Astronomers using the Fiber-fed Extended Range Optical Spectrograph (FEROS) on the MPG/ESO 2.2-m telescope at ESO’s La Silla Observatory have found a stellar-mass black hole in HR 6819 (also known as HD 167128), a naked-eye stellar system located 1,119 light-years away in the constellation of Telescopium. The system is made up of an inner binary with a B3-type star and an unseen companion — the closest known black hole to the Sun — and a classical Be star in a wide orbit.
An artist’s impression of the HR 6819 triple system. Image credit: L. Calçada / ESO.
“We were totally surprised when we realised that this is the first stellar system with a black hole that can be seen with the unaided eye,” said Dr. Petr Hadrava, Emeritus Scientist at the Academy of Sciences of the Czech Republic in Prague and co-author of the research.
“This system contains the nearest black hole to Earth that we know of,” added Dr. Thomas Rivinius, an astronomer at ESO.
The researchers originally observed HR 6819 as part of a study of binary stellar systems. However, as they analysed their observations, they were stunned when they revealed a third, previously undiscovered body in HR 6819.
The observations with the FEROS instrument showed that one of the two stars in the HR 6819 system orbits an unseen object every 40 days, while the second star is at a large distance from this inner pair.
The black hole in HR 6819 is one of the very first stellar-mass black holes found that do not interact violently with their environment and, therefore, appear truly black. But the scientists could spot its presence and calculate its mass by studying the orbit of the star in the inner pair.
“An invisible object with a mass at least 4.2 times that of the Sun can only be a black hole,” Dr. Rivinius said.
The discovery provides clues about where the many hidden black holes in the Milky Way might be.
“There must be hundreds of millions of black holes out there, but we know about only very few. Knowing what to look for should put us in a better position to find them,” Dr. Rivinius said.
“Finding a black hole in a triple system so close by indicates that we are seeing just the tip of an exciting iceberg,” said Dr. Dietrich Baade, an astronomer at ESO.
“We realised that another system, called LB-1, may also be such a triple, though we’d need more observations to say for sure,” said Dr. Marianne Heida, a postdoctoral researcher at ESO.
“LB-1 is a bit further away from Earth but still pretty close in astronomical terms, so that means that probably many more of these systems exist.”
“By finding and studying them we can learn a lot about the formation and evolution of those rare stars that begin their lives with more than about 8 times the mass of the Sun and end them in a supernova explosion that leaves behind a black hole.”
The discovery is reported in a paper in the journal Astronomy & Astrophysics.
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Th. Rivinius et al. 2020. A naked-eye triple system with a nonaccreting black hole in the inner binary. A&A 637, L3; doi: 10.1051/0004-6361/202038020
