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An artist’s concept depicts a never-before-seen superkilonova event. . | Credit: Caltech/K. Miller and R. Hurt (IPAC)
Scientists may have witnessed a massive, dying star split in two and then collapse back together, triggering an unprecedented double explosion. The explosion sent ripples through space-time and forged some of the universe’s heaviest elements.
Most massive stars reach the end of their lives by collapsing and exploding ca Supernovaseeding the cosmos with elements like carbon and iron. Another type of cataclysm, known as a kilonova, occurs when the ultradense remnants of dead stars, called neutron stars, collide, forging even heavier elements like gold.
The newly identified event, dubbed AT2025ulz, appears to combine these two types of cosmic explosions in a way that scientists have long hypothesized but not observed until now.
If confirmed, it could represent the first example of a “superkilonova,” a rare hybrid explosion in which a single object produces two distinct but equally dramatic explosions.
“We don’t know for sure that we have found a superkilonova, but the event is an eye-opener nonetheless,” lead author of the study. Mansi Kasliwala professor of astronomy at Caltech, said in a statement.
The findings are detailed in a study published Dec. 15 in The Astrophysical Journal Letters.
A two-in-one combo
AT2025ulz first caught the attention of astronomers on August 18, 2025, when gravitational wave detectors operated by the US Laser Interferometer Gravitational-Wave Observatory (LIGO) and its European partner, Virgo, have recorded a subtle signal consistent with the merger of two compact objects.
Shortly thereafter, the Zwicky Transient Facility at California’s Palomar Observatory observed a rapidly fading red point of light in the same region of the sky, according to the statement. The behavior of the event closely resembled that of GW170817 – the only confirmed kilonova, which was observed in 2017 — with the red glow consistent with freshly minted heavy elements such as gold and platinum.
Instead of fading as astronomers usually expect, AT2025ulz began to brighten again, the study reported. Subsequent observations from a dozen observatories around the world, including the Keck Observatory in Hawaii, showed that the light shifts to bluer wavelengths and reveals hydrogen fingerprints, a hallmark of a supernova rather than a kilonova.
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This data helped the researchers confirm the presence of hydrogen and helium, indicating that the massive star lost most of its hydrogen-rich outer layers before detonating, the paper notes.
To explain the astonishing sequence, the team proposed that a massive, rapidly spinning star collapsed and exploded as a supernova. But instead of forming a single neutron star, its core split into two smaller neutron stars. Those newborn remnants came back together and collided within hours, triggering a kilonova inside the expanding remnants of the supernova.
The combined effect is a hybrid explosion in which the supernova initially masks the kilonova signature, accounting for the unusual observations, the researchers wrote in the paper.
Clues from the gravitational wave data support this idea. Although the signal cannot precisely determine the individual masses of the two merging neutron stars, it rules out scenarios where both were heavier than the sun, the new paper notes.
The researchers found a 99 percent chance that at least one of the objects was less massive than the sun—a result that challenges conventional stellar physics, which predicts that neutron stars should not weigh less than about 1.2 solar masses. Such light neutron stars can only form when a rapidly spinning star collapses, according to the scenario proposed for AT2025ulz, according to the statement.
However, the study noted that the complexity of the overlapping signals makes it difficult to rule out the possibility that the signals originated from unrelated events that happened to occur close together. Ultimately, the only way to test the theory will be to find more such events using next-generation sky surveys, such as those from the Vera C. Rubin Observatory and NASA’s upcoming Nancy Grace Rome Space Telescope, the researchers said.
“If superkilonovae are real, we will eventually see more of them,” co-author of the study Antonella Palmesean assistant professor of astrophysics and cosmology at Carnegie Mellon University in Pennsylvania, said in another statement. “And if we keep finding associations like this, then maybe this was the first.”