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Astronomers find 'truly black' black hole lurking in a nearby star system

Wednesday, May 6th 2020, 5:29 pm - This discovery of a black hole hiding in a star system so close to us was quite the surprise for astronomers.

An international team of astronomers has located the closest known black hole to Earth, hidden in a star system just 1,000 light-years away. The discovery of this nearby black hole was not only a welcome surprise, but it may help astronomers to find more of these lurking out in space.

It may come as quite the surprise to stargazers in the southern hemisphere, but a black hole has been hiding in plain view to them, every night.

HR 6819 is a fairly non-descript star in the night sky, tucked away in the corner of the southern constellation of Telescopium. Although easily spotted by the unaided eye from anywhere with good dark-sky conditions, it's unlikely to have attracted much attention, until now.

Initially, HR 6819 was identified as a double-star system, with two giant blue stars, one orbiting the other. Then a team of five astronomers, led by Thomas Rivinius with the European Southern Observatory, took a closer look while surveying 'binary' star systems. What they found came as a shock. The inner star of the system appeared to be circling an unseen, third object!

With that realization, they had just made three simultaneous discoveries! HR 6819 was a triple system rather than a double system, the unseen component was a newfound black hole, and this was the closest black hole to Earth ever found!

Closest-BlackHole-labelled-eso2007aThis artist's impression of the HR 6819 star system shows the three members of this trinary - the roughly four stellar-mass black hole, and its two giant blue star companions. Credit: ESO/L. Calçada/Scott Sutherland

"This system contains the nearest black hole to Earth that we know of," Rivinius said in an ESO press release.

"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," study co-author Petr Hadrava, Emeritus Scientist at the Academy of Sciences of the Czech Republic in Prague and co-author of the research, told the ESO.

Co-author Dietrich Baade, Emeritus Astronomer at ESO in Garching, Germany, called the discovery of this hidden black hole "the tip of an exciting iceberg."


HR-6819-NightSky-eso2007bThe location of HR 6819 in the southern constellation of Telescopium. Credits: ESO, IAU and Sky & Telescope

So, how, for a star so close by and easily spotted in the southern night sky, could a black hole have remained hidden there for so long?

This particular black hole is 'truly black'.

Black holes are called 'black' because they - themselves - do not emit any light or radiation. Thus, a black hole, all by itself, will not give out any direct clues to its existence to astronomers here on Earth. The only way to find one of these is to look for the effects of its gravitational pull, which is strong enough to even warp the shape of spacetime around it.

Most black holes that have companion stars, however, end up surrounded by a disk of material that was pulled off the outer layers of its companion. This is known as an 'accretion disk'. As the material in the disk gets closer and closer to the black hole, it is heated to incredible temperatures, causing it to emit hard radiation in the form of X-rays. These x-rays are usually how astronomers find these black holes, as they are picked up by special telescopes.

Stellar-black-hole-eso1028aThis artist's impression of a stellar-mass black hole shows it pulling matter off a red giant companion star, forming an accretion disk around it. Twin jets of high-energy particles blast from the 'poles' of the black hole, due to the matter swirling into it. Credit: Credit: ESO/L. Calçada/M.Kornmesser

In this case, however, the black hole in the HR 6819 system doesn't have an accretion disk. This could be due to the fact that, between the black hole and its blue companion, the star appears to be the heavier of the two objects. The astronomers on the discovery team have estimated that this star is around five times more massive than the Sun, while the black hole is most likely around 4.2 solar masses.

Thus, with the two objects circling each other, they apparently keep enough distance between them that the black hole can't draw matter off the companion. As a result, the HR 6819 black hole remained 'truly black', emitting no light or radiation, and was therefore invisible to direct detection by telescopes. It was only found by spotting the 40-day 'wobble' of the companion.

The truly exciting part of this discovery, for the astronomers, is how it may lead to even more discoveries!

"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," Rivinius said.

Source: ESO


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