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.

Now, astronomers are reporting evidence for a possible second kilonova event, but the case is not closed. In fact, this situation is much more complex because the candidate kilonova, named AT2025ulz, is thought to have stemmed from a supernova blast that went off hours before, ultimately obscuring astronomers' view.