A flash of energetic light lasts only ten seconds, yet it has completely turned around our understanding of the timeline for star deaths. A burst, known as GRB 250314A, detected on March 14, 2025, by the Francophren-Chinese satellite SVOM, occurred in the very early Universe, only 730 million years old, in the era of reionization. Reionization was the phase between the Dark, hydrogen-rich Universe and the shining Universe, where the first stars and galaxies appeared. After the gamma-ray flash, the supernova, with its rapid increase in brightness, left its mark, now known to be the oldest supernova yet detected.
Gamma-Ray Bursts (GRBs) are some of the most extreme events in astrophysics, which in a few seconds would radiate more energy than the Sun emits throughout its entire lifetime. Long GRBs, including GRB 250314A, are commonly associated with the collapse of massive stars into black holes with relativistic jets. The first alert was followed at a very rapid pace by a series of observations. NASA’s Neil Gehrels Swift satellite localized the position of the X-ray counterpart in ninety minutes, and subsequently, the infrared spectra measured with ground-based observatories, including the Nordic Optical Telescope and the European Southern Observatory’s Very Large Telescope, revealed an extreme redshift at z ≃ 7.3.
The crucial evidence arrived 110 days after the event of its own when James Webb Space Telescope’s Near-Infrared Camera resolved the light of the fading supernova from its underlying host. “Only Webb could provide direct evidence that the light is coming from a supernova, and this is how massive stars end their lives,” explained Andrew Levan of the Radboud University. The sensitivity in infrared observations is essential in this case because of time dilation and because the light is shifted to longer instead of optical wavelengths for similar distances.
Spectral analysis showed a surprising similarity to SN 1998bw, a local broad-lined Type Ic supernova known to be linked to a GRB. This similarity existed despite the strongly differing conditions of the early universe, during which the metallicity, or the relative abundance of elements heavier than helium, would have been smaller. Although calculations suggested that ‘primordial’ stars could cause more luminous or blue events, GRB 250314A’s supernova conforms to the same physical processes that occur in contemporary core collapse. The observations also eliminated a possible superluminous supernova, which could shine with a luminosity above −21 mag and with a time evolution that could span a more extended period.
It is crucial to understand the type: broad-lined Type Ic supernovae arise from massive stars that have been stripped of their hydrogen and helium envelopes, for example, due to stellar winds, before the collapse into a black hole. The collapse may result in the production of relativistic jets, causing the gamma-ray burst in cases when they are aligned with Earth’s line of vision. In the case of GRB 250314A, the supernova and the associated afterglow curve matched this process.
The host galaxy, resolved by Webb to a small pixelated smear, is representative of galaxies during the reionization era: small, actively star-forming, and prematurely ejecting ions into the intergalactic medium. Analyses of these galaxies have indicated carbon to oxygen ratios that are lower than those observed at later times, marking the onset of active star-forming activity and metal enrichment. Identifying a supernova explosion within a reionization-era galaxy connects the demise of a massive star to an active process of reionization.
The expansion of the cosmos not only shifts the wavelength of light towards the red end of the spectrum but also stretches the time scale. While a two-week journey to reach the peak brightness in the past would become a three-and-a-half-month journey in the case of GRB 250314A, this enabled scientists to schedule the observation time on the Webb space telescope.
The finding highlights the importance of GRBs as beacons too. By observing the prompt onset and tracing the multi-wavelength follow-up emission and identifying the SN, scientists can target and observe galaxies that are otherwise invisible. “This light will allow Webb to see further and give us a ‘fingerprint’ of a galaxy,” Levan said in a statement. The scientists are preparing a follow-up observation campaign using JWST when the SN has faded by over two magnitudes in brightness.
GRB 250314A represents more than the breaking of a record it’s the connection between the dawn of the universe and the present, proving that the process of certain star deaths has been quite unchanged for the last 13 billion years. In the first galaxies, massive stars were still synthesizing heavy elements and exploding in bursts whose underlying physics are the same today.
