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Artist’s concept depicts an unprecedented superkilonova event. . |Image credit: Caltech/K. Miller and R. Hurt (IPAC)
Scientists may have witnessed a massive dying star split in half and then crash back together, triggering a never-before-seen double explosion. The explosion sent ripples through space and time and formed some of the heaviest elements in the universe.
Most massive stars end their lives by collapsing and exploding supernovaseeding the universe with elements like carbon and iron. Another type of catastrophe called a kilonova occurs when the ultra-dense remnants of dead stars, called neutron stars, collide, forming heavier elements such as gold.
The newly discovered event, named AT2025ulz, appears to combine these two types of cosmic explosions in a way that scientists have long hypothesized but never observed.
If confirmed, it could represent the first example of a “superkilonova,” a rare hybrid explosion in which a single object produces two different but equally violent explosions.
Study lead author: “We’re not sure if we’ve discovered a superkilonova, but the event is still eye-opening.” Mansi KasliwalCaltech astronomy professor said in a report statement.
The research results are detailed in study Published December 15 in The Astrophysical Journal Letters.
2-in-1 combination
AT2025ulz first came to the attention of astronomers on August 18, 2025, when Gravitational waves Detectors operated by the US Laser Interferometer Gravitational-Wave Observatory (LIGO) and its European partner Virgo recorded a subtle signal consistent with the merger of two compact objects.
Shortly after, the Zwicky Transient Facility at Palomar Observatory in California spotted a rapidly fading red spot in the same area of the sky, the statement said. The event behaves very similarly to GW170817 – the only confirmed kilonova, Observed in 2017 – Its red glow is consistent with newly forged heavy elements such as gold and platinum.
Rather than fading away, as astronomers would normally expect, AT2025ulz is starting to brighten again, the study reports. Follow-up observations from more than a dozen observatories around the world, including the Keck Observatory in Hawaii, showed the light shifted toward blue wavelengths and revealed a fingerprint of hydrogen, a sign of a supernova rather than a kilonova.
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The paper noted that the data helped researchers confirm the presence of hydrogen and helium, indicating that the massive star had shed most of its hydrogen-rich outer layers before exploding.
To explain the puzzling sequence, the team proposed that a massive, rapidly spinning star collapsed and exploded as a supernova. But instead of forming a neutron star, its core split into two smaller neutron stars. These nascent remnants then spiraled together and collided within hours, triggering kilonovae in the supernova’s expanding debris.
The combined effect, a hybrid explosion in which the supernova initially obscured the signature of the kilonova, explains the unusual observations, the researchers wrote in their paper.
Clues from gravitational wave data support this idea. The new paper notes that while the signal cannot precisely determine the individual masses of the two merging neutron stars, it does rule out the possibility that both are heavier than the sun.
The researchers found that there was a 99% 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 weigh no less than about 1.2 solar masses. Such a lightweight neutron star could only form when a very rapidly rotating star collapses, matching the scenario proposed by AT2025ulz, the statement said.
However, the complexity of the overlapping signals makes it difficult to rule out the possibility that they come from unrelated events that happen to occur at the same time, the study noted. Ultimately, the researchers say, the only way to test the theory is 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 Roman Space Telescope.
“If superkilonovae are real, we will eventually see more superkilonovae,” study co-author Antonella PalmeseAssistant Professor of Astrophysics and Cosmology at Carnegie Mellon University in Pennsylvania said in a separate article statement. “If we keep finding associations like this, this could be the first.”
