Astronomers have found the closest black hole to Earth yet.
And despite being an invisible "truly black" hole, it can - in a way - be seen with the naked eye.
HR 6819 is only 1000-light years away - close by galactic standards - and until now was thought to be a binary star system.
But measurements of the two stars' orbits and the light they were giving off didn't match up. Using the European Southern Observatory's (ESO) La Silla Observatory in Chile, astronomers worked out there was a third body in the system with a mass about four times that of our sun - but it was invisible.
"An invisible object with a mass at least four times that of the Sun can only be a black hole," said ESO scientist Thomas Rivinius.
Last year astronomers took the first-ever picture of a black hole - specifically an enormous one in a nearby galaxy called M87 - showing an empty black sphere which light cannot escape, surrounded by hot swirling gases. But with HR 6819's stars visible through a basic backyard telescope or binoculars, the black hole went undetected until now because it doesn't have any.
Its relative proximity to our solar system suggests nearby black holes might be far more common than previously believed, the astronomers behind the discovery said. So far only a "couple of dozen" have been found, "nearly all of which strongly interact with their environment and make their presence known by releasing powerful X-rays" - which HR 6819 doesn't.
"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," said Dr Rivinius.
Black holes are objects with such huge amounts of density and gravity; nothing can escape their pull - not even light. For example, if the sun were a black hole with exactly the same gravitational pull it has now, it would only need to be six kilometres across.
HR 6819 is in the constellation Telescopium, and from this distance looks like a single star when viewed through a telescope.
"It's remarkable that a black hole has been found so close to Earth," University College London astronomer and black hole expert Dr Ziri Younsi told The Guardian.
The Earth is about 25,000 light-years from the centre of the galaxy and its supermassive black hole, Sagittarius A*.
The next-closest confirmed black hole to Earth is 3000 light-years away. But last year scientists studying the anomalous orbits of the outer planets theorised there might be one orbiting the sun, perhaps as small as a tennis ball.
"If conventional searches fail to find Planet Nine and the evidence for TNO anomalies continues to grow, the primordial black hole Planet Nine hypothesis will become a compelling explanation," that paper said.
The latest findings were published in the journal Astronomy & Astrophysics.
