First-Ever ‘Black Hole Triple’ Formed Quietly Without a Massive Explosion

CAMBRIDGE, Mass. — For decades, astronomers believed all compact objects like neutron stars and black holes received violent “natal kicks” during their formation. That theory just suffered a fatal blow thanks to an unassuming star quietly orbiting the famous black hole V404 Cygni, a star that simply shouldn’t be there if conventional wisdom about black hole formation were correct.

V404 Cygni, the first widely accepted black hole in a low-mass X-ray binary, turns out to be part of a triple star system, with a distant companion that has remained bound to it for billions of years. This revelation, published this month in Nature, provides compelling evidence that this black hole formed with virtually no “natal kick,” the violent push that’s thought to accompany the birth of most compact objects.

“We think most black holes form from violent explosions of stars, but this discovery helps call that into question,” says study author Kevin Burdge from the MIT Department of Physics, in a statement. “This system is super exciting for black hole evolution, and it also raises questions of whether there are more triples out there.”

An Astronomical Discovery Hidden in Plain Sight

The discovery happened almost by accident when researchers were examining optical images of V404 Cygni. They noticed a star just 1.43 arcseconds away, about 3500 times the distance from Earth to Pluto, that shared nearly identical movement through space with the black hole system.

This arrangement simply shouldn’t exist if black holes form through violent supernova explosions as traditionally theorized. The third companion star is so loosely bound that even a small kick during the black hole’s formation would have sent it flying out of the system.

“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,” says Burdge.

Burdge calculated that the outer star is 3,500 astronomical units (AU) away from the black hole (1 AU is the distance between the Earth and Sun). The odds of finding a star at this distance with matching motion is about one in ten million, making a chance alignment virtually impossible.

Rewriting Black Hole Birth

Black holes are formed when massive stars die, and astronomers have long debated how violent this process is. Most evidence suggests neutron stars (smaller stellar remnants) receive significant kicks during formation, but evidence for black hole kicks has remained limited.

V404 Cygni’s stable triple configuration shows that at least some black holes form through a process more like an implosion than an explosion, with virtually no ejected mass or momentum transferred to the black hole itself.

“Imagine you’re pulling a kite, and instead of a strong string, you’re pulling with a spider web,” explains Burdge. “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.”

To confirm their discovery wasn’t just chance, the team measured the companion star’s velocity through space. Sure enough, it perfectly matched the previously established velocity of V404 Cygni.

The research team ran computer simulations of various formation scenarios to figure out what would have happened during the black hole’s birth. They found that to keep the third companion star bound to the system, the black hole must have received a kick of less than 5 kilometers per second, which is practically nothing in cosmic terms. To put it into perspective, that’s slower than a car on the highway.

Billions of Years in the Making

The companion star is slightly evolved, indicating the system formed 3-5 billion years ago. This means the black hole has had plenty of time to strip material from its immediate companion, explaining why the current donor star in the inner binary appears to have lost significant mass.

This discovery also backs up a long-standing theory that some black hole systems form as part of three-star setups, not just pairs. That idea has been hard to prove, but it could explain why some systems that should fall apart or merge actually survive. In traditional two-star models, the stars are often so different in mass that they usually collide or merge instead of forming a stable orbit.

By looking more closely at the system, the researchers found that the third star, the one orbiting farthest out, has a mass about 1.2 times that of our Sun. It’s also starting to age, expanding to nearly twice its original size. The team figured this out by studying the star’s light spectrum, which acts like a fingerprint, and comparing it to known models of how stars evolve over time.

V404 Cygni is one of the most thoroughly studied black holes in astronomy, documented in over 1,300 scientific papers since being confirmed as a black hole in 1992. Yet somehow, the fact that it is part of a triple star system remained hidden in plain sight until now.

This triple system has survived billions of years, a silent witness to an alternative path of black hole formation we never knew existed. Astronomers continue to find that the cosmos refuses to fit neatly into our theoretical boxes. V404 Cygni’s triple system changes how we think about the death of massive stars and the birth of the universe’s most enigmatic objects.