An Explosion of Twin Dead Stars May Signal the First Superkilonova Ever Seen in Space 

Few spectacles in space are as phenomenal as supernovae, but a “superkilonova” may give them a run for their money. This type of cosmic outburst is thought to arise when a supernova cascades into a kilonova, an explosion that isn’t as bright as a supernova, but forges heavier elements like gold and uranium. And such a unique mashup may have recently been observed for the first time.

A new study published in The Astrophysical Journal Letters describes a stellar event that may have been a superkilonova, something that has never been seen before. Researchers think it all started when a faraway supernova led to the birth of two neutron stars that then collided to trigger a follow-up kilonova.

Read More: A Light Echo From a Supernova Has Illuminated Interstellar Gas and Dust

A Neutron Collision

Kilonovae occur when a pair of neutron stars spiral into each other and explode; supernovae, on the other hand, can be caused by a few different factors, but they mainly take place when a supermassive star’s core collapses near the end of its life.

According to the ESA, if the stellar core that remains after a supernova has a mass less than about three times the sun’s mass, it forms into an extremely dense neutron star.

Kilonovae are rare events, seeing as only one has been unambiguously confirmed to date, according to a press release on the new study. This kilonova, known as GW170817, was observed in 2017 when observatories detected gravitational waves and light from the collision of two neutron stars.

Seeing Red, Then Blue

On August 18, 2025, several observatories around the world picked up a gravitational wave signal from space that looked to be quite special. The signal pointed to a merger between two objects, one of which was unusually tiny.

Further observations showed a rapidly fading object 1.3 billion light-years away, glowing at red wavelengths. This is the same type of glow that 2017 kilonova GW170817 had given off eight years earlier, its red colors originating from heavy elements like gold — the atoms of heavier elements have more electron energy levels than lighter elements, blocking blue light and letting red light pass through.

The momentum of the new kilonova candidate, called AT2025ulz, was abruptly cut short when the object began to turn blue and show hydrogen in its spectra, suggesting that it may have been a supernova instead.

Signs of a Superkilonova

At this point, many researchers believed that AT2025ulz may have just been a regular supernova after all. But the team behind the new study realized that the object didn’t behave like an average supernova either, all because one of the neutron stars initially observed in the merger appeared to be less massive than the sun.

There are two potential explanations for the one neutron star’s small stature, both of which could have transpired following a supernova explosion involving a rapidly spinning star; either this star split into two sub-solar neutron stars, or it was surrounded by a lumpy disk of material that coalesced into a tiny neutron similar to how planets form.

The researchers ultimately theorize that a supernova from one massive star may have spawned twin baby neutron stars that then merged to create a kilonova. The kilonova would’ve produced the red light seen by telescopes at first, and then expanding debris from the earlier supernova would have taken over astronomers’ view.

"The only way theorists have come up with to birth sub-solar neutron stars is during the collapse of a very rapidly spinning star," said study co-author Brian Metzger of Columbia University. "If these 'forbidden' stars pair up and merge by emitting gravitational waves, it is possible that such an event would be accompanied by a supernova rather than be seen as a bare kilonova."

The researchers, however, note that this superkilonova theory hasn’t been confirmed by evidence just yet. The only way to confirm it, they say, will be to look for future kilonova events with new and improved space telescopes.

Read More: First-Ever Binary White Dwarf System Destined to Explode as Type 1a Supernova

Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• This article references information from a study published in The Astrophysical Journal Letters: ZTF25abjmnps (AT2025ulz) and S250818k: A Candidate Superkilonova from a Subthreshold Subsolar Gravitational-wave Trigger
• This article references information from the ESA: Neutron Star