Scientists have uncovered a black hole with two stars as companions for the first time.

Until now, black holes have been directly detected paired up with just one other object such as a star or another black hole.

The "black hole triple", called V404 Cygni, was originally thought to be a binary system, but US researchers stumbled across a third body, a star, almost by chance.

The finding, published in Nature today, is so unusual that it might cause a rethink about how black holes are formed, according to Kevin Burdge, an astrophysicist from the Massachusetts Institute of Technology (MIT) and one of the researchers on the new study.

“We think most black holes form from violent explosions of stars, but this discovery helps call that into question,” Dr Burdge said.

Instead, the team suggest, sometimes black holes may form from a "gentle" collapse inwards.

From one companion to two

V404 Cygni, which lies nearly 8,000 light-years away from Earth in the constellation of Cygnus, has long been of interest to astronomers.

It was one of the first confirmed black holes, with astronomers validating it in 1992.

Before the new study, scientists thought the system comprised one small black hole (known as a stellar black hole) with a mass about nine times our Sun, and a red giant star, which orbited each other every six-and-a-half days. 

The system is also known as a nova — it has been caught brightening and dimming multiple times since its discovery in 1938. The brightening is caused by the black hole consuming part of the star.

While trawling through archival data from multiple telescopes, Dr Burdge saw what appeared to be two blobs of light, surprisingly close to each other.

While the first star was already known, the second star was previously thought to be temporarily passing through, and called an interloper.

“The fact that we can see two separate stars over this much distance actually means that the stars have to be really very far apart,” Dr Burdge said.

The binary system and its stellar companion are about 90 times further apart than the Sun is from Pluto.

The team used 10 years of data from the Gaia space telescope to confirm that despite being far apart, the distant star did orbit the black hole/star pair.

They calculated the odds of it moving that way without being gravitationally connected at just one in 10 million.

According to Fiona Panther, an astrophysicist at the University of Western Australia who was not involved in the paper, the team did an impressive job of showing the additional star does, in fact, orbit the binary system.

"It's really quite a nice result," she said. 

"This shows there could be a non-negligible number of systems out there where there's interaction with a third companion star.

A 'gentle' black hole birth

The newly confirmed companion star being part of the system creates other problems.

Stellar black holes are thought to form in the violent explosion of a star, known as a supernova.

With its current companions, the black hole couldn't have violently exploded when it turned from a star into a black hole.

Unlike explosions on Earth, stars don't usually explode in a neat, symmetrical way. 

Instead, they sometimes end up exploding more in one direction than the other, and this pull would have caused the weak gravity connecting the three stars to "snap".

If this connection was lost, it would have created a rogue star wandering the Universe.

"All those forces have to be balanced. This is basic mechanics," Dr Panther said.

So, without an explosion, how did the black hole form? 

The researchers ran thousands of simulations, and found the easiest way to produce this type of system was to create a black hole from a collapse instead of an explosion.

The simulations suggest the dying star would have to collapse at least half its mass in on itself to not kick out its companions. The team suggest it may have even completely imploded, with no kick at all.

This is considered a "gentle" black hole formation.

"If there's no explosion, there's no unbalanced force that needs to be countered by the black hole moving off in the opposite direction," Dr Panther said.

How many more are out there? 

While this is the first time a triple system like this has been found, it probably won't be the last, according to Dr Panther.

Dr Panther works on detecting ripples in space-time called gravitational waves from unusual black hole systems.

She said this new finding could lead to entirely new understanding of gravitational waves too. 

"I think these are going be very important when we think about gravitational waves going forward."