“An invisible object with a mass at least 4 times that of the Sun can only be a black hole...”
Astronomers say they have found a black hole that is part of a triple system that can be seen with the naked eye from the southern hemisphere on a dark and clear night.
A team of astronomers from the European Southern Observatory (ESO) have discovered a black hole ‘just’ 1,000 light-years from Earth. This would be the closest black hole to the Solar System ever detected to date. This black hole is part of a triple system that can be seen with the naked eye from Earth. The team found evidence of the presence of the black hole by tracking its two companion stars with the 2.2-meter MPG / ESO Telescope, installed at ESO’s La Silla Observatory in Chile.
“We were totally surprised when we realized that this is the first stellar system with a black hole that can be seen with the unaided eye,” says Petr Hadrava, scientist emeritus at the Academy of Sciences of the Czech Republic in Prague, and co-author of the study.
Located in the Telescopium constellation, the system is so close to Earth that its stars can be seen from the southern hemisphere on a dark, clear night without binoculars or telescope.
“This system contains the closest black hole to Earth we know of,” confirms ESO scientist Thomas Rivinius, who led the study published in the journal ‘Astronomy & Astrophysics’ in a statement.
Initially, the team studied the system, called ‘HR 6819’, as part of a study of double-star systems. However, upon analyzing their observations, they were surprised to discover a third body, previously unknown in ‘HR 6819’: a black hole.
Observations with the FEROS spectrograph, installed on the 2.2-meter MPG / ESO telescope at La Silla, showed that one of the two visible stars orbits an invisible object every 40 days, while the second star is located at a much greater distance away from the two inner objects.
As Dietrich Baade, ESO astronomer emeritus at Garching and co-author of the study points out, the observations needed to determine the 40-day period had to span several months.
“This was made possible by ESO’s pioneering observing service scheme, whereby ESO staff make observations on behalf of scientists who need them,” he says.
The hidden black hole in ‘HR 6819’ is one of the first stellar-mass black holes discovered to not violently interact with its surroundings and thus appear truly black. Despite this, the team was able to detect its presence and calculate its mass by studying the orbit of the star located in the inner pair. “An invisible object with a mass of at least 4 times that of the Sun can only be a black hole,” concludes Rivinius.
To date, astronomers have detected just a couple of dozen black holes in the Milky Way, and almost all of them interact with their environment and make their presence known by releasing powerful X-rays. But scientists estimate that, throughout the life of the Milky Way galaxy, many more stars ended up collapsing like black holes at the end of their lives.
The discovery of a black hole in ‘HR 6819’ provides clues as to where the numerous black holes hidden in the Milky Way might be. “There must be hundreds of millions of black holes out there, but we know very few. Knowing what to look for should make it easier for us to find them,” says Rivinius. This system could be just the “tip of an exciting iceberg,” Baade adds, as many more black holes similar to this could be discovered in the future.
Astronomers believe their discovery may already shed some light on a second system. “We realized that another system, called ‘LB-1‘, can also be a triple system composed of two stars orbiting a black hole, although we would need more observations to confirm our theory,” explained Marianne Heida, an ESO postdoctoral fellow, and co-author of the study.
“LB-1 ‘is a little further away from Earth, but still close enough in astronomical terms, which means that there are probably many more systems like this,” says Heida, adding that by finding and studying them, we can learn much about the formation and evolution of those stars that begin their lives with more than 8 times the mass of the Sun and end in a supernova explosion that leaves behind a black hole.