Astronomers find closest black hole to Earth

There's a black hole lurking nearby, apparently, but you can breathe easy: It's not a threat to Earth. 

Astronomers believe they have found the closest black hole to our solar system, just 1,000 light years away, which in astronomical terms is right in our neighbourhood.

The black hole — which is roughly four times the mass of our sun — is joined by two stars, making it a triple system, called HR 6819. The stars are visible with the naked eye in the southern hemisphere, in the constellation Telescopium.

Astronomers at the European Southern Observatory (ESO) in La Silla, Chile, found that one star orbited an unseen object every 40 days. The second, which lies farthest from the invisible object, hasn't been observed moving at all (it's likely on a slower orbit).

Both stars appeared to wobble, indicating that there was something nearby. But whatever it was, it couldn't be seen with the observatory's MPG/ESO 2.2-metre telescope.

The observations were made with a spectrograph, where light is broken down into its individual colours. This allows astronomers to measure their motion, and in this case, the stars appeared to wobble, indicating that there was something nearby. But whatever it was, it couldn't be seen.

Though it's estimated our galaxy could contain hundreds of millions of black holes, only a couple dozen have been found, and that's why this potential discovery is so important.

"Black holes aren't rare; they're just very hard to find," said Thomas Rivinius, an astronomer at the ESO and a corresponding author of the study, published Wednesday in the journal Astronomy and Astrophysics on the discovery.

Most black holes, those considered "active," have been detected by their interaction with matter, such as stars that wander too close. Just before the stellar matter falls into the black hole, never to escape, it releases powerful X-rays, allowing astronomers to "see" them. 

But "inactive" black holes are invisible. However, if there is a star close — but not too close — astronomers can detect it by the wobble of the star.

Astronomers are hoping to find others in this manner.

"The fact that it's so close by, means that there must be more of these out there, way more, because if there's only one in the Milky Way, it would be a little bit too convenient if it's right next door to us," said Marianne Heida, an astronomer with ESO in Garching bei München, Germany, also a corresponding author of the study.

Seeded the universe

Black holes form when a massive star dies and collapses into itself, which first causes a supernova, a spectacular stellar explosion. It's believed that these types of explosions seeded the universe with most of the elements we know. So they're an important piece of the puzzle when it comes to understanding how most matter — including ourselves — have come to be.

"Black holes are cool," Heida said. "And these massive stars are the stars that produced basically all the elements that we are made of."

Priya Natarajan, an astronomer and professor at Yale University who was not involved in the study, said the findings are extremely helpful in better understanding how astronomers might be able to find these invisible galactic phenomena.

"It makes a huge difference for our understanding of what matters, like what the constituents of our galaxy are," she said. 

Accidental discovery

The black hole was discovered by accident.

In 2004, astronomers were looking for a particular type of binary star system. HR 6819 was a candidate — except there was something a little off about it. It seemed there was a third, unseen, object. 

After the leader of this investigation, Stan Stefl, died in a car accident in 2014, the work fell by the wayside.

However, last November, a separate study suggested the existence of an unusually massive black hole, roughly 70 times the mass of our sun. The finding was controversial, as astronomers believe the upper limit for a stellar-mass black hole (as opposed to supermassive black holes which lie at the centre of most galaxies) should max out somewhere around 30 times the mass of the sun.

That study took Rivinius back to HR 6819. He thought that perhaps it, too, was home to a stellar-mass black hole.

Follow-up observations suggested that was likely the case. 

"The cool thing about this system is that it's so close by, that we can hopefully do what's called optical interferometry, so then you get like really, really, really, really, high resolution images, basically," Heida said. "If we get those observations, then we will be able to prove that there is a black hole in there."

Optical interferometry involves telescopes that are widely separated, even around the globe, are combined to make precise observations. It is a method similar to that which was used to take the first photograph of a black hole, found at the centre of the galaxy Messier 87.

The findings aren't a slam dunk: Heida said that some have suggested there could still be something else not seen in the spectrum, but that could be observed visually. That's why they are anxiously awaiting visual observations. 

Could there be even closer black holes?

"It's fairly likely that there are black holes even closer, but if they're lonely without a companion we will never find them," Rivinius said.