Thanks to a team of astronomers led by the European Southern Observatory (ESO), the closest black hole to Earth has been discovered. Using the ESO’s La Silla Observatory in Chile, the black hole was found in a triple system, around 1000 light-years (9,500,000,000,000,000,000 kilometers) away from Earth.  

Black holes are regions in space where matter has been so condensed, and the force of gravity is so strong, that nothing including light has the velocity to escape once it has entered a region known as the ‘event horizon’. Black holes were first predicted from Einstein’s theory of relativity, where mathematically it can be shown that a sufficiently compact mass can deform spacetime to form a black hole. 

There are two main types of black hole: supermassive and stellar. A supermassive black hole is thought to be at the centre of most galaxies, and it can range in size from 1 million to 40 billion solar masses. A stellar black hole is formed from the collapse of a large star and typically ranges from 3-100 solar masses; the newly discovered black hole is of this kind.  

The black hole was discovered during research into binary star systems. While observing the HR 6819 system in the Telescopium constellation, the scientists found evidence of a third unseen object which was approximately 4.2 times the size of the sun. This mass is within the range of a stellar black hole. However, unlike the few dozen stellar-mass black holes that are known in the Milky-Way, HR 6819 is the first to be discovered that does not interact violently with its environment and so is truly invisible. Its presence was confirmed by studying the effect it has on the orbit of the inner stellar companion of the binary. 

By assuming the presence of a third object, the team showed how the mass of the black hole could be better-constrained and more consistent with known stellar black holes. Marianne Heida, a postdoctoral fellow at ESO and co-author of the paper, quoted in the papers release: “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 eight times the mass of the Sun and end them in a supernova explosion that leaves behind a black hole.”

And so, this discovery has opened a new area of research into stellar-mass black holes within our galaxy. Particularly as research into triple systems could provide additional insight into the kind of violent mergers that result in strong gravitational waves.  

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