An international team of astronomers has achieved a first in probing the early universe, using the James Webb Space Telescope (JWST), detecting a supernova – the explosive death of a massive star – at an unprecedented cosmic distance.
The explosion, designated SN in GRB 250314A, occurred when the universe was only about 730 million years old, placing it deep in the era of reionisation. This remarkable discovery provides a direct look at the final moments of a massive star from a time when the first stars and galaxies were just beginning to form.
The event, which has been reported on in the recently published academic paper ‘JWST reveals a supernova following a gamma-ray burst at z ≃ 7.3,’ (Astronomy & Astrophysics, 704, December 2025), was initially flagged by a bright burst of high-energy radiation, known as a long-duration Gamma-Ray Burst (GRB), detected by the space-based multi-band astronomical Variable Objects Monitor (SVOM) on March 14, 2025. Follow-up observations with the European Southern Observatory’s Very Large Telescope (ESO/VLT) confirmed the extreme distance.
The key finding came from targeted observations with JWST's Near-Infrared Camera (NIRCAM) approximately 110 days after the burst. Scientists were able to separate the light of the explosion from its faint, underlying host galaxy.
Co-author, and astrophysicist at UCD School of Physics, Dr Antonio Martin-Carrillo said: “The key observation, or smoking gun, that connects the death of massive stars with gamma-ray bursts is the discovery of a supernova emerging at the same sky location. Almost every supernova ever studied has been relatively nearby to us, with just a handful of exceptions to date. When we confirmed the age of this one, we saw a unique opportunity to probe how the Universe was there and what type of stars existed and died back then.
“Using models based on the population of supernovae associated with GRBs in our local universe, we made some predictions of what the emission should be and used it to proposed a new observation with the James Webb Space Telescope. To our surprise, our model worked remarkably well and the observed supernova seems to match really well the death of stars that we see regularly. We were also able to get a glimpse of the galaxy that hosted this dying star.”
The data indicate that the distant supernova is surprisingly similar in brightness and spectral properties to the prototype GRB-associated supernova, SN 1998bw, which exploded in the local universe.
This similarity suggests that the massive star that collapsed to create GRB 250314A was not significantly different from the progenitors of GRBs observed locally, despite the vastly different physical conditions (such as lower metallicity) in the early universe. The observations also ruled out a much more luminous event, such as a Superluminous Supernova (SLSN).
The findings challenge the assumption that the stars of the early universe, formed under extremely low-metallicity conditions, would lead to markedly different, perhaps brighter or bluer, stellar explosions than those seen today.
While this discovery provides a powerful anchor point for understanding stellar evolution in the early universe, it also opens new questions about the observed uniformity.
The research team plans to secure a second epoch of JWST observations in the next one to two years. By that time, the supernova light is expected to have faded significantly (by over two magnitudes), allowing the team to completely characterise the properties of the faint host galaxy and confirm the supernova's contribution.