V404 Cygni, a black hole 8,000 light-years from Earth, was already famous in astronomical circles. But a recent discovery has just made it extraordinary. For the first time, scientists have found a black hole with not one, but two companion stars—reshaping our understanding of how black holes form and evolve.
However, a new discovery is reshaping this picture, introducing a new cosmic configuration.
In a study published in Nature, physicists at MIT and Caltech have revealed the first “black hole triple”—a system where, instead of the usual binary pair, a black hole is accompanied by not one but two stars.
The black hole at the system’s core is steadily consuming a star that circles it every six and a half days. But lurking farther out, far beyond the drama of this close encounter, is something completely unexpected: another star orbiting the black hole. Its orbit is so vast that it takes 70,000 years to complete just one loop.
A quiet black hole birth
“We think most black holes form from violent explosions of stars, but this discovery helps call that into question,” says study author Kevin Burdge, a Pappalardo Fellow in MIT’s Department of Physics.
Burdge says the reason for this challenge lies in the nature of the distant companion. If the black hole had formed through a supernova—the explosive death of a star that collapses into a black hole—the resulting blast should have been powerful enough to disrupt any weakly bound objects nearby. In this case, the second, outer star should have been flung far away, unable to remain gravitationally connected to the system.
Yet, this distant star remains in orbit. The discovery suggests that the black hole may have formed through a gentler process known as “direct collapse,” where a massive star collapses inward under its own gravity without the dramatic outburst typical of a supernova. This quiet formation would allow distant objects to stay in place, relatively undisturbed.
“Imagine you’re pulling a kite, and instead of a strong string, you’re pulling with a spider web,” Burdge explains. “If you tugged too hard, the web would break, and you’d lose the kite. Gravity is like this barely bound string that’s really weak, and if you do anything dramatic to the inner binary, you’re going to lose the outer star.”
In this analogy, the distant star remains tied to the system by the delicate thread of gravity, which would snap in the event of a supernova but stays intact in the more gradual process of direct collapse.
Burdge and his team ran tens of thousands of simulations to test this idea, exploring different scenarios for forming this triple system. They experimented with models of supernovae and direct collapse, each time altering the amount and direction of energy released. The simulations overwhelmingly supported the direct collapse hypothesis.
But Burdge said this discovery offers more than just a new perspective on black hole formation. Including the outer star provides a rare opportunity to estimate the age of the system. This distant star is in the process of becoming a red giant—a stage that occurs at the end of a star’s life. Based on this phase of its evolution, the researchers estimate the star to be about four billion years old. Given that neighboring stars typically form around the same time, the entire triple system, including the black hole, is likely the same age.
“We’ve never been able to do this before for an old black hole,” Burdge said. “Now we know V404 Cygni is part of a triple, it could have formed from direct collapse, and it formed about four billion years ago, thanks to this discovery.”
A chance discovery
The discovery itself came about through a combination of curiosity and uncertainty. Burdge and his colleagues were searching for new black holes within the Milky Way galaxy using Aladin Lite, a tool that gathers astronomical images from telescopes worldwide. As part of their investigation, they examined V404 Cygni, one of the first black holes ever confirmed in 1992 and a subject of extensive study in over 1,300 scientific papers. While most previous studies focused on the black hole and its inner, closely orbiting star, Burdge noticed something curious—another source of light farther away, which had been largely overlooked.
Burdge soon realized that this second source of light was not a random object but a distant star orbiting the black hole. To determine whether the two stars were gravitationally linked, the team turned to data from the Gaia satellite, which has been tracking the motion of stars across the galaxy since 2014. The Gaia data revealed that the inner and outer stars moved in tandem, indicating they were gravitationally bound to the same black hole. The odds of such a configuration occurring by chance were astronomically low, at about one in 10 million.
“It’s almost certainly not a coincidence or accident,” Burdge said. “We’re seeing two stars that are following each other because they’re attached by this weak string of gravity. So this has to be a triple system.”
This discovery opens new avenues for exploring how black holes form and evolve, particularly in systems where violence and chaos may not play the central role previously assumed. The idea that black holes can form through quieter, more gradual processes challenges traditional stellar evolution models, suggesting that more gently formed black holes may be hidden throughout the galaxy.
