to improve the way we process specific datasets compared to the previous data release (DR3), so we expect to uncover many more black holes in DR4,\u201d said Berry Holl of the University of Geneva, in Switzerland, member of the Gaia collaboration.<\/p>\n \u201cThis discovery should also be seen as a preliminary teaser for the content of Gaia DR4, which will undoubtedly reveal other binary systems hosting a BH,\u201d the authors conclude. <\/p>\n Gaia DR4 is scheduled to be released no sooner than the end of 2023. If past data releases are any indication, the data will be full of new discoveries. If there are enough binary stellar mass black holes in the data, astronomers may get closer to understanding where they come from and if massive stars behaved differently in the early Universe. <\/p>\n\n
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