Black holes are tough to find; they’re black, after all. The easiest way to find them is if they orbit a normal star like the Sun. If the two are close enough together then the black hole can draw matter off the star, which swirls around the black hole before falling in. It gets so hot from friction that it glows fiercely in X-rays, making it possible to spot using X-ray observatories.

V404 Cygni is one such black hole. It’s roughly 8,000 light-years from Earth, and has a mass of 9 times the Sun’s. It’s orbited by a star with about 0.7 times the Sun’s mass, and the two are close enough together that they circle each other every 6.5 days.

While looking into information about the black hole, though, astronomers found a weird thing: In an old image, the light from the companion star can be seen, but very nearby, nearly overlapping, is another star to the north [link to journal paper]. That could be coincidence, but using measurements from ESA’s Gaia satellite they were able to determine the two stars are moving through space in the same direction and at the same velocity, indicating they are indeed together! This means the black hole is not in a binary, but a trinary: a three-star system!

The third star is much farther out from the binary, about 535 billion km, or 3,500 times the distance from Earth to the Sun. That’s a long way, but still close enough to be gravitationally bound to the other two.

But that in itself is interesting. Black holes form when massive stars run out of fuel in their core to fuse. The core collapses, initiating a colossal supernova explosion which tears away all the outer layers of the star. Such an explosion is almost never perfectly spherically symmetric, meaning it has more force in one direction than another. That off-center explosion can give a ferocious kick to the newly formed black hole, which in turn accelerates it to high speed, sometimes tens of thousands of kilometers per hour.

But that literally cannot have happened here! If V404 Cygni got that big a kick, it wouldn’t have stayed in the trinary system. So either it got a small kick (unlikely) or it formed in a different way. Sometimes, the massive star doesn’t explode but instead kind of peters out and collapses in on itself. This is called a direct collapse supernova, and is somewhat rare. However it explains this system pretty neatly.

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