Did Astronomers See a Star Explode Twice?

Astronomers might have noticed their first superkilonova — a star that’s exploded not as soon as, however twice.

When huge stars die, they detonate in a celestial fireworks show often called a supernova. Typically this leaves behind a neutron star — a dense, city-sized kernel. A spoonful of its materials weighs greater than everybody on Earth put collectively. When two neutron stars collide, they produce a special spectacular present: a kilonova.

Astronomers presently spot a number of supernovae daily, however they’ve solely ever seen one confirmed kilonova. That was in 2017, when gravitational waves from the cataclysm washed up on Earth similtaneously a number of telescopes have been bathed within the gentle of the explosion.

Then, on August 18, 2025, each detectors of the Laser Interferometer Gravitational-wave Observatory (LIGO) within the U.S. and the Virgo detector in Europe, picked up a brand new gravitational wave sign.

“The eruption regarded identical to the primary kilonova in 2017,” says Mansi Kasliwal (Caltech). A dozen telescopes all over the world swung into motion, coaching their mirrors on the patch of sky the gravitational waves had travelled from.

The Zwicky Transient Facility (ZTF) was the primary to pinpoint a quickly fading object 1.3 billion light-years away. Astronomers would ultimately identify it AT2025ulz. It glowed crimson, the results of heavy components similar to gold blocking gentle on the blue finish of the seen spectrum. That is precisely how the 2017 kilonova appeared.

Then one thing curious occurred. Days after the preliminary blast, AT2025ulz started to brighten, flip blue and present hydrogen in its spectra. These are all calling playing cards of a supernova, not a kilonova. “Some astronomers misplaced curiosity,” says Kasliwal. “Not us.”

The gravitational waves contained a tantalizing clue: one of many merging objects appeared to have a mass decrease than our personal Solar. If true, this might be a breakthrough discovery.

Neutron stars often have lots between 1.2 and three Suns, however theorists have speculated that “sub-solar” neutron stars may exist. Maybe a heavier neutron star spins so quickly that it fractures into two lighter ones. Alternatively, a star may go supernova, making a disk of fabric that then collapses right into a tiny neutron star.

“If these ‘forbidden’ stars pair up and merge by emitting gravitational waves, it’s potential that such an occasion can be accompanied by a supernova reasonably than be seen as a naked kilonova,” says Brian Metzger (Columbia College).

In a paper revealed within the December twentieth Astrophysical Journal Letters, a group led by Kasliwal and Metzger speculates that that is what might have occurred with AT2025ulz. The group is eager to emphasize, nevertheless, that extra proof is required to agency up the declare.

Matt Nicholl (Queen’s College Belfast, UK), who was not concerned within the analysis, isn’t satisfied. “In my view, AT2025ulz seems to be most certainly to be a traditional supernova that exploded by probability at round the fitting time and in the fitting area of the sky.” In different phrases, AT2025ul and the gravitational-wave sign have been two utterly separate occasions.

James Gillanders (College of Oxford, UK), who was additionally not concerned within the analysis, is equally skeptical. “The info recorded within the first few days do resemble that of a kilonova, however this habits is a identified property of Sort IIb supernovae,” Gillander says.

“If [AT2025ulz] have been found and noticed in absence of the gravitational-wave sign, I think nobody would interpret it as a superkilonova,” he says. Like Nicholl, Gillanders believes the gravitational-wave sign is unrelated.

Whether it is even a sign in any respect. “There’s solely a 29% chance that the sign is actual, whereas there’s a 71% probability that it’s some glitch or noise sign,” Gillanders says.

Taken collectively, these counterpoints forged vital doubt over whether or not astronomers have actually found a superkilonova — and spotlight a number of the difficulties in understanding stellar explosions. Nevertheless, if additional work clears the doubt, the discover might be one for the historical past books.