The closest black hole to Earth is not a black hole at all. Instead, what scientists thought was a stellar triplet (two stars and a black hole) is actually a pair of stars caught in a single stage of evolution.
In May 2020, a team of astronomers reported that the star system HR 6819 was likely made up of a massive, bright star locked in a tight 40-day orbit with an unseen black hole that doesn’t feed plus a second star orbiting further out. . About 1,000 light-years from Earth, that would make this black hole closest to us (Serial number: 5/6/20). But over the next few months, other teams analyzed the same data and came to a different conclusion: the system hosts only two stars and no black holes.
Now the original team and one of the follow-up teams have teamed up and observed HR 6819 with more powerful telescopes that collect a different type of data. The new data can distinguish finer details in the sky, allowing astronomers to definitely see how many objects are in the system and what kind of objects they areteams report in March astronomy and astrophysics.
“Ultimately, it was the binary system that best explained everything,” says astronomer Abigail Frost of KU Leuven in Belgium.
Previous observations of HR 6819 showed it as a unit, so astronomers couldn’t tell the objects in the system or their masses apart. To pin down the true nature of HR 6819, Frost and his colleagues turned to the Very Large Telescope Array, a network of four interconnected telescopes in Chile that can essentially see the stars separately.
“It allowed us to definitively tease out that original signal, which is really important in determining how many stars were in it and whether one of them was a black hole,” says Frost.
Scientists believe that one of the stars is a massive bright blue star that has been pulling material from its companion star’s puffy atmosphere. That companion star now has little gaseous atmosphere left. “It has already gone through its main life, but because the exterior has been stripped away and only the exposed core is visible, it has a temperature, luminosity and radius similar to that of a young star,” says Kareem El-Badry, an astrophysicist at the Center. Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. El-Badry was not involved in the new study, but suggested in 2021 that HR 6819 is a binary system.
The color and brightness of this deviated star’s core could fool astronomers looking at older data into thinking it’s a young star with 10 times as much mass. Originally it seemed as if this star was orbiting something massive but invisible: a black hole.
Once the researchers unraveled the details of the system, they realized that it is unique, showing astronomers a never-before-seen phase between systems with massive stars. “It’s a missing link in the evolution of binary stars,” says astrophysicist Maxwell Moe of the University of Arizona in Tucson, who was also not part of the new study.
For years, astronomers have seen binary systems in which one star is actively pulling gas from the other, and they have seen systems in which the donor star is simply a bare stellar core. But in HR 6819, the donor star has stopped giving mass to the other. “It still has a little bit of shell left, but it’s rapidly contracting, evolving to become a remnant core,” says Moe.
Frost and his colleagues are using the Very Large Telescope Array to monitor HR 6819 for a year to precisely track how the stars are moving. “We want to really understand how the individual stars in the system work,” she says. The team will then use that information in computer simulations of the evolution of binary stars. “[It’s] exciting to now have a system that we can use as a kind of cornerstone to investigate this in more detail,” says Frost.
Although HR 6819 doesn’t have the closest black hole to Earth, it appears to have something more useful to astronomers.