The alleged 'nearest black hole' to Earth isn't a black hole

In stellar evolution, a contested multiple star system is a missing link

The closest black hole to Earth isn't actually a black hole. Instead, scientists discovered that what they thought was a stellar triplet — two stars and a black hole — is actually a pair of stars at a unique stage of evolution.

A team of researchers stated in May 2020 that the star system HR 6819 consisted of a bright, massive star bound in a tight 40-day orbit with a nonfeeding, unseen black hole, as well as a second star orbiting further away. With a distance of about 1,000 light-years from Earth, this black hole would be the closest to us (SN: 5/6/20).

However, other teams evaluated the same data over the following months and came to a different conclusion: the system only has two stars and no black hole.

Now, the original team and one of the follow-up teams have teamed up to examine HR 6819 using more powerful telescopes that collect different types of data. The new data allows astronomers to see finer features on the sky, allowing them to determine how many objects are in the system and what type of objects they are, according to the teams' paper in Astronomy & Astrophysics in March."In the end, it was the binary system that best explained everything," says KU Leuven astronomer Abigail Frost.

Previous observations of HR 6819 revealed it to be a single object, making it impossible for astronomers to distinguish between the members in the system or their masses. Frost and colleagues used the Very Large Telescope Array, a network of four interconnected telescopes in Chile that can see the individual stars, to determine HR 6819's true nature.

"It allowed us to definitively untangle that original signal," Frost adds, "which is critical for determining how many stars were in it and whether one of them was a black hole."

One of the stars is thought to be a big bright blue star that has been siphoning material from its companion star's swollen atmosphere, according to the astronomers. That companion star's gaseous atmosphere is rapidly dwindling. "It's already gone through its main life," says Kareem El-Badry, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "But because the outside has been stripped off, and you only see the exposed core, it has similar temperature, luminosity, and radius to a young star." El-Badry was not engaged in the current research, but in 2021, he suggested that HR 6819 is a binary system.

The core hue and brightness of this siphoned star may trick researchers viewing at older data into thinking it was a newborn star with 10 times the mass. This star appeared to be orbiting something big but invisible – a black hole — at first.

Once the researchers figured out the system's parameters, they concluded it was a one-of-a-kind system, exhibiting a phase that astronomers had never seen previously in big star systems. "It's a missing link in double star evolution," says Maxwell Moe of the University of Arizona in Tucson, who was not involved in the new research.

Frost and her colleagues are using the Very Large Telescope Array to trace the movement of the stars in HR 6819 over the course of a year. "We really want to know how the individual stars in the system are ticking," she explains. The knowledge will subsequently be used in computer simulations of binary star evolution. "It's fantastic to now have a framework that we can utilize as a cornerstone to look into this further," Frost says.

Despite the fact that HR 6819 lacks the closest black hole to Earth, it appears to have something more helpful for astronomers.