Cosmic Double Trouble: A Star Explosion Like No Other

In the vast universe, stars meet their end in grand explosions, scattering elements like carbon and iron. These events, known as supernovae, are well-known. But there's another type of explosion, called a kilonova, which happens when two dense, dead stars, or neutron stars, collide. This collision creates heavy elements like gold and uranium, essential for forming stars and planets. So far, only one kilonova has been confirmed, in 2017, when two neutron stars crashed into each other, sending out gravitational waves and light waves. This event, called GW170817, was a big deal in astronomy. Now, astronomers think they might have spotted another kilonova, but it's not so straightforward. This event, named AT2025ulz, seems to have started with a supernova explosion just hours before the kilonova. This makes it tricky to study, as the supernova's aftermath might be hiding the kilonova's details. At first, the event looked like the 2017 kilonova, but after a few days, it started to resemble a supernova. This shift confused some astronomers, but a team led by Mansi Kasliwal from Caltech kept investigating. They believe this might be a rare "superkilonova, " where a kilonova is triggered by a supernova. This idea has been suggested before but never seen in action. The story begins on August 18, 2025, when gravitational wave detectors picked up a signal from a merger between two objects, one of which was unusually small. Astronomers around the world turned their telescopes towards the event, including the Zwicky Transient Facility, which spotted a rapidly fading red object 1. 3 billion light-years away. Other telescopes confirmed that the light was fading fast and glowing red, just like the 2017 kilonova. But then, a few days later, the event started to brighten again, turn blue, and show signs of hydrogen, all signs of a supernova, not a kilonova.This left some astronomers thinking that AT2025ulz was just a typical supernova and not related to the gravitational wave signal. But Kasliwal and her team weren't convinced. They pointed out that the event didn't look like an average supernova either. Plus, the gravitational wave data showed that at least one of the neutron stars was less massive than our sun, hinting that one or two small neutron stars might have merged to produce a kilonova. Neutron stars are the leftover cores of massive stars that explode as supernovae. They're usually around the size of a city but with masses ranging from 1. 2 to about three times that of our sun. However, some theorists think neutron stars could be even smaller, with masses less than the sun's. No one has observed this yet, but it's a possibility. There are two main ideas about how a neutron star could be that small. In one, a rapidly spinning massive star goes supernova, then splits into two tiny neutron stars. In the other, called fragmentation, the rapidly spinning star goes supernova, and a disk of material forms around the collapsing star. This disk then clumps together to form a tiny neutron star, similar to how planets form. With the detection of at least one sub-solar neutron star, it's possible that two newly formed neutron stars spiraled together and crashed, causing a kilonova that sent gravitational waves through the cosmos. As the kilonova created heavy metals, it would have initially glowed red, as observed by telescopes. The expanding debris from the initial supernova blast would have then obscured the view of the kilonova. In other words, a supernova might have birthed twin baby neutron stars that then merged to make a kilonova. But while this idea is exciting, the research team says there's not enough evidence to make firm claims. The only way to test the superkilonova theory is to find more events like this.