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‘Closest black hole’ isn’t actually a black hole, but a stellar vampire

In 2020, astronomers announced they had found the closest black hole ever, located just 1,000 light-years away. However, recent research suggests that the object in question is not actually a black hole, but is rather a rare form of two-star “vampire” system.

The system, called HR 6819, was originally thought to be a three system, with two stars orbiting a black hole — one star near to the black hole and the other much further away. But other researchers suggested a different interpretation of the data, with two stars orbiting each other and no black hole, where one of the stars was “stripped” down to a much lower mass.

This artist’s of an oblate star with a disc around it and B-type star that has been stripped of its atmosphere (background).
New research using data from ESO’s Very Large Telescope and Very Large Telescope Interferometer has revealed that HR 6819, previously believed to be a triple system with a black hole, is in fact a system of two stars with no black hole. This artist’s impression shows what the system might look like; it’s composed of an oblate star with a disc around it (a “vampire” star; foreground) and a B-type star that has been stripped of its atmosphere (background). ESO/L. Calçada

The two teams of researchers, both those who made the original finding and those who proposed the alternative theory, joined forces to research the possibilities. New data gathered using the European Southern Observatory (ESO)’s Very Large Telescope shows that there are two stars orbiting each other closely, supporting the idea of a stripped star. “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,” said lead researcher Abigail Frost in a statement.

“Our best interpretation so far is that we caught this binary system in a moment shortly after one of the stars had sucked the atmosphere off its companion star. This is a common phenomenon in close binary systems, sometimes referred to as ‘stellar vampirism’ in the press,” explained Julia Bodensteiner, co-author of the new study. “While the donor star was stripped of some of its material, the recipient star began to spin more rapidly.”

Even though the system turns out not to host a black hole, it is an exciting finding all the same as it gives researchers a chance to study this stellar vampirism at a crucial time. “Catching such a post-interaction phase is extremely difficult as it is so short,” said Frost. “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, and in turn the formation of their associated phenomena including gravitational waves and violent supernova explosions.”

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