On August 18, 2025, gravitational wave detectors across the globe picked up something unusual: two neutron stars colliding 1.3 billion light-years away. What followed was a cosmic mystery that's still unfolding.
When neutron stars collide, they typically create a kilonova—a blast so bright it can briefly outshine an entire galaxy. We've only confidently identified one before: GW170817, spotted in 2017. But this new event, catalogued as AT2025ulz, didn't follow the expected script.
A Blast That Changed Its Mind
Mansi Kasliwal and her team at Caltech's Palomar Observatory caught the explosion in real time. For the first three days, it looked textbook: a rapidly fading red glow, just like that 2017 kilonova. Then it did something unexpected. The light brightened again, shifted to blue, and started behaving like a conventional supernova—the kind of explosion that happens when a single massive star dies, not when two neutron stars merge.
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Start Your News DetoxMost astronomers moved on. Kasliwal's team didn't.
The gravitational wave data kept nagging at them. The signals suggested something theoretically forbidden was happening: at least one of the colliding neutron stars was less massive than our sun. Neutron stars are supposed to be heavier than that. They're city-sized remnants of dead stars, typically ranging from 1.2 to three times the sun's mass. Finding one smaller violated what physicists thought was possible.
The Superkilonova Hypothesis
Kasliwal and Columbia University astronomer Brian Metzger developed a theory. What if a rapidly spinning star went supernova, and during that explosion, two sub-solar neutron stars were born? Those newborns could then orbit each other, eventually collide, and trigger a kilonova. The supernova's expanding shell would have initially hidden the kilonova's red light, then as it spread outward, the supernova's blue wavelengths became visible. Two explosions, one after the other, seen as a single event.
It's a scenario theorists have hypothesized for years but never observed. If AT2025ulz is indeed this "superkilonova," it would be the first confirmed case.
But Kasliwal, Metzger, and their colleagues are careful. This remains a theory. The data fits, but the universe is good at surprising us. What makes this moment significant isn't certainty—it's that we're finally equipped to ask these questions at all. A decade ago, we didn't have the gravitational wave detectors or the rapid-response telescope networks to catch something this subtle.
The search continues. More candidates will emerge. Each one brings us closer to understanding how the universe's densest objects behave when they collide, and what rare configurations of stellar death are actually possible.
