In 2020, a team led by astronomers from the Southern European Observatory (ESO) reported the closest black hole to Earth, which is only 1,000 light-years away. But other researchers have challenged their findings. After further observations from the ground and space, both the original researchers and their critics came to the conclusion that there is no black hole.
In 2020 a team led by astronomers from the Southern European Observatory (ESO) reported the closest black hole to Earth, which is only 1000 light-years away in HR 6819. But other researchers have challenged their findings, including an international team from KU Leuven University in Belgium. In an article published today, these two teams have teamed up to report that there is actually no black hole in HR 6819, which is instead a two-star “vampire” system that is in a rare and short stage in its evolution.
The original study on HR 6819 has garnered much attention from both the press and scientists. Thomas Rabinius, an ESO astronomer working in Chile and lead author of that article, was not surprised by the astronomers’ reaction to the discovery of their black hole. “Not only is it normal, so it should be, results need to be scrutinized carefully,” he says, “and a result that reaches headlines even more.”
Rabinius and colleagues were convinced that the best explanation for the data they had, obtained using the 2.20-meter MPG / ESO telescope, was that HR 6819 is a triple system, with one star orbiting a black hole every 40 days and a second star in a much wider orbit. But a study led by Julia Bodensteiner, then a doctoral student at KU Leuven University in Belgium, suggested a different explanation for the same data: HR 6819 could also be a system with only two stars in a 40-day orbit and no black hole at all. This alternative scenario requires that one of the stars be “exposed”, meaning that in an earlier period it lost a large part of its mass to the other star.
“We have reached the limit of existing data, so we had to resort to a different observation strategy to decide between the two scenarios proposed by the two teams,” said KU researcher Leuven Abigail Frost, who led the new study published today in the journal Astronomy & Astrophysics.
To solve the mystery, the teams worked together to obtain new and sharper data on the HR 6819 using ESO’s very large telescope (VLT) and the VLT (VLTI) interferometer. “VLTI was the only means that could give us the crucial data we needed to differentiate between the two explanations,” says Dietrich Bada, who co-authored the original study on HR 6819 and also the new article in Astronomy & Astrophysics. Since there was no point in requesting the same observation twice, the two teams joined forces, and this allowed them to pool their resources and knowledge to discover the true nature of this system.
“The scenarios we were looking for were quite clear, very different and easy to differentiate with the right device,” says Rabinius. “We agreed that there are two light sources in the system, so the question was whether they orbit each other close, as in the exposed star scenario, or are they as far apart as in the black hole scenario.”
To differentiate between the two propositions, the astronomers used VLTI’s GRAVITY instrument and ESO’s MUSE instrument.
“MUSE has confirmed that there is no bright companion star in a wider orbit, while GRAVITY’s high spatial resolution has been able to distinguish two bright sources whose distance is only a third of the distance between the Sun and Earth,” says Frost. “This data was the last part of the hut, and allowed us to conclude that HR 6819 is a dual system without a black hole.”