When a giant star reaches the end of its life, it explodes and emits an enormous amount of light. That is a supernova. For weeks to months, it shines as bright as an entire galaxy. A kilonova is also a phenomenon in which dead stars called neutron stars collide, producing intense light around them.
Astronomers have observed for the first time a phenomenon in which two rare cosmic explosions occur in succession. It is a superkilonova, in which a single object proceeds directly from a supernova to a kilonova. Given that various elements that form stars and planets come from cosmic explosions, the discovery is expected to provide key clues to uncovering the process of cosmic evolution.
◇ Detecting gravitational waves and light from a double explosion
A team led by Mansi Kasliwal, a professor of astronomy at the California Institute of Technology, said on the 16th that it had witnessed for the first time a cosmic explosion likely to be a superkilonova, a combination of a supernova and a kilonova. The results were published in The Astrophysical Journal Letters.
At the end of its life, a star explodes as a supernova, filling the universe with heavy elements such as carbon and iron. Another cosmic explosion, a kilonova, occurs when neutron stars left behind right after a supernova explosion collide with each other. In this process, even heavier elements such as gold and uranium are created. These heavy elements go on to form stars, planets, and life.
The team said that on Aug. 18, gravitational waves from a cosmic explosion combining a supernova and a kilonova were detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States, Virgo in Italy, and KAGRA in Japan. Gravitational waves are ripples of gravitational energy that spread out like waves when colossal events such as black hole mergers or stellar explosions occur.
So far, the only kilonova confirmed with certainty is GW170817, which occurred in 2017. At the time, two neutron stars collided, sending ripples through spacetime. Along with gravitational waves, light waves spread across the universe. LIGO and Virgo detected the gravitational waves, and dozens of ground- and space-based telescopes captured the light waves.
The team believes the newly captured cosmic explosion could be confirmed as the second kilonova on record. The situation is far more complex than before. The candidate kilonova, AT2025ulz, was estimated to have originated from a supernova explosion that occurred just a few hours earlier. In other words, it was not a simple kilonova but a superkilonova combined with a supernova.
◇ A star dies, becomes neutron stars, and triggers a double explosion
Kasliwal said, "For about the first three days, this explosion looked exactly like the first kilonova in 2017," and added, "As it gradually began to resemble a supernova over time, some astronomers lost interest, but we did not." The team presented evidence that the gravitational waves and light observed this time could be from a superkilonova, a kilonova triggered by a supernova. Such events had existed only as a hypothesis and had never been observed.
First, LIGO in Louisiana and Washington and Virgo in Italy captured a gravitational-wave signal. A few hours later, the Palomar Observatory in San Diego detected, at the location of the gravitational-wave source, a red object rapidly fading. The light came from 1.3 billion light-years away (one light-year is the distance light travels in one year, about 9.46 trillion kilometers).
Twelve telescopes around the world, including the W. M. Keck Observatory in Hawaii and the Wendelstein Observatory in Germany, collected additional information there. Observations showed the light's explosion glowed in the red band, the same as the GW170817 kilonova observed eight years ago. The red color of GW170817 came from heavy elements such as gold and uranium.
The team's explanation is as follows. A giant star first exploded as a supernova and created neutron stars. A neutron star is formed when, after a supernova explosion, the core of the remaining star is compressed so that atomic nuclei and electrons inside atoms combine to become neutrons. Neutron stars are the densest objects after black holes. Their mass is two to three times that of the sun, but their diameter is compressed to 25 kilometers—about the size of San Francisco.
After the star collapsed into a supernova, it typically creates one neutron star, but this time it produced two. The team said, "This can happen when the core of a giant star splits in two," and explained, "Two dead stars—neutron stars—collided to create a kilonova." The two neutron stars approached each other in a spiral, orbited, then merged, generating gravitational waves.
◇ Still under debate, confirmation possible with further observations
However, a few days after the explosion, AT2025ulz brightened again and showed a blue hue of hydrogen. On this basis, some astronomers concluded that AT2025ulz was triggered by an ordinary supernova and was unrelated to the gravitational-wave signal. But Kasliwal said several clues hinted that something unusual had occurred.
AT2025ulz differs from the classical kilonova GW170817, but it also differed from an ordinary supernova. In addition, the LIGO–Virgo gravitational-wave data revealed that at least one of the merged neutron stars has a mass smaller than our sun. The team said this suggests the possibility that two small neutron stars merged to create a kilonova.
The only way to test the superkilonova theory is to find more cases. Kasliwal said, "Future kilonova events may look different from GW170817 in 2017 and could be mistaken for supernovas," adding, "When trying to unravel nature's creative mysteries, you need to keep your eyes wide open." In other words, approaches different from the conventional ones, like superkilonovae, are needed.
Korea has also jumped into the race to observe kilonovae. Seoul National University installed a multiwavelength optical telescope at the El Sauce Observatory in the Andes, about 480 kilometers from Santiago, Chile. The Gravitational-Wave Universe Research Group, led by Lee Hyeongmok, a professor in the Department of Astronomy at Seoul National University, aims to use this telescope to capture kilonovae—the light that emerges right after gravitational waves occur—faster than anyone in the world.
References
The Astrophysical Journal Letters (2025), DOI: https://doi.org/10.3847/2041-8213/ae2000
Nature (2017), DOI: https://doi.org/10.1038/nature24303