This artist’s animation shows a hypothesized event known as a superkilonova. First, the massive star ends its life in a supernova, generating elements like carbon and iron. In the aftermath, 2 neutron stars are born, at least 1 of which is likely less massive than our sun. The neutron stars spiral together, sending gravitational waves rippling through the cosmos, before merging in a dramatic kilonova. Astronomers observed the first superkilonova candidate event, named AT2025ulz, in 2025. The LIGO and Virgo observatories first spotted gravitational waves, while the Zwicky Transient Facility (ZTF) at Palomar Observatory was the first to detect light from the stellar fireworks. Video via Keck Observatory.
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- Astronomers might have witnessed a star going superkilonova. It’s the first time astronomers have witnessed such a double explosion in a star.
- The star first went supernova at the end of its life. A supernova is when a star blows its outer layers into space while becoming extremely bright.
- Then, scientists think two neutron stars created in the supernova collided in a kilonova. Kilo means a thousand, and a kilonova can be 1,000 times greater than a nova but only a fraction as bright as a supernova.
The W. M. Keck Observatory first published this original story on December 16, 2025. Edits by EarthSky.
A ‘superkilonova’ double explosion?
A team of astronomers using numerous telescopes, including the W. M. Keck Observatory on Maunakea, Hawaii Island, have discovered a possible superkilonova that exploded not once but twice. The evidence shows this oddball event may be a first-of-a-kind superkilonova, or a kilonova spurred by a supernova. Astronomers have hypothesized such an event, but it’s never been seen before.
Mansi Kasliwal, Caltech professor of astronomy and director of Palomar Observatory, is lead author of a new study describing the findings. The researchers published the peer-reviewed study in The Astrophysical Journal Letters on December 15, 2025.
Supernovas and kilonovas
When the most massive stars reach the ends of their lives, they blow up in spectacular supernova explosions, which seed the universe with heavier 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. The heavy elements created by both of these explosions 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 National Science Foundation’s Laser Interferometer Gravitational-Wave Observatory (LIGO) and its European partner Virgo detected the gravitational waves from the blast. And dozens of ground-based and space telescopes around the world detected the explosion in light waves.
A supernova came first
The curious case of candidate AT2025ulz is complex. It’s thought to have stemmed from a supernova blast that went off hours before, ultimately obscuring astronomers’ view and making the case more complicated.
Lead author Kasliwal of Caltech and the Palomar Observatory said:
At first, for about three days, the eruption looked just like the first kilonova in 2017. Everybody was intensely trying to observe and analyze it, but then it started to look more like a supernova, and some astronomers lost interest. Not us.
Gravitational effects
In August 2025, LIGO and Virgo picked up a new gravitational-wave signal. Within minutes, an alert went out to the astronomical community, containing a rough map of the source and signaling to researchers that gravitational waves had been registered from what appeared to be a merger between two objects, with at least one of them being unusually tiny.
After first being identified by the Zwicky Transient Facility at Palomar Observatory, Kasliwal coordinated with Keck Observatory staff astronomer Michael Lundquist to launch a rapid Target of Opportunity (ToO) observation of AT2025ulz. This process allows scientists to request immediate access for short-lived cosmic events. Mansi’s ToO request enabled the immediate spectroscopic follow-up using the Low-Resolution Imaging Spectrograph (LRIS).
Lundquist said:
Keck Observatory provided the imagery and spectroscopy via our Low-Resolution Imaging Spectrograph (LRIS) instrument to measure the host extinction and redshift of the galaxy as well as looking at the spectroscopic evolution. This highlights Keck Observatory’s Target of Opportunity capability to rapidly respond to transient alerts and deliver the spectroscopic data needed to explore potential multi-messenger associations.
The explosion faded fast
The observations confirmed that the eruption of light had faded fast and glowed at red wavelengths, just as GW170817 had done eight years earlier. In the case of the GW170817 kilonova, the red colors came from heavy elements like gold. These atoms have more electron energy levels than lighter elements, so they block blue light but let red light pass through.
Then, days after the blast, AT2025ulz started to brighten again, turn blue and show hydrogen in its spectra. Those are all signs of a supernova, not a kilonova (specifically a “stripped-envelope, core-collapse” supernova). Supernovas from distant galaxies are generally not expected to generate enough gravitational waves to be detectable by LIGO and Virgo, whereas kilonovas are. This led some astronomers to conclude that AT2025ulz was triggered by a typical, ho-hum supernova, and not in fact related to the gravitational-wave signal.
What might be going on?
Kasliwal said several clues tipped her off that something unusual had taken place. Though AT2025ulz did not resemble the classic kilonova GW170817, it also did not look like an average supernova. Additionally, the LIGO–Virgo gravitational-wave data had revealed that at least one of the neutron stars in the merger was less massive than our sun. And that was a hint that one or two small neutron stars might have merged to produce a kilonova.
Neutron stars are the leftover remains of massive stars that explode as supernovas. Astronomers think they are around the size of San Francisco (about 22-30 km or 14-18 miles across). And their masses range from 1.2 to about 3 times that of our sun. Some theorists have proposed ways in which neutron stars might be even smaller, with masses less than the sun’s, but no one has observed one that small so far.
Theorists invoke two scenarios to explain how a neutron star could be that small. In one, a rapidly spinning massive star goes supernova, then splits into two tiny, sub-solar neutron stars through fission. In the second scenario – fragmentation – the rapidly spinning star again goes supernova. But this time a disk of material forms around the collapsing star. The lumpy disk material coalesces into a tiny neutron star in a manner similar to how planets form.
Neutron stars collide
With LIGO and Virgo having detected at least one sub-solar neutron star, it is possible, according to theories proposed by co-author Brian Metzger of Columbia University, that two newly formed neutron stars could have crashed into each other. This would have caused the kilonova eruption that sent gravitational waves rippling through the cosmos. As the kilonova churned out heavy metals, it would have initially glowed in red light, as ZTF and other telescopes observed. The expanding debris from the initial supernova blast would have obscured the astronomers’ view of the kilonova. In other words, a supernova may have birthed twin baby neutron stars that then merged to make a kilonova.
Metzger said:
The only way theorists have come up with to birth sub-solar neutron stars is during the collapse of a very rapidly spinning star. If these ‘forbidden’ stars pair up and merge by emitting gravitational waves, it is possible that such an event would be accompanied by a supernova rather than be seen as a bare kilonova.
But while this theory is tantalizing and interesting to consider, the research team stresses that there is not enough evidence to make firm claims. The only way to test the superkilonova theory is to find more.
Kasliwal said:
Future kilonova events may not look like GW170817 and may be mistaken for supernovas. We can look for new possibilities in data like this, but we do not know with certainty that we found a superkilonova. The event, nevertheless, is eye opening.
Bottom line: Astronomers have detected a strange double explosion of a star and hypothesize that it is a kilonova following a supernova: a superkilonova.
Source: ZTF25abjmnps (AT2025ulz) and S250818k: A Candidate Superkilonova from a Subthreshold Subsolar Gravitational-wave Trigger
Via W. M. Keck Observatory