The black hole closest to the Earth is not a black hole at all. In contrast, what scientists think of as a trinity of stars — two stars and a black hole — is actually a pair of stars at a unique stage in evolution.
In May 2020, a team of astronomers reported that the HR 6819 star system likely consisted of a bright, large star locked in a narrow 40 -day orbit with an invisible black hole that did not feed and a second star orbiting further away. At a distance of about 1000 light-years from Earth, this black hole will be closest to us (SN: 5/6/20). But in the months that followed, other teams analyzed the same data and came to different conclusions: the system had only two stars and no black holes.
Now the original team and one of the subsequent teams have joined forces and looked at the HR 6819 with a more powerful telescope that collects different types of data. The new data could reveal finer details in the sky, allowing astronomers to finally see how many objects are in the system and what types of objects are.
“Ultimately, it’s the binary system that explains everything best,” said astronomer Abigail Frost of KU Leuven in Belgium.
Previous observations of HR 6819 showed it as a single entity, so astronomers could not distinguish between an object in a system and its mass. To determine the true nature of the HR 6819, Frost and colleagues turned to the Very Large Telescope Order, a network of four interconnected telescopes in Chile that can essentially see individual stars.
“This allows us to decipher with certainty this early signal, which is very important for determining the number of stars in it and whether one of them is a black hole,” Frost said.
Scientists believe that one of the stars is a large bright blue star that sucks material from the inflatable atmosphere of the companion star. This companion star has little gas atmosphere remaining. “It has gone through its main life cycle, but because its outer shell has been torn off and you only see the core open, its temperature, luminosity and radius are similar to those of a young star,”said Karim El-Badri, an astrophysicist. at Harvard University. Smithsonian Center for Astrophysics in Cambridge, Massachusetts. El-Badri was not involved in the new study, but suggested in 2021 that HR 6819 is a binary system.
The color and brightness of the core of this pumped star can trick astronomers looking at old data into thinking it’s a young star with 10 times its mass. At first, the star seemed to rotate around something large but invisible – a black hole.
Once the researchers uncovered the details of the system, they realized that the system was unique, showing astronomers a phase never seen before among systems with large stars. “This is a missing link in the evolution of binary stars,” said astrophysicist Maxwell Moe of the University of Arizona at Tucson, who was also not involved in the new study.
Astronomers have for years observed binary systems in which one star actively draws gas from another, and they have seen systems in which the donor star is simply the core of an empty star. But in HR 6819, the donor star stopped donating mass to others. “It still has some shells left, but it’s quickly shrinking into a residual core,” Mo said.
Frost and his colleagues use a very large array of telescopes to observe HR 6819 throughout the year to track exactly how the stars move. “We want to really understand how the individual stars in the system work,” he said. The team will then use this information in a computer simulation of the evolution of binary stars. “[It’s] great because we now have a system that we can use as a basis for more detailed research,” Frost said.
Although HR 6819 does not have the closest black hole to Earth, it does seem to have something more useful for astronomers.