Free Astronomy Magazine May-June 2022

30 MAY-JUNE 2022 ASTRO PUBLISHING for the data they had, obtained with the MPG/ESO 2.2-metre tele- scope, was that HR 6819 was a triple system, with one star orbiting a black hole every 40 days and a sec- ond star in a much wider orbit. But a study led by Julia Bodensteiner, then a PhD student at KU Leuven, Belgium, proposed a different ex- planation for the same data: HR 6819 could also be a system with only two stars on a 40-day orbit and no black hole at all. This alternative scenario would require one of the stars to be “stripped”, meaning that, at an earlier time, it had lost a large fraction of its mass to the other star. “We had reached the limit of the existing data, so we had to turn to a different observational strategy to decide between the two scenarios proposed by the two teams,” says KU Leuven researcher Abigail Frost, I n 2020 a team led by European Southern Observatory (ESO) as- tronomers reported the closest black hole to Earth, located just 1000 light-years away in the HR 6819 system. But the results of their study were contested by other re- searchers, including by an interna- tional team based at KU Leuven, Belgium. In a paper, these two teams have united to report that there is in fact no black hole in HR 6819, which is instead a “vampire” two-star system in a rare and short- lived stage of its evolution. The original study on HR 6819 received significant attention from both the press and scientists. Thomas Riv- inius, a Chile-based ESO astronomer and lead author on that paper, was not surprised by the astronomy community’s reception to their dis- covery of the black hole. “Not only is it normal, but it should be that re- sults are scrutinised,” he says, “and a result that makes the headlines even more so.” Rivinius and his colleagues were convinced that the best explanation “Closest black hole” system found to contain no black hole by ESO - Bárbara Ferreira who led the new study published in Astronomy & Astrophysics . To solve the mystery, the two teams worked together to obtain new, sharper data of HR 6819 using ESO’s Very Large Telescope (VLT) and Very Large Telescope Interferometer (VLTI). “The VLTI was the only facil- ity that would give us the decisive data we needed to distinguish be- tween the two explanations,” says Dietrich Baade, author on both the original HR 6819 study and the new Astronomy & Astrophysics paper. Since it made no sense to ask for the same observation twice, the two teams joined forces, which allowed them to pool their resources and knowledge to find the true nature