Webb Detects Earliest-Known Supernova Explosion

Astronomers using the NASA/ESA/CSA James Webb Space Telescope discovered a supernova explosion accompanying the gamma-ray burst event GRB 250314A at a redshift of 7.3, when the Universe was only 730 million years old. The previous record-breaking supernova event occurred when the Universe was 1.8 billion years old. The discovery is reported in two articles in the journal Astronomy and astrophysics.

Webb identified the source of a very bright flash of light known as a gamma-ray burst: a supernova that exploded when the Universe was only 730 million years old. Image credit: NASA / ESA / CSA / STScI / A. Levan, IMAPP / A. Pagan, STScI.

“Only Webb could directly show that this light comes from a supernova, a massive collapsing star,” said Dr Andrew Levan, an astronomer at Radboud University and the University of Warwick, and lead author of one of the two papers.

“This observation also demonstrates that we can use Webb to find individual stars when the Universe was only 5% of its current age.”

While a gamma-ray burst typically lasts a few seconds to a few minutes, a supernova brightens rapidly for several weeks before slowly fading.

On the other hand, the supernova associated with GRB 250314A has become brighter over the months.

Since its explosion so early in the history of the Universe, its light has stretched as the cosmos has expanded over billions of years.

As light stretches, so does the time it takes for events to unfold.

Webb’s observations were intentionally taken 3.5 months after the end of the GRB 250314A event, as the underlying supernova was expected to be brightest at that time.

“Webb provided the fast and sensitive tracking we needed,” said Dr. Benjamin Schneider, an astronomer at the Laboratoire d’Astrophysique de Marseille.

Gamma-ray bursts are incredibly rare. Those that last a few seconds can be caused by the collision of two neutron stars, or by the collision of a neutron star and a black hole.

Longer bursts like this, which lasted about 10 seconds, are frequently linked to explosions of massive stars.

On March 14, 2025, the SVOM mission, a Franco-Chinese telescope launched in 2024 and designed to detect fleeting events, detected a gamma-ray burst coming from a very distant source.

Within an hour and a half, NASA’s Neil Gehrels Swift Observatory pinpointed the location of the X-ray source in the sky. This allowed subsequent observations that would determine the distance to Webb.

Eleven hours later, the Nordic Optical Telescope was queued and revealed a gamma-ray burst afterglow of infrared light, an indication that the gamma ray might be associated with a very distant object.

Four hours later, ESO’s Very Large Telescope estimated that the object existed 730 million years after the Big Bang.

“Over the past 50 years, only a handful of gamma-ray bursts have been detected during the first billion years of the Universe,” Dr. Levan said.

“This particular event is very rare and very exciting.”

Since it is the oldest and most distant supernova detected to date, researchers compared it to nearby modern supernovae. The two turned out to be very similar, which surprised them.

For what? We still know little about the first billion years of the Universe.

The first stars probably contained fewer heavy elements, were more massive, and lived shorter lives.

They also existed during the era of reionization, when the gas between galaxies was largely opaque to high-energy light.

“Webb showed that this supernova looks exactly like modern supernovae,” said Professor Nial Tanvir of the University of Leicester.

“Webb’s observations indicate that this distant galaxy is similar to other galaxies that existed at the same time,” said Dr Emeric Le Floc’h, an astronomer at CEA Paris-Saclay.

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AJ Levan and others. 2025. JWST reveals a supernova following a gamma-ray burst at z ≃ 7.3. A&A 704, L8; doi: 10.1051/0004-6361/202556581

B. Cordier and others. 2025. SVOM GRB 250314A at z ≃ 7.3: An explosive star in the era of reionization. A&A 704, L7; doi: 10.1051/0004-6361/202556580