The existence of a black hole just a thousand light-years away has been disputed, but has now been debunked.

In 2020, astronomers came up with big news. They had spotted a real black hole just a thousand light-years away. A special discovery. Because at that distance, this was the closest black hole found so far. Yet this discovery raised the eyebrows of many astronomers. Because was it really a black hole that the astronomers had seen?

HR 6819

The alleged closest black hole would be part of a triple system known as HR 6819. One of the stars would complete one orbit around the black hole in a fortnight, while a second star orbits it in a much wider orbit. Located in the southern constellation Telescope, the system is so close that the two companion stars can be seen in the night sky on a dark, clear evening without a telescope or binoculars.

This map shows the position of the triple system HR 6819 in the constellation Telescopium (Telescope), which is said to contain ‘the closest black hole observed to date’. The map shows almost all stars that are visible to the naked eye under good conditions. HR 6819 is circled in red. Although the black hole itself is invisible, both stars in the system are visible from the Southern Hemisphere on a dark, clear night without binoculars or telescopes. Image: ESO, IAU and Sky & Telescope

Not surprisingly, this discovery received a lot of attention, both from the press and from scientists. Some astronomers even started to question it. Chilean ESO astronomer and research leader Thomas Rivinius was not very surprised by this. “It is not only normal, but also necessary for results to be looked at critically,” he says, “especially when a result makes headlines.”

disputed

One of the research groups that disputed the discovery was an international team from KU Leuven in Belgium. Based on the same data, researcher Julia Bodensteiner proposed a completely different explanation: HR 6819 could also be a system without a black hole, consisting of only two stars that revolve around each other in forty days. This alternate scenario would require one of the stars to be “dismantled,” meaning it has lost much of its mass to the other star in the recent past.

Resit

In order to solve the mystery once and for all, the two teams decided to team up and play together a new study to search for the truth. Using ESOs Very Large Telescope (VLT) and Very Large Telescope Interferometer (VLTI) they re-examined HR 6819. “The scenarios we were looking for are very different and easily distinguishable with the right instrument,” says Rivinius. “We agreed that the system contains two light sources. However, the question was whether they are orbiting closely, as in the decommissioned star scenario, or far apart, as in the black hole scenario.”

No black hole

The research leads to a perhaps somewhat sobering conclusion. Because the alleged black hole in HR 6819 turns out to be a mirage. The closest ‘system with a black hole’ appears to contain no black hole at all. Powerful instruments confirmed that there is no bright companion in wider orbit and that the two bright light sources are only a third of the Earth-Sun distance from each other. This means that HR 6819 is a binary star without a black hole.

Rare double star

It is true that this is a special binary star, which is in a rare and short-lived phase of evolution. “As far as we know, we’re dealing with a star system in which one star has ‘sucked’ the atmosphere away from its companion,” Bodensteiner said. “This is a common phenomenon in compact binary stars, also known as ‘stellar vampirism’. While the donor star lost some of its material, the receiving star has started spinning faster.” The chances of witnessing the aftermath of such an interaction are extremely slim because it lasts so short. And that’s what makes HR 6819 so interesting: it’s a perfect candidate for investigating how this vampirism influences the evolution of massive stars, and associated phenomena such as gravitational waves and violent supernova explosions.

Further research is already in the offing. The plan is to take a closer look at HR 6819 to better understand its evolution. That knowledge can be used to learn more about other binary star systems. As for the search for black holes? The team remains optimistic. Stellar-mass black holes remain very elusive by their nature, Rivinius says. Rough estimates, however, suggest that there must be tens to hundreds of millions of black holes in our Milky Way Galaxy alone. It’s probably only a matter of time before astronomers uncover them.