Have you ever wondered what happens when the universe decides to break its own rules?
Welcome to FreeAstroScience.com, where we break down the most complex scientific discoveries into something you can actually enjoy reading. Today, we're exploring a discovery that's got astronomers around the world scratching their heads—and maybe rewriting textbooks.
On August 18, 2025, telescopes captured something nobody had ever seen before: what might be a supernova and a kilonova happening together, from the same dying star. Scientists are calling it a "superkilonova," and it's as wild as it sounds.
Grab your coffee. This one's going to blow your mind. Stick with us to the end, because the implications of this discovery could change how we understand stellar death itself.
What Exactly Are We Talking About? Supernova vs. Kilonova
Before we dive into the double explosion, let's get our cosmic vocabulary straight.
A supernova happens when a massive star runs out of fuel and collapses. The result? A catastrophic explosion that can outshine entire galaxies. Astronomers spot about 20,000 of these every year . They're dramatic, violent, and leave behind either a neutron star or a black hole.
A kilonova is different. It happens when two neutron stars—those ultra-dense remnants of dead stars—spiral into each other and collide. These events are rarer and fainter in visible light, but they light up gravitational wave detectors like fireworks . We've only confirmed one kilonova before now: GW170817, detected in August 2017 .
Here's a quick comparison:
| Feature | Supernova | Kilonova |
|---|---|---|
| Cause | Massive star core collapse | Two neutron stars merging |
| Frequency | ~20,000 per year observed | Only 1 confirmed (until now) |
| Detection | Bright in visible light | Strong gravitational waves |
| Heavy elements | Some produced | Major source (gold, platinum, uranium) |
| Remnant | Neutron star or black hole | Black hole (usually) |
Now imagine both happening at once. That's the puzzle we're trying to solve.
The Discovery: August 18, 2025
At 01:20:06 UTC on August 18, 2025, the LIGO-Virgo-KAGRA collaboration detected something unusual: gravitational waves rippling through spacetime . The signal, named S250818k, came from what looked like two neutron stars merging about 399 million light-years away .
But here's where things got weird.
The signal suggested something impossible. At least one of the merging objects appeared to be less massive than our Sun . For context, neutron stars typically weigh between 1.2 and 2 solar masses. A neutron star below one solar mass? That's not supposed to happen—at least, not according to our current understanding of stellar evolution.
Within hours, the Zwicky Transient Facility (ZTF) at Palomar Observatory swung into action. They mapped the region of sky where the gravitational waves originated and found an optical transient—a new, bright source of light—that appeared exactly where and when the merger should have been .
They named it AT2025ulz. And for three days, it looked exactly like the 2017 kilonova.
The Plot Twist: When a Kilonova Becomes a Supernova
"At first, for about three days, the eruption looked just like the first kilonova in 2017," said Mansi Kasliwal, director of Caltech's Palomar Observatory. "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."
Here's what the team observed:
Days 1-3: The light was red and fading fast, with featureless spectra. Classic kilonova behavior. The object was about 0.5-1.0 magnitudes brighter than GW170817, peaking at an absolute magnitude of around -17 .
Days 7-10: A prominent P Cygni profile appeared in the spectra—a distinctive pattern showing hydrogen gas expanding at 17,000 km/s. This screams supernova, not kilonova .
After Day 10: The light curve showed a second peak, something you'd expect from a Type IIb supernova where radioactive nickel-56 decay takes over .
The color evolution was particularly strange. The object showed unusual red-to-infrared colors before the second peak that didn't match typical Type IIb supernovae . Something extra was going on.
Why This Matters: The Superkilonova Model
So we have gravitational waves suggesting a neutron star merger with impossibly light components, combined with optical observations showing features of both kilonovae and supernovae. How can both be true?
The research team, led by Kasliwal, proposes a bold explanation: the superkilonova .
Here's the idea. When a rapidly spinning massive star collapses, it doesn't just leave behind one neutron star. Instead, something stranger happens.
Two Possible Mechanisms
1. Core Fission
If the collapsing core is spinning fast enough, it might split into two neutron stars instead of one . Think of it like a water droplet spinning so fast it breaks apart. These twin neutron stars, born in the same explosion, could then spiral together and merge within hours.
2. Disk Fragmentation
When a rapidly rotating star collapses, the outer material doesn't fall straight onto the neutron star. Instead, it forms an accretion disk—a swirling ring of matter. If this disk is massive enough and cools quickly, it can become gravitationally unstable and fragment .
Picture it like planet formation in reverse, but instead of planets forming around a star, you get small neutron stars forming in a disk around a bigger one. These fragments can be tiny—potentially less than one solar mass—because the disk material is neutron-rich, which lowers the mass threshold for collapse .
