First ‘superkilonova’ double star explosion surprises astronomers

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Scientists may have seen a giant, dying star split in two and then crash back together, triggering a never-before-seen double explosion. The explosion sent ripples through space-time and created the heaviest elements in the universe.

Most massive stars come to the end of their lives as collapses and explosions SupernovaThe universe is peppered with elements like carbon and iron. A different type of cataclysm, called a kilonova, occurs when the ultradense remnants of dead stars, called neutron stars, collide, forming heavier elements such as gold.

The newly identified event, called AT2025ulz, appears to link these two types of cosmic explosions in a way that scientists have long hypothesized but never observed before.

If confirmed, it could represent the first example of a “superkilonova,” a rare hybrid explosion in which the same object produces two different but equally dramatic explosions.

“We don’t know for sure that we found a superkilonova, but the event is an eye-opener,” said the study’s lead author. Mansi KasliwalA professor of astronomy at Caltech, said in one statement.

The results are detailed in a Study Published Dec. 15 in Astrophysical Journal Letters.

A two-in-one combo

AT2025ulz first caught the attention of astronomers on August 18, 2025, when Gravitational waves Detectors operated by the US-based Laser Interferometer Gravitational-Wave Observatory (LIGO) and its European partner, Virgo, registered a subtle signal consistent with the merger of the two compact objects.

Soon after, the Zwicky Transient Facility at Palomar Observatory in California spotted a rapidly fading red dot in the same area of ​​the sky, according to the statement. The behavior of the event was similar to that of GW170817 – the only confirmed kilonova, which was Observed in 2017 – Its red glow is consistent with freshly forged heavy elements such as gold and platinum.

Instead of fading as astronomers had generally expected, AT2025ulz began to brighten again, the study reported. Follow-up observations from a dozen observatories around the world, including Hawaii’s Keck Observatory, showed light shifting to blue wavelengths and revealing hydrogen fingerprints, characteristic of a supernova rather than a kilonova.

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That data helped the researchers confirm the presence of hydrogen and helium, indicating that the giant star shed most of its hydrogen-rich outer layers before exploding, the paper noted.

To explain the startling sequence, the team proposed that a massive, rapidly rotating 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 then swirled together and collided within hours, triggering a kilonova within the expanding debris of the supernova.

The combined effect is a hybrid explosion in which the supernova initially masks the signature of the kilonova, accounting for the unusual observations, the researchers wrote in the paper.

Clues from gravitational-wave data reinforce this idea. While the signal cannot definitively determine the individual masses of the two merging neutron stars, it rules out scenarios in which both were more massive than the Sun, the new paper notes.

The researchers found a 99% chance that at least one object was less massive than the Sun—a result that challenges conventional stellar physics, which predicts that neutron stars should not be less than about 1.2 solar masses. According to the statement, such light neutron stars can only form after the collapse of a very fast rotating star, matching the scenario proposed for AT2025ulz.

However, the study noted that the complexity of the overlapping signals makes it difficult to rule out the possibility that the signals came from unrelated events that occurred close together. Ultimately, the only way to test the theory is with next-generation sky surveys such as Vera C. Using the Rubin Observatory and NASA’s upcoming Nancy Grace Roman Space Telescope to find more events, the researchers said.

“If superkilonovae are real, we will eventually see more of them,” study co-authors Antonella palmaceaesaid assistant professor of astrophysics and cosmology at Carnegie Mellon University in Pennsylvania. statement. “And if we keep looking for associations like this, maybe this was the first.”