A double explosion of dying stars could be the first observed case of a long-hypothetical and never-proven “superkilonova.” Even though astronomers are still looking for concrete answers, a study published in Letters from the astrophysical journal could detail the historic explosion about 1.3 billion light years from Earth.
Most massive stars in the universe end their lives in a blaze of glory as supernovae, but that’s not always the case. Sometimes the end of the path is a more spectacular event called a kilonova. These explosions are generally thought to occur after two dense neutron stars collide and produce an exponentially larger explosion. While supernovae help spread heavy elements like carbon and iron across the cosmos, kilonovae create even denser remnants, including uranium and gold.
The most definitive example of a kilonova in astronomy, GW170817, was not discovered until 2017. At the time, observation networks such as the National Science Foundation’s Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo Gravitational-Wave Detector in Europe detected gravitational and light waves dating back to a collision of two neutron stars. Since then, professional and amateur astronomers have reported numerous possible kilonovae, but our understanding of these events remains relatively thin.
A team at Caltech’s Palomar Observatory thinks they have another kilonova candidate, but the situation is a little more complex. On August 18, 2025, LIGO and Virgo detected gravitational wave signals, triggering a warning system for the global astronomy community. Researchers at Palomar Observatory’s Zwicky Transient Facility quickly identified a rapidly fading red body about 1.3 billion light years away. Later classified as AT2025ulz, the object displayed red wavelengths similar to those of GW170817.
“At first, for about three days, the eruption resembled the first kilonova of 2017,” Mansi Kasliwal, director of the Palomar Observatory and co-author of the study, said in a statement.
However, AT2025ulz began to brighten again a few days later, this time turning blue to indicate the presence of hydrogen. To many, this proved that the explosion was not another kilonova, but an ordinary supernova.
Kasliwal suspected something else was at play. The cumulative data on AT2025ulz certainly didn’t look like kilonova GW170817, but it didn’t match a typical supernova either. Additionally, gravitational waves suggested that at least one of the two neutron stars was less massive than the Sun. Neutron stars are inherently small – only about 24 kilometers wide – and their mass ranges from 1.2 to three times that of our sun.
How could a neutron star be even smaller? Kasliwal’s team offered two possible explanations. In the first scenario, a rapidly rotating star becomes a supernova before splitting into two subsolar neutron stars. A second theory involves the same onset as a supernova, but instead of fission, a debris disk begins to form around the collapsing star. This material eventually combines to form a small neutron star, much like the process of early planet formation.
Given the possibility of a subsolar neutron star implied by the gravitational wave data, the researchers hypothesize that the two newborn neutron stars from a supernova orbited each other to generate a separate kilonova. This could explain the early red wavelengths, since kilonovas generate red-spectrum heavy metals. As the supernova grew, its blue-spectrum waves eventually obscured the kilonova.
“The only way theorists have found the birth of subsolar neutron stars is during the collapse of a very rapidly rotating star,” added Brian Metzger, a Columbia University astronomer and co-author of the study. “If these ‘forbidden’ stars combine and merge by emitting gravitational waves, it is possible that such an event would be accompanied by a supernova rather than being considered a simple kilonova.”
Metzger, Kasliwal and their colleagues emphasize that their theory remains just a theory. Still, this possibility is intriguing enough to continue the search for additional candidates in hopes of unequivocally identifying a superkilonova. In the meantime, Kasliwal explained the importance of continuing to study possible suspects, even if they start to look like a regular supernova.
“Everyone was intensely trying to observe and analyze it, but then it started to look more like a supernova, and some astronomers lost interest,” she said. “Not us.”
