A newly discovered black hole is closer than any other to Earth, scientists say.
It is so nearby that the stars that swirl around it can be see with the naked eye, they write in a new study.
And the object could be just the "tip of the iceberg" with many other similar black holes being hidden and waiting to be found, the astronomers say.
The black hole is a mere 1,000 lightyears from Earth, and was spotted using a telescope at the European Southern Observatory's facility in Chile. But if a person is in the southern hemisphere, they may be able to see the stars of its system on a dark, clear night, without need for a telescope.
"We were totally surprised when we realised that this is the first stellar system with a black hole that can be seen with the unaided eye," said Petr Hadrava, Emeritus Scientist at the Academy of Sciences of the Czech Republic in Prague and co-author of the research.
The system was first observed as part of a study on double-star systems, with scientists watching it to better understand those solar systems that have two suns. But they were shocked to find that one particular example appeared to be hiding a black hole, which had previously not been discovered.
It is one of the first black holes to be found that does not interact violently with its environment, meaning that it is truly black, and not visible due to its impact on other visible objects like stars. Instead, astronomers spotted its presence by watching the orbit of the system's inner star – which seemed to be disturbed by some large, mysterious object.
"An invisible object with a mass at least 4 times that of the Sun can only be a black hole," said ESO scientist Thomas Rivinius, who led the study.
One of the two stars seems to orbit around the unseen object every 40 days, while the other sits at a further distance from both. That meant that astronomers had to watch the system over a longer period, allowing them to get a picture of the orbits over several months of observations.
The fact that the black hole was not easily spotted means that there could be many more such systems waiting to be found, and that the new discovery could be just the "tip of the iceberg, according to Rivinius.
"There must be hundreds of millions of black holes out there, but we know about only very few," he said. "Knowing what to look for should put us in a better position to find them."
The fact that the black hole is so close indicates that there are probably many more, further away and more difficult to discover, the scientists say.
Already, astronomers have speculated that another system known as LB-1 that is also relatively close to Earth might also be such a triple system, since observations have led to "stunningly similar" data, the researchers write in the new paper. Further observations are needed to confirm that discovery.
So far, astronomers have only seen a relatively small number of black holes in our galaxy. Almost all of them had strong interactions with their environment, meaning that they could be detected by the powerful X-rays that behaviour flings through space.
The new paper, 'A naked-eye triple system with a nonaccreting black hole in the inner binary', is published in Astronomy & Astrophysics.
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