The mathematical relationship here involves the Chandrasekhar mass, which scales with the electron fraction:
Where Ye is the electron fraction. In neutron-rich environments, Ye drops significantly, allowing much smaller objects to collapse into neutron stars .
The Evidence: What Supports This Theory?
The superkilonova model explains several puzzling observations:
The low chirp mass. The gravitational wave signal gave a chirp mass of 0.87 solar masses—the lowest ever detected for such an event . If subsolar neutron stars formed through disk fragmentation, this suddenly makes sense.
The Type IIb supernova features. The hydrogen and helium signatures suggest the progenitor star had been stripped of most of its outer envelope by a binary companion . This kind of star would have retained enough angular momentum for disk formation during collapse.
The unusual color evolution. The strange red-to-infrared colors before the second peak could indicate heavy r-process elements—products of neutron star mergers—mixed into the supernova ejecta . It's like finding gold dust in a fireworks explosion.
The active star-forming host galaxy. AT2025ulz sits in a region of active star formation, exactly where you'd expect massive stars to be collapsing .
The Aha Moment: We Might Be Wrong About Stellar Death
Here's where I want you to pause for a second.
For decades, we've treated supernovae and kilonovae as completely separate phenomena. One is about single stars dying. The other is about dead stars colliding. They happen in different places, at different times, through different physics.
But what if that boundary is artificial?
What if some stellar deaths are messy enough to produce both? What if a single collapsing star can give birth to multiple neutron stars that immediately crash together, forging heavy elements in a cosmic alchemist's dream?
This isn't confirmed yet. The chance of AT2025ulz being an unrelated supernova that just happened to appear in the same patch of sky at the same time is about 3-5% . That's not nothing. It could be cosmic coincidence.
But if it's not coincidence, we're looking at an entirely new class of astronomical events. The universe just got weirder—and more wonderful.
What Comes Next?
The only way to confirm superkilonovae exist is to find more of them. The astronomical community is now on high alert for similar events .
Future observations that could strengthen the case include:
- Late-time light curve monitoring: Checking whether the fading matches nickel-56 decay
- Infrared spectroscopy with JWST: Directly measuring the composition of ejected material
- Radio and X-ray observations: Looking for relativistic jets that might punch through the expanding debris
The team is also calling for improvements in how gravitational wave detectors flag low-mass events. S250818k was technically "subthreshold"—meaning it almost didn't trigger follow-up observations . Without the dedication of researchers who decided to look anyway, we might have missed this discovery entirely.
The Bigger Picture: Multimessenger Astronomy Grows Up
AT2025ulz represents something profound for astronomy: the power of combining different types of cosmic signals.
Since 2015, when gravitational waves were first detected, scientists have dreamed of studying sources using multiple "messengers"—gravitational waves, light, neutrinos, and cosmic rays . GW170817 proved this was possible. AT2025ulz shows that multimessenger astronomy can reveal phenomena we never imagined.
"We encourage the community to approach the future of multimessenger astrophysics with a wide-open view to new possibilities," the researchers wrote. "Future multimessenger events may not look like GW170817, will likely be much farther away, and may even show similarities to supernovae."
Conclusion: The Sleep of Reason Breeds Monsters
We've just walked through one of the most exciting astronomical discoveries of the year. A dying star, possibly spinning so fast that it gave birth to twin neutron stars in its death throes. Those twins, crashing together within hours, producing gravitational waves and heavy elements. A supernova wrapped around a kilonova. A superkilonova.
Is it real? We don't know for sure yet. But the evidence is tantalizing, and the implications are staggering.
If subsolar neutron stars can form in the violent aftermath of stellar collapse, it opens new questions about star formation, element production, and the physics of matter under extreme conditions. Every gold atom in your wedding ring might have been forged in events like this.
At FreeAstroScience.com, we believe in keeping your mind active and curious. Because as the Spanish painter Goya warned us, the sleep of reason breeds monsters. Don't let your curiosity sleep.
Come back soon. The universe has more surprises waiting.
Sources
Kasliwal, M.M., et al. (2025). "ZTF25abjmnps (AT2025ulz) and S250818k: A Candidate Superkilonova from a Subthreshold Subsolar Gravitational-wave Trigger." The Astrophysical Journal Letters, 995:L59. Published December 15, 2025. https://doi.org/10.3847/2041-8213/ae2000
Johnston, S. (2025). "Did Astronomers Just Find a 'Superkilonova' Double Explosion? Maybe." Universe Today, December 19, 2025.
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