The James Webb Space Telescope (JWST) has detected a possible supernova in the early universe just days after the star exploded. Called AT 2026abck, the transient lies in a star-forming galaxy at a redshift of 5.017 and was found without the help of gravitational lensing. JWST observations suggest the event was caught only about four to 10 days after the explosion. The paper was posted to the arXiv preprint server on Sept. 29.
The farther we go, the less we see
Supernovae in the early universe offer a direct view of how massive stars lived and died when galaxies were young, metal-poor and forming stars in bursts. Their rate also gives an independent measure of the cosmic star-formation history. But they are difficult to observe because the light they emit gets stretched and redshifted to infrared wavelengths by the time it reaches us.
The James Webb Space Telescope, with its exceptional infrared vision, has made it possible to detect over 200 potential supernova events with host galaxy redshifts out to around 6. Beyond redshift 4 (when the universe’s age was 1.5 billion years), most candidates have been observed only once or a few times, and few have been caught within days of exploding. At redshifts greater than 5, only one supernova has been confirmed based on its spectra, and only because it was gravitationally magnified up to 30 times by a galaxy cluster.
In this work, astronomers led by Ming-Yang Zhuang of the University of Illinois Urbana-Champaign report the discovery of one such young “transient” detected at redshift 5, in a field that is not lensed or magnified by any galaxy or galaxy cluster. It appeared in observations from NEXUS, a JWST Treasury program that revisits the same patch of sky every two months.
On May 28, 2026, the spot was empty. About 71 observed days later, it had appeared at roughly magnitude 27.4, a brightening of more than 3.6 times in the F200W filter.
Hotter than the rest
Modeling gave a temperature of about 17,000 Kelvin, a radius of about 46 times the Earth–sun distance and a luminosity of about 3 × 10⁴³ erg/s. The team estimates the explosion happened four to 10 rest-frame days before discovery.
It sits between two components of its host galaxy, roughly 3,600 and 4,200 light-years from the center. This argues against the scenario of a star shredded by a black hole or an active galactic nucleus flare, which typically happens at the host galaxy’s center. The ultraviolet-bright emission, high temperature and brightness so early are also inconsistent with a Type Ia explosion.
Type II supernovae are core-collapse explosions, in which a massive star dies in a violent blast after its core collapses rapidly. Among 34 well-observed nearby Type II supernovae, none had a temperature above about 15,000 Kelvin beyond six days or reached this luminosity after four days. “AT 2026abck is hotter and more luminous than normal SNe II at comparable phases,” the team writes in the paper.
This or that?
The brightness instead matches Type II supernovae surrounded by dense, confined circumstellar material. The team says this points to extra early-time power, most plausibly from shock cooling of an extended envelope or interaction