Star system thought to contain Earth’s ‘closest black hole’ doesn’t actually have it, study finds

A star system located just 1,000 light-years away, which was thought to contain the closest black hole to Earth, does not actually contain a black hole, according to a new study.

Researchers led by European Southern Observatory (ESO) astronomers reported in 2020 that the closest black hole to Earth in the HR 6819 system was 1,109 light-years away.

They had found theoretical evidence of the invisible cosmic entity by tracking two stars using the MPG/ESO 2.2m telescope at ESO’s La Silla Observatory in Chile, but these results were later contested by other researchers, including by an international team based at KU Leuven, Belgium.

The ESO astronomers were initially convinced that the best explanation for the data they had from telescope observation was that HR 6819 was a triple system, with one star orbiting a black hole every 40 days, and the second star in a much wider orbit.

In a new study, published last month in the journal Astronomy and Astrophysics, the two teams from ESO and KU Leuven have united to report that there is in fact no black hole in the star system HR 6819.

They say it is instead a “vampire” two-star system in a rare and short-lived stage of its evolution, in which one of the stars sucked the atmosphere off its companion star.

The new study argues that there could be only two stars in the system and no black hole, with one of the stars being “stripped,” meaning it had lost a large fraction of its mass to the other star at an earlier time.

“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,” KU Leuven researcher Abigail Frost, who led the new study, said in a statement.

The two teams of scientists 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 facility that would give us the decisive data we needed to distinguish between the two explanations,” Dietrich Baade, author of both the HR 6819 studies, said.

“We agreed that there were two sources of light in the system, so the question was whether they orbit each other closely, as in the stripped-star scenario, or are far apart from each other, as in the black hole scenario,” study lead author Thomas Rivinius, a Chile-based ESO astronomer, said.

New telescope observations confirmed that there was no bright companion in a wider orbit and the two bright sources were separated by only one-third of the distance between the earth and the sun.

“These data proved to be the final piece of the puzzle, and allowed us to conclude that HR 6819 is a binary system with no black hole,” Dr Frost added.

The new interpretation of data suggests that the scientists had caught the star system in a moment shortly after one of the stars had sucked the atmosphere off its companion star – a common phenomenon in close binary systems, sometimes referred to as “stellar vampirism”.

“While the donor star was stripped of some of its material, the recipient star began to spin more rapidly,” Julia Bodensteiner, another co-author, explained.

Researchers say catching such a post-interaction phase of two stars is extremely difficult as it is short-lived.

“This makes our findings for HR 6819 very exciting, as it presents a perfect candidate to study how this vampirism affects the evolution of massive stars,” Dr Frost said.

The scientists next plan to conduct another joint study of the star system over time, to better understand its evolution, constrain its properties, and use that knowledge to learn more about other binary systems.

“We conclude that HR 6819 is a binary system and that no BH is present in the system. The unique nature of HR 6819, and its proximity to Earth make it an ideal system for quantitatively characterising the immediate outcome of binary interaction,” scientists wrote in the study.