A baffling cosmic event, designated AT2025ulz, was detected by LIGO and Virgo and is now considered a candidate for a never-before-seen phenomenon: a superkilonova. This oddball event, which took place 1.3 billion light-years away, initially resembled a kilonova—an explosion caused by the merger of two dense neutron stars. Kilonovae are known to forge the heaviest elements, such as gold and uranium.

However, after about three days, AT2025ulz started to look more like a supernova, brightening, turning blue, and showing hydrogen in its spectra. The gravitational-wave data indicated that at least one of the colliding objects was less massive than a typical neutron star.

Astronomers hypothesize that this "superkilonova" was a kilonova spurred by a prior supernova blast. The leading theory suggests that a rapidly spinning, massive star went supernova, birthing two "forbidden" sub-solar mass neutron stars. These newborn stars may have then spiraled together and merged, creating a kilonova. This scenario would explain why the event displayed features of both a supernova and a kilonova, potentially obscuring the initial merger. This potential cosmic rarity challenges our understanding of stellar death and the formation of heavy elements.