The cosmic record breaker has now been confirmed by astronomers, who see that the James Webb Space Telescope has detected what could possibly be the earliest supernova, the light of which was observed simply 730 million years after the Big Bang. The supernova has been associated with the long-duration gamma-ray burst GRB 250314A, a burst that was noticed on the 14th of March 2025, by the Space-based multi-band Astronomical Variable Objects Monitor, which was jointly developed by the French space agency CNES and the Chinese academy of Sciences. The duration of the gamma-ray burst was 10 seconds, making it a long GRB, which normally results when massive stars undergo a supernova, turning either a neutron star or a black hole.
The scarcity of observations of this type of GRB at high redshift it is a very select group, with no more than a handful observed during the first billion years of the universe’s existence made it a golden opportunity to study the life cycles and death throes of these primeval stars. The problem is that the GRB’s afterglow, which emits characteristic radiation in the infrared, optical, and X-ray parts of the spectrum and typically provides the clue to unlocking the meaning of the precursor explosion, would be too faint by the time the JWST’s instrument, called NIRCam, was trained on the phenomenon in July 2025 to distinguish it from the signature of the associated supernova and its parent galaxy.
Infrared imaging was essential for the JWST. With the redshift ‘z’ at 7.3, the light emitted by the explosion has been stretched by the expanding universe. Although the nearby supernovae increase in brightness over weeks, the event in that distant galaxy takes much longer in this case, 110 days after the initial GRB trigger, or 13 days after the explosion in the rest frame.
To properly model the separation of the light contributions, a detailed comparison was made with the known spectral energy distributions of typical GRB-associated supernovae, in particular the well-studied type Ic supernova with large velocities, SN 1998bw, whose properties are used as a model for this type of supernovae. The similarity was impressive, as the observed absolute magnitude of M_B ~ -19.4 and the forward spectrum increase fitted very well to a model of a 1998bw-like supernova at this extraordinary distance. More luminous or bluer, like super luminous supernovae or pair-instability supernovas from some metal-free Population III stars, could be excluded on the basis of the observations.
The host galaxy itself is a faint, compact object with MUV ~ −17.8, typical of high redshift gamma-ray burst hosts but smaller and fainter than Lyman break galaxies at redshifts around 7. The resolution was only just an extension in the shorter wavelength filters, implying a greater contribution in the blue from the galaxy itself, though the redder flux is dominated by the supernova. Other models, such as being due to the light from the host alone, were assessed using population synthesis models, but these models required highly implausible assumptions, such as a galaxy dominated by an old stellar population at redshifts of 21.
Gamma-ray bursts can act as highly valuable tools in understanding early star formation because their luminosity enables them to be traced back even if the host galaxy was not luminous enough to be detected in the emission lines. Long GRBs, associated with the collapse of massive stars, can be traced back to their stellar properties directly. The uniformity in supernovae associated with GRBs in the local universe, in terms of their maximum brightness (MV = −19.2 ± 0.4), raised doubts regarding whether these uniform properties would have been observed at all epochs. The resemblance of GRB 250314A’s supernova to present-day ones indicates that early massive stars which led to GRBs may not have been significantly different in either their internal composition or their explosions compared to those of recent times despite lower metallicities of their gas, as well as differing interstellar environments in the Era of Reionization.
The discovery also reveals the capability of JWST to be a revolutionary facility for transient astronomy. The instrument’s sensitivity in the near- to mid-infrared wavelength range allows it to directly observe the individual bursts of supernovae occurring when the Universe was merely 5% of its present age. The measurement of the warm echo of GRBs allows JWST to isolate the so-called “fingerprints” of the host galaxy, which include details of its composition, star-formation rates, and dust content. A follow-up observation is also scheduled for 2026, when the brightness of the supernova will have reduced by more than two magnitudes, thus separating the light of the host and the supernova.
For aficionados of space science and researchers alike, the significance of the GRB 250314A goes beyond the fact that it is the biggest explosion ever recorde it is essentially a testing ground for the models of stellar evolution that would have occurred in the primordial universe. The remarkable consistency between the model and the JWST photometric data reveals that even at the early stages of the universe’s development, there were stars that lived and died according to the principles of modern astrophysics.
