Two years ago, astronomers thought they had seen signs of a black hole 1000 light years from Earth – but it turned out to be a star being consumed by its neighbor.
A team of astronomers in 2020 reported the discovery of the closest black hole to Earth. A new study shows they had misidentified a cosmic phenomenon. The team from the European Southern Observatory (ESO) reported the detection of a black hole just 1000 light-years away from Earth in the HR 6819 system.
In a new paper, an international team based at KU Leuven, Belgium has contested the finding and revealed that there is in fact no black hole in HR 6819, instead, it’s a “vampire” two-star system in a rare and short-lived stage of its evolution. Astronomers have taken a good look at what happens when a “vampire” star sucks the outer layers of material from a companion star.
THE ORIGINAL PROPOSAL
The original study in 2020 was led by Thomas Rivinius, a Chile-based ESO astronomer, who welcomed the scrutiny and said, “Not only is it normal, but it should be that results are scrutinized, and a result that makes the headlines even more so.” The team under Rivinius was confident 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.
However, Julia Bodensteiner, then a Ph.D. student at KU Leuven proposed that HR 6819 could also be a system with only two stars on a 40-day orbit and no black hole at all. Now the researchers say they were mistaken: the black hole does not exist.
Dietrich Baade, an emeritus astronomer at European Southern Observatory (ESO) and a co-author of the work, said just one blob of light was previously detected, containing the hallmarks of two stars.
Since both stars are of similar brightness and the same age, they would normally have the same mass and would whirl each other around with similar, high velocities.
“Since we saw that only one of the stars was whirled around at high velocity by some massive object, which we didn’t see, we assumed this unseen massive object to be a third body, namely a black hole,” he said. Abigail Frost, a researcher at KU Leuven and the lead author of the new study, says that they had to turn to a different observational strategy to decide between the two scenarios proposed by the two teams.
The two teams worked together to conduct new observations of the system and gather fresh data HR 6819 using ESO’s Very Large Telescope (VLT) and Very Large Telescope Interferometer (VLTI). The new data confirmed that two companion stars were not accompanied by a black hole as was previously proposed.
The two stars instead exist as a binary system, gravitationally married in an orbit lasting about 40 days. While binary systems are common, what makes this one so unique is that it has provided a rare glimpse of the immediate aftermath of what is dubbed stellar vampirism.
“What we mean by stellar vampirism is that one star has sucked the outer material off of another star,” said astronomer Abigail Frost. The team explained that caught this binary system in a moment shortly after one of the stars had sucked the atmosphere off its companion star. While the donor star was stripped of some of its material, the recipient star began to spin more rapidly.
“Catching such a post-interaction phase is extremely difficult as it is so short. 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,” Abigail said in a statement.
Astronomers say it’s difficult to catch stellar vampirism in action since it’s a short-lived phenomenon, but these findings may prove useful for understanding the evolution of massive stars, and the formation and behavior of associated phenomena like gravitational waves and violent supernova explosions. Insights into these processes ultimately help give scientists a better picture of how the entire universe formed and how it continues to change over time.
The ESO-Leuven team plans to continue monitoring HR 6819 to follow its lifecycle and learn more about binary systems. They haven’t given up yet on finding stellar black holes, which are a bit of an elusive species. But as astronomers estimate there may be 40 quintillion black holes hiding in the dark recesses of space, one is almost certain to turn up.
Astronomers are now planning to monitor HR 6819 more closely using the VLTI’s GRAVITY instrument to better understand its evolution.