An unexpected discovery in space: a black hole leads to a love triangle

Astronomers know of many black holes that have an ordinary star as a partner. Now a research team has accidentally discovered that a black hole forms a system shared with not just one star, but even two stars.

What's particularly surprising is that the second star orbits the black hole at a very great distance, according to the group in the journal Nature.

A powerful boost to the star's body

Black holes form when massive stars exhaust their nuclear energy reserves and collapse. At the same time, stars explosively expel a large portion of their outer crust into space. The dying stars then shine brightly as a “supernova” for some time. Meanwhile, depending on its mass, the star's interior collapses into a neutron star or black hole.

Since the explosion of the star's outer atmosphere is asymmetric, the resulting star's body receives a strong shock. This “kick” from the supernova's bounce catapults the neutron star or black hole onto a new path through the Milky Way.

If the dying star is a member of a nearby binary system, the kick drags its stellar partner, bound to the black hole by gravity, with it. In fact, astronomers know of many neutron stars and black holes that form close pairs with regular stars and move in orbits that indicate a strong kick in the formation.

An unexpected discovery in space

The situation is different if the dying star has a partner in a very wide orbit. Here the gravitational bond is very weak, and this star will lose its partner due to the effect of black hole formation. That's why it was surprising when Kevin Burdge from the Massachusetts Institute of Technology in the United States of America and his colleagues encountered a stellar companion in the black hole V404 Cygni at a distance of 3,500 times Earth's distance from the Sun. This is the first time that astronomers have found not only one star, but two stars that are partners to a black hole.

This was a somewhat accidental discovery, Berg reported. The team searched archive data from several observatories for previously unknown black holes in the Milky Way. Out of pure curiosity, Burdge also looked at images of V404 Cygni, a black hole known since 1992 that has a mass nine times the mass of our Sun and is 8,000 light-years from Earth.

Every six and a half days, this black hole is rotated in a narrow orbit by an ordinary star. Because the orbit is so narrow, V404 and its companions appear as just a single point of light, even in images taken with large telescopes. But to Burdge's surprise, there was another point of light right next to him, which piqued the researcher's curiosity.

A new theory about the formation of black holes

V404 Cygni is one of the most studied black holes, with more than 1,300 research papers published on the object, Burdge explains. But it seems that until now no one has thought about the point of light near it. Berridge and his team investigated the mystery, and based on more archival data from the European astronomical satellite Gaia, they determined that it was not a star that happened to be in the same direction, but rather a companion to the black hole.

Based on available data and other observations, the team was able to determine that the star has an orbital period of about 70,000 years around the black hole and is about four billion years old.

“We previously thought that most black holes form when stars explode, but this discovery calls that view into question,” Burdge explains. It completely exploded without exploding. “This raises the question of whether there are more such triple systems,” the researcher says. The team therefore recommends carefully searching known pairs of black holes and ordinary stars for more distant partners.