Astronomers may have just found a first-of-its-kind phenomenon buried in the shadow of one of the Milky Way's most famous stellar explosions: two stars that died in separate supernovas, after spending millions of years orbiting each other.

Using 16 years of data from NASA's Fermi Gamma-ray Space Telescope, researchers detected high-energy gamma rays coming from a faint, previously overlooked supernova remnant called G189.6+3.3, which sits right beside the well-studied remnant IC 443 (nicknamed the Jellyfish Nebula). Their analysis suggests that the two remnants aren't just neighbors by coincidence. Instead, they may be the leftover wreckage of a binary star system, making this the first confirmed case of two supernova remnants traced to the same stellar pair.

The findings were published July 21 in the journal Nature Communications.

"We weren't really looking for a binary supernova remnant," study first author Miltiadis Michailidis, an astrophysicist and postdoctoral fellow at Stanford University, told Live Science. The team originally set out to characterize G189.6+3.3, which had long been overshadowed by its brighter neighbor. IC 443 is one of the best-studied remnants in the galaxy and among the first ever confirmed to accelerate protons — one of the key ingredients thought to produce cosmic rays, the high-energy particles that constantly strike Earth's atmosphere.

By combining Fermi's gamma-ray data with X-ray, radio, ultraviolet and optical observations, the team teased out G189.6+3.3's faint signal and found something strange: Instead of glowing uniformly, its northern half was dominated by accelerated protons, while its southern half was dominated by electrons. It's the first time such a clean split has been spotted within a single remnant.

The explanation, they found, came down to the environment: The northern edge of the remnant presses against a dense cloud of hydrogen gas, which gives fast-moving protons something to collide with, thereby producing gamma rays, Michailidis said. The southern half lacks that dense material, letting a different, electron-driven process take over instead. Ultraviolet observations showed that the northern shock wave had slowed after slamming into the cloud, backing up that interpretation.

Earlier work had already shown that IC 443 interacts with the same cloud, which implies that both remnants sit at roughly the same distance from Earth. To rule out coincidence, the researchers simulated 1 million hypothetical binary star systems and calculated how often two unrelated remnants would wind up this close together purely by chance. Depending on the method used, the odds came out to somewhere between 1 in 1,000 and 1 in 100. In other words, it's highly likely that the two supernova remnants are related.

The team also estimated when each star likely exploded, finding that the two blasts were probably separated by tens of thousands of years. That finding is consistent with one star in a binary pair going supernova first, with its companion following suit long afterward.

Rather than relying purely on computer models, astronomers can use this real-world system to test long-standing theories about how massive binary stars live, interact and eventually explode.

Michailidis said the team's next step is to search for similar binary remnant pairs elsewhere in the galaxy, to understand why this system has stood alone until now. He pointed to one particular payoff: Measuring the distance between two remnants' explosion centers can reveal how much energy a supernova actually released — a quantity that, until now, astronomers could only estimate from theory. "Now we can actually test it," he said.