Oldest Supernova Ever Seen – Webb Telescope Discovery

Rapid observations from the James Webb Space Telescope (also called Webb or JWST) made it possible to detect the host galaxy of a supernova, a massive star at the end of its life. These observations thus verify the data collected by telescopes around the world, which in mid-March followed the indicator of the explosion of this star, called a “gamma burst”. What is exceptional is that the star identified by Webb went out when the Universe was only 730 million years old, or 5% of its current age! This is the first time that the telescope, for which CNES provided the Mirim imager, has observed such a distant and ancient event.

The supernova phenomenon is usually visible for several weeks, before slowly fading away. The light from the supernova observed by James Webb remained visible for several months! The satellite observations were intentionally carried out three and a half months after the end of the gamma-ray burst, which only lasts a few seconds to a few minutes, the supernova then being at its peak.

If this phenomenon lasted so long, it is because the explosion of this star occurred very early in the history of the Universe. Its light therefore diffused as the cosmos expanded over billions of years. However, the more the light stretches, the longer the time during which the event remains visible.

Teamwork with SVOM and terrestrial telescopes

Gamma-ray bursts are extremely rare. The shortest ones usually come from the collision of two neutron stars, or a neutron star and a black hole. The longest ones, like this one, are often linked to the disappearance of massive stars.

It was the Franco-Chinese satellite SVOM, launched in 2024 to detect gamma-ray bursts in the Universe, which issued the first alert, on May 14, 2025. In an hour and a half, the NASA Neil Gehrels Swift Observatory pointed out the source in the sky, in order to determine its distance from the JWST. Then it was the Nordic Optical Telescope in the Canary Islands which detected an infrared afterglow from a gamma-ray burst, revealing that this observation was extremely distant. Finally, the European Very Large Telescope in Chile estimated the object’s distance at 730 million years after the Big Bang. An exceptional observation: over the last 50 years, only a few rare gamma-ray bursts dating from the first billion years of the Universe have been spotted.

Striking similarities with neighboring and more recent supernovae

Not only is this the most distant and oldest supernova ever observed, but it also bears uncanny similarities to closer and more recent supernovae. What intrigued the researchers… and remains a mystery to this day.

We do not know much about the first billion years of the Universe. Early stars probably contained fewer heavy elements, were more massive, and had shorter lifespans. These massive stars also played a role during the so-called “reionization” process of the Universe, when a fog of light-absorbing gas existed between the galaxies: they gradually ionized this opaque matter, allowing light to circulate in space.

But for researchers to determine why such an old supernova is similar to nearby supernovas, more data is needed to highlight the differences between them. The team received permission to reuse James Webb to observe other ancient gamma-ray bursts in hopes of using their afterglow as a tracer of primitive galaxies. To be continued!