Astronomers using the NASA/ESA/CSA James Webb Space Telescope have detected a supernova explosion accompanying the gamma-ray burst GRB 250314A at redshift 7.3 when the Universe was just 730 million years old. The previous chart-topping supernova event occurred when the universe was 1.8 billion years old. The discovery is reported in two journal articles magazine Astronomy and astrophysics.
Webb identified the source of the super-bright flash of light known as a gamma-ray burst: a supernova that exploded when the universe was just 730 million years old. Image credit: NASA/ESA/CSA/STScI/A. Levan, IMAPP/A. Pagan, STScI.
“Only Webb could directly show that this light came from a supernova – a collapsing massive 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.”
Although a gamma-ray burst typically lasts from a few seconds to minutes, a supernova quickly brightens over a period of weeks before slowly dimming.
In contrast, the supernova associated with GRB 250314A became brighter over several months.
Because it exploded so early in the history of the universe, its light was stretched as the cosmos expanded over billions of years.
As light is stretched, the time required for events to unfold increases.
Webb's observations were deliberately taken 3.5 months after the end of GRB 250314A because the underlying supernova was expected to be brightest at this time.
“Webb provided we need fast and sensitive observations,” said Dr. Benjamin Schneider, an astronomer at the Laboratory of Astrophysics in Marseille.
Gamma-ray bursts are incredibly rare. Those that last a few seconds may be caused by two neutron stars or a collision between a neutron star and a black hole.
Longer flares like this one, lasting about 10 seconds, are often associated with the explosions of massive stars.
On March 14, 2025, the SVOM mission, a French-Chinese telescope launched in 2024 and designed to detect fleeting events, discovered gamma-ray burst from a very distant source.
Within an hour and a half, NASA's Neil Gehrels Swift Observatory located the X-ray source in the sky. This allowed for follow-up observations that would determine the distance to Webb.
Eleven hours later, the Northern Optical Telescope was in line and detected the afterglow of an infrared gamma-ray burst, indicating that the gamma rays may 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 last 50 years, only a few gamma-ray bursts have been detected that were detected in the first billion years of the Universe,” Dr. Levan said.
“This particular event is very rare and very exciting.”
Because it is the earliest and most distant supernova discovered to date, the researchers compared it to modern nearby supernovae. The two turned out to be very similar, which surprised them.
Why? Little is still known about the first billion years of the Universe's existence.
Early stars likely 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 University of Leicester professor Nial Tanveer.
“Webb's observations show that this distant galaxy is similar to other galaxies that existed at the same time,” said Dr. Aymeric Le Floc, an astronomer at CEA Paris-Saclay.
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Also Levan etc.. 2025. JWST detects a gamma-ray burst supernova at z ≃ 7.3. A&A 704, L8; two: 10.1051/0004-6361/202556581
B. Cordier etc.. 2025. SVOM GRB 250314A at z ≃ 7.3: an exploding star in the era of reionization. A&A 704, L7; doi:10.1051/0004-6361/202556580
