Closest black hole is actually just one star eating another

Two years ago, astronomers thought they had seen signs of a black hole 1000 light years from Earth – but it turns out to be a star being consumed by its neighbour

By Alex Wilkins

Strange signals coming from 1000 light years away were once thought to be produced by the closest black hole to Earth, but further investigation shows they are actually from a pair of stars in a rare “vampire”-like system, where one star strips the other of its mass.

Two years ago, astronomers observed strange spectral lines from the star system HR 6819, just 1000 light years from Earth, and concluded that the likely explanation was a black hole four times bigger than the sun that affected two orbiting stars.

Now, those same researchers and a larger team led by Abigail Frost at KU Leuven in Belgium have observed the star system using more detailed spectroscopy from the Very Large Telescope (VLT) in Chile and interferometry, which can measure positions very precisely.

The researchers wanted the extra data so they could test whether the system was in fact just two stars very close together. A three-object system, of two stars and a black hole, would require one of the stars to be on a wide orbit far from the second star and the black hole. “What these two data sets allowed us to do was to distinguish between the two [scenarios],” says Frost.

The researchers observed nothing on a wide orbit, eliminating the scenario with a black hole.

“This is a really good example of how the scientific method works,” says Frost. “You propose an idea, someone else has another idea and you discuss it amongst yourselves and you think, OK, how could we push this further and actually decide which is the best explanation?”

Not only were the stars close together, but one of them appeared to be sucking material from the other, creating what is known as a Be star. Understanding the evolution of such stars could help us learn about how they become neutron stars or produce gravitational wave events.

“We think [HR 6819] is a very rare evolutionary stage of binary systems,” says Frost. “This binary interaction between these stars has the potential to completely change their evolution.”

Journal reference: Astronomy & Astrophysics, DOI: 10.1051/0004-6361/202143004