A new study says scientists have discovered a “black hole triple” in space for the first time. The system comprises a black hole at its centre, currently in the process of consuming a small star spiralling very close to it. There is also a second star, which appears to be circling the black hole but is actually far away. The discovery of the system, located about 8,000 light years away from Earth (one light year is the distance light travels in a year, 9.5 trillion kilometre), has raised questions about how black holes are formed. A black hole is a region in space where the pull of gravity is so strong that no matter or light can escape it. Astronomers believe most black holes are formed after massive stars explode at the end of their lives — known as a supernova. However, the triple system suggests a gentler process. The study, ‘The black hole low-mass X-ray binary V404 Cygni is part of a wide triple’, was carried out by researchers at the California Institute of Technology and Massachusetts Institute of Technology, and was published in Nature last month. The triple system Many black holes discovered until now have been part of binary systems, consisting of a black hole and a secondary object (such as a star or another black hole). But the black hole triple not only has one star which orbits the black hole about every 6.5 days, but also a more far-off star which orbits it every 70,000 years. Situated in the constellation of Cygnus, the system features one of the oldest known black holes, the V404 Cygni, which is nine times as big as the Sun in our solar system. Researchers accidentally discovered the distant star while looking through a repository of astronomical observations taken by telescopes. Kevin Burdge, a research fellow at the MIT Department of Physics and one of the study authors, told MIT News, “It [black hole triple] is almost certainly not a coincidence or accident. We’re seeing two stars that are following each other because they’re attached by this weak string of gravity. So this has to be a triple system.” ‘Failed supernova’ Burdge and his colleagues have proposed that V404 Cygni has two stars around it as the black hole did not arise from a supernova, which typically kicks away outer stars in the explosion. Instead, it was formed through another process called “direct collapse”, where the star caves in after expending all its fuel, but does not explode. “We call these events a ‘failed supernova’. Basically, the gravitational collapse just acts too quickly for the supernova to be able to trigger and you get an implosion instead — which sounds super dramatic and awesome but it is ‘gentle’ in the sense that you do not expel any matter,” Burdge told Reuters. However, the black hole triple will not have three members forever, as V404 Cygni is consuming the nearer star. This suggests that some already discovered binary systems could have been triple systems at some point, with the black hole later devouring one of its members.
