On March 14, 2025, satellite instruments detected a brief burst of gamma rays lasting about 10 seconds, originating from a source so remote it dates back to the universe's infancy, before it reached one billion years old.
Within hours, ground- and space-based observatories swiftly turned their attention to the fading event. Initial X-ray data swiftly pinpointed its coordinates, followed by detection of an infrared afterglow. Observations identified the burst as GRB 250314A, situated at an astonishingly vast distance, prompting astronomers to reserve time on the most advanced infrared observatories in orbit.
Key follow-up observations by the James Webb Space Telescope identified this event as the earliest supernova ever recorded, originating from a massive star that exploded when the cosmos was barely 730 million years old. Webb also captured the faint galaxy hosting this explosion. NASA’s detailed report emphasizes that this finding shatters the previous supernova record, which dated back to about 1.8 billion years after the Big Bang.
Locating GRB 250314A at Redshift 7.3
The initial alert came from SVOM, a collaborative French-Chinese mission designed to detect cosmic transient phenomena. Within roughly 90 minutes, NASA’s Neil Gehrels Swift Observatory locked onto the X-ray source's precise location, enabling other telescopes to conduct follow-up observations.
About 11 hours later, the Nordic Optical Telescope positioned in the Canary Islands detected infrared emissions. Subsequently, the Very Large Telescope in Chile estimated the event occurred approximately 730 million years post-Big Bang.
Further analysis by the Stargate collaboration established the event’s redshift at 7.3. According to the Paris Observatory’s overview, GRB 250314A ranks as the third most distant gamma-ray burst measured through spectroscopy, taking place during the universe’s reionization epoch.
The approximately 10-second duration of this gamma-ray burst hinted at the cataclysmic demise of an enormous star, typical of long-duration bursts.
With these insights, astronomers could strategically schedule observations with Webb timed to capture the supernova at peak brightness.
Webb’s Follow-Up Observations After Several Months
On July 1, approximately three and a half months post-burst, Webb trained its instruments on the target. The scheduling accounted for cosmic expansion effects, which stretch both light and the perceived event duration, causing this supernova to brighten over months instead of weeks as is common nearby.
Webb’s near-infrared imaging directly revealed the supernova responsible for the gamma-ray burst and identified its dim host galaxy, which appeared only as a few faint pixels, limiting detailed study of its structure.
Benjamin Schneider, a postdoctoral researcher at the Laboratoire d’Astrophysique de Marseille and co-author, noted, “Webb offered the essential sensitivity and promptness needed for this study.”
Prior to this, the farthest known supernova detected by Webb was from a period when the universe was 1.8 billion years old. This discovery pushes the boundary back to an epoch just 730 million years after the Big Bang.
Remarkable Similarities to Nearby Supernovae Found
Scientists compared this ancient supernova to recent ones in the nearby universe and found a striking resemblance despite the vast temporal gap.
Early stars likely differed significantly in composition, possessing fewer heavy elements and exhibiting distinct lifespans and masses, as noted in the European Space Agency’s release. More observations are necessary to discern subtle distinctions between GRB 250314A’s explosion and those nearer to us.
Professor Nial Tanvir from the University of Leicester, a co-author, remarked, “We approached it without assumptions, and Webb revealed this supernova is remarkably similar to those observed today.”
Additionally, Webb provided the first-ever glimpse of a host galaxy containing a supernova at such a distant epoch, which resembles other galaxies from that era though its image is limited to just a few pixels.
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