Webb Spots Earliest Known Supernova After Gamma-Ray Burst From Cosmic Dawn

On a March afternoon last year, a ten-second flash of high-energy light from an obscure patch of sky set off alarms aboard a new Chinese-French space telescope.

The satellite, called SVOM, had just caught a long gamma-ray burst — one of the most powerful kinds of explosions in the universe. Within hours, telescopes from orbit and on the ground were racing to the fading afterglow. By the following day, astronomers knew they were looking almost all the way back to the dawn of time.

Months later, the James Webb Space Telescope delivered the real surprise: nestled inside that distant fireball was the earliest confirmed supernova ever seen, and it looked uncannily like explosions in galaxies next door.

Scientists say the discovery shows that massive stars were already living fast and dying in familiar ways just 730 million years after the Big Bang, when the universe was barely 5% of its current age.

“It’s the earliest time we’ve been able to directly see a massive star explode,” said Andrew Levan, an astronomer at Radboud University in the Netherlands and the University of Warwick in the United Kingdom, who led one of the two studies describing the event. “Only Webb can actually separate the light of the supernova from the surrounding galaxy and afterglow.”

The results, published in the journal Astronomy & Astrophysics in December, cap a global observing campaign that began with a brief burst of gamma rays on March 14, 2025, and ended more than three months later with a faint point of infrared light in Webb’s cameras.

A message from cosmic dawn

The chain of events started March 14 at 12:56:42 Coordinated Universal Time, when SVOM’s wide-field ECLAIRs instrument detected a sudden spike in gamma rays lasting about 10 seconds. The event was cataloged as GRB 250314A, marking it as a long-duration gamma-ray burst, the type typically linked to the collapse of a massive star.

SVOM — short for Space-based multi-band astronomical Variable Objects Monitor — is a joint mission of the China National Space Administration and the French space agency CNES. Launched in 2024, it is designed to spot fleeting, high-energy phenomena and broadcast alerts within seconds through a low-latency ground network.

Those alerts triggered a relay of follow-up observations. NASA’s Neil Gehrels Swift Observatory turned its X-ray telescope toward the source and localized the afterglow within about 90 minutes. Roughly 11 hours after the burst, the Nordic Optical Telescope in Spain’s Canary Islands picked up a faint afterglow in infrared light, but saw almost nothing at visible wavelengths — an early sign that the explosion was extremely distant and its light heavily redshifted.

About four hours later, the European Southern Observatory’s Very Large Telescope in Chile obtained a spectrum of the fading afterglow. The data showed a redshift of about 7.3, meaning the light had been stretched by the expansion of the universe to more than eight times its original wavelength.

In cosmic terms, that places the explosion roughly 13 billion light-years away, during a period known as the epoch of reionization, when the first generations of stars and galaxies were transforming the foggy early universe into the transparent cosmos seen today.

On its website, the SVOM team described the event as “a message from the depths of time,” noting that the gamma rays had traveled nearly 13 billion years before striking the satellite’s detectors.

Waiting for a supernova that already happened

For astronomers who study gamma-ray bursts, a long-duration event at such a large redshift carried a clear implication: somewhere behind that glare, a massive star had likely just died.

In the nearby universe, many long gamma-ray bursts have been tied to energetic core-collapse supernovae, often from stripped, rapidly rotating stars at least 20 to 30 times the mass of the Sun. But confirming a similar explosion at redshift 7.3 would require an instrument powerful enough to pick out a faint, slowly evolving point of light in a distant galaxy.

That is where the James Webb Space Telescope came in.

Because of cosmic expansion, time in such a remote galaxy appears stretched by a factor of around eight. A supernova that would brighten and fade over weeks in the local universe instead plays out over months when seen from Earth. To catch it near maximum brightness, Levan and his colleagues requested Director’s Discretionary Time on Webb, a fast-track mechanism reserved for particularly urgent or compelling targets.

Webb’s Near-Infrared Camera, or NIRCam, observed the site of GRB 250314A on July 1, 2025 — about 110 days after the initial burst in Earth’s frame of reference. The telescope imaged the region in several infrared filters, covering wavelengths from roughly 1.5 to 4.4 microns.

The images revealed a tiny, reddish smudge of a galaxy, typical of star-forming systems at that era, along with a brighter point of light offset from its center. By measuring the brightness and colors across different filters, the team concluded that the point source was best explained as a supernova riding on top of the host galaxy’s glow.

Nial Tanvir, an astronomer at the University of Leicester in the United Kingdom and co-author on the Webb study, said the team intentionally did not assume the explosion would behave like those seen nearby.

“We went in with open minds,” Tanvir said in a statement released by NASA. “What Webb showed us is that this supernova looks remarkably similar to modern examples.”

A familiar death in an unfamiliar universe

In their analysis, Levan and colleagues compared the observed light to models based on SN 1998bw, a well-studied supernova associated with a nearby gamma-ray burst that has become a prototype for such events.

After adjusting for the extreme distance and redshift, they found the brightness and spectral energy distribution of the new event closely matched that of SN 1998bw. That suggests the explosion in GRB 250314A was a broad-lined Type Ic-like core-collapse supernova — a category of powerful “hypernovae” where a massive star’s core collapses into a black hole and launches jets that power a gamma-ray burst.

The finding runs against some expectations about the first generations of stars. Early in the universe, gas contained far fewer heavy elements, or “metals,” than it does today. Many theoretical models predict that such pristine, metal-poor stars could grow far more massive and might explode in unusually bright or exotic ways, such as pair-instability supernovae.

Instead, at least in this case, the death of a star in a young, metal-poor galaxy looks very much like the deaths of some massive stars in the modern universe.

“The supernova is surprisingly similar in brightness and properties to SN 1998bw,” Levan and co-authors wrote, arguing that the data provide direct evidence for “broadly similar” explosion mechanisms at a much earlier cosmic epoch.

The team cautions there are still uncertainties. Some of the light in Webb’s images likely comes from the host galaxy itself or from lingering afterglow from the gamma-ray burst. A second round of Webb observations, planned for a year or two later, will allow astronomers to subtract the galaxy’s constant light and isolate the supernova contribution more precisely.

Even so, space agencies including NASA and the European Space Agency have described the event as the earliest confirmed supernova yet identified, surpassing a previous record-holder at redshift about 4.3, seen by Webb in 2023.

A tiny galaxy and a bigger picture

Beyond the supernova, GRB 250314A offers a rare glimpse at its host galaxy, one of the faintest and youngest systems ever linked to a stellar explosion.

Webb’s NIRCam images resolved a compact, irregular galaxy with multiple knots of star formation. Its properties appear typical of galaxies at similar redshifts seen in deep-field surveys, but such faint objects are difficult to find and study without a bright beacon to mark their position.

Gamma-ray bursts provide that beacon.

“Bursts like this act as signposts to star-forming galaxies we might otherwise miss,” Levan said. By tracing where these events occur, he added, astronomers can build a more complete map of how and where stars were forming during the first billion years after the Big Bang.

Core-collapse supernovae also play a crucial role in how those galaxies evolve. When massive stars explode, they forge and eject heavy elements such as oxygen, silicon and iron — the raw materials for rocky planets and, eventually, life. Observing such explosions deep in cosmic time helps scientists estimate how quickly galaxies enriched their gas and how early the building blocks of planets became available.

Collapsar-type gamma-ray bursts like GRB 250314A are also believed to leave behind stellar-mass black holes. Detecting them at redshift 7.3 provides data for models of how quickly black holes formed and grew in the early universe, a key piece of the puzzle in understanding the origin of the supermassive black holes found at the centers of galaxies today.

A global relay for a fleeting event

The campaign around GRB 250314A also underscores how modern astronomy depends on rapid data sharing and international cooperation.

SVOM is operated jointly by China and France. Swift is run by NASA in partnership with the United Kingdom and Italy. The Nordic Optical Telescope is funded by a consortium of Northern European countries, while the Very Large Telescope is an ESO facility in Chile supported by European and partner nations. Webb itself is a collaboration between NASA, the European Space Agency and the Canadian Space Agency.

Alerts about the burst and its afterglow were distributed in near real time through global networks, allowing telescopes that are often in competition for observing time to act as parts of a coordinated system.

“This is a textbook example of why we build these facilities and link them together,” Tanvir said. “No single observatory could have done this alone.”

As Webb continues to scan the distant universe, astronomers expect more such events to appear — and perhaps, eventually, the kind of truly exotic explosions theorists have long predicted for the very first stars.

For now, the death of the star behind GRB 250314A offers a different kind of lesson. At a time when galaxies were just beginning to carve the first clearings in the cosmic fog, at least some of the universe’s earliest massive stars seem to have lived and died in ways that look strikingly familiar.

Their light, crossing most of cosmic history, is only now arriving — and revealing that, even close to the beginning, the universe was already running some of the same plays it uses today.