A team of astronomers from the European Southern Observatory (ESO) and other institutions have discovered a black hole that is just 1000 light-years or so from our planet. That means it is the closest black hole to Earth discovered to date. It is part of a triple star system whose stars can be seen with the naked eye from the southern hemisphere. Members of the team found evidence of the dark object’s existence by meticulously tracking its two space companions using the MPG/ESO telescope at the La Silla Observatory in Chile. Experts believe the system may represent just the proverbial tip of the iceberg and that many more black holes of this type may be found in the future.
“We were really surprised when we realised that this was the first stellar system with a black hole that can be seen with the naked eye,” said Petr Hadrava, Emeritus Scientist at the Astronomical Institute of the Czech Academy of Sciences and co-author of the research. The system is located in the Telescope constellation (Telescopium) and is so close to the Earth that it is visible from the southern hemisphere on a dark, clear night without the need for binoculars or a telescope. “The system includes the nearest back hole to Earth known to date,” notes Thomas Rivinius, an ESO researcher and the primary author of a paper that has been published in the prestigious journal Astronomy & Astrophysics.
The scientists had originally focused on the HR 6819 system as part of their research into binary stars. However, an analysis of the data surprisingly revealed a third, yet unknown object – a black hole. Observations using FEROS spectrograph and MPG/ESO 2.2-metre telescope at La Silla observatory in Chile revealed that one of the stars in fact orbits an invisible object with a period of 40 days while the other star in the system is located at a great distance from this central pair.
The unseen object within the HR 6819 system is one of the first stellar-mass black holes found that do not interact with their surroundings in any way. This makes it appear truly ‘black’. However, astronomers were able to detect its presence and calculate its mass by studying the orbit of an ordinary star that makes the other member of the pair. “An unseen object with a mass of at least four times that of our Sun can be nothing else but a black hole,” says Thomas Rivinius.
Astronomers have discovered only a few dozen black holes in our galaxy to date. Most of them strongly interact with their accretion discs, revealing their existence by powerful X-ray emissions. Experts estimate that many stars collapsed into black holes at the end of their lives during the formation of the Milky Way. The discovery of the silent, invisible black hole in the HR 6819 system shows researchers where to look for such hidden objects. “There must be hundreds of millions of black holes, but we have found just a few so far,” notes Thomas Rivinius. “The discovery of a black hole in a system of three objects near our planet suggests that we have only seen the proverbial tip of the iceberg,” adds Dietrich Baade.
Astronomers believe that the discovery could help them find another similar system. “We have found that another system, known as LB-1, could potentially also be a triple star system. However, we need more observations to know for sure,” says Marianne Heida, co-author of the paper and a postdoctoral fellow at ESO. “LB-1 is farther away from Earth but still quite close on the astronomical scale. That means that there likely exist many more such systems. When we discover and study them, we can learn much about the formation and evolution of relatively rare types of stars, meaning those that start their life with a mass eight times that of our Sun and die as supernovae in an explosion creating a black hole.”
Discoveries of triple star systems comprising an inner pair and a distant third object can also provide clues to understanding violent stellar collisions accompanied by emissions of gravitational waves intensive enough to be detected from Earth. Some astronomers believe that such collisions may occur within systems with a similar configuration as that of HR 6819 and LB-1, but where the inner pair comprises two black holes or a black hole and a neutron star. The distant third element may create such gravitational pull as to cause the inner pair to collide, emitting gravitational waves during the event. Although HR 6819 and LB-1 contain only one black hole and no neutron star, they may still help scientists understand the mechanics of collisions in triple star systems.
Source: Astronomical Institute of the Czech Academy of Sciences / ESO