A superkilonova is a hypothesized, extremely powerful cosmic event involving two explosions: a massive star's supernova birth of two neutron stars, followed quickly by those neutron stars merging into a kilonova, creating a unique signature seen by astronomers (like in the recent ZTF25abjmnps event) that blends a supernova's glow with a kilonova's heavy element production (gold, platinum), offering insights into black hole formation and heavy element origins. 

When the most massive stars reach the ends of their lives, they blow up in spectacular supernova explosions, which seed the universe with heavy elements such as carbon and iron. Another type of explosion—the kilonova—occurs when a pair of dense dead stars, called neutron stars, smash together, forging even heavier elements such as gold and uranium. Such heavy elements are among the basic building blocks of stars and planets.

So far, only one kilonova has been unambiguously confirmed to date, a historic event known as GW170817, which took place in 2017. In that case, two neutron stars smashed together, sending ripples in space-time, known as gravitational waves, as well as light waves across the cosmos. The cosmic blast was detected in gravitational waves by the National Science Foundation's Laser Interferometer Gravitational-wave Observatory (LIGO) and its European partner, the Virgo gravitational-wave detector, and in light waves by dozens of ground-based and space telescopes around the world.

Double Cosmic Explosion

This artist's animation shows a hypothesized event known as a superkilonova. A massive star explodes in a supernova, collapsing into a stellar core that forms two neutron stars. The neutron stars spiral together and merge, sending gravitational waves rippling through the cosmos and seeding the universe with heavy elements, such as gold and platinum. Animation credit: Caltech/K. Miller and R. Hurt (IPAC)

Evidence for the possible rarity first came on August 18, 2025, when the twin detectors of LIGO in Louisiana and Washington, as well as Virgo in Italy, picked up a new gravitational-wave signal. Within minutes, the team that operates the gravitational-wave detectors (an international collaboration that also includes the organization that runs the KAGRA detector in Japan) sent an alert to the astronomical community letting them know that gravitational waves had been registered from what appeared to be a merger between two objects, with at least one of them being unusually tiny. The alert included a rough map of the source's location.

The observations confirmed that the eruption of light had faded fast and glowed at red wavelengths—just as GW170817 had done eight years earlier. In the case of the GW170817 kilonova, the red colors came from heavy elements like gold; these atoms have more electron energy levels than lighter elements, so they block blue light but let red light pass through.

Then, days after the blast, AT2025ulz started to brighten again, turn blue, and show hydrogen in its spectra—all signs of a supernova not a kilonova (specifically a "stripped-envelope core-collapse" supernova). Supernovae from distant galaxies are generally not expected to generate enough gravitational waves to be detectable by LIGO and Virgo, whereas kilonovae are. This led some astronomers to conclude that AT2025ulz was triggered by a typical ho-hum supernova and not, in fact, related to the gravitational-wave signal.

Neutron stars are the leftover remains of massive stars that explode as supernovae. They are thought to be around the size of San Francisco (about 25 kilometers across) with masses that range from 1.2 to about three times that of our Sun. Some theorists have proposed ways in which neutron stars might be even smaller, with masses less than the Sun's, but none have been observed so far. The theorists invoke two scenarios to explain how a neutron star could be that small. In one, a rapidly spinning massive star goes supernova, then splits into two tiny, sub-solar neutron stars in a process called fission.

In the second scenario, called fragmentation, the rapidly spinning star again goes supernova, but, this time, a disk of material forms around the collapsing star. The lumpy disk material coalesces into a tiny neutron star in a manner similar to how planets form.

With LIGO and Virgo having detected at least one sub-solar neutron star, it is possible, according to theories proposed by co-author Brian Metzger of Columbia University, that two newly formed neutron stars could have spiraled together and crashed, erupting as a kilonova that sent gravitational waves rippling through the cosmos. As the kilonova churned out heavy metals, it would have initially glowed in red light as ZTF and other telescopes observed. The expanding debris from the initial supernova blast would have obscured the astronomers' view of the kilonova.

In other words, a supernova may have birthed twin baby neutron stars that then merged to make a kilonova.

But while this theory is tantalizing and interesting to consider, the research team stresses that there is not enough evidence to make firm claims.

1. Massive Star Collapse: A very large, rapidly spinning star collapses under its own gravity, going supernova but forming two neutron stars instead of one, possibly through a process called fission.
2. Neutron Star Merger: These newly born neutron stars spiral inward and merge, just like in a standard kilonova.
3. Combined Explosion: The initial supernova explosion's debris might temporarily hide the kilonova, but both explosions happen in quick succession, creating a unique, energetic signal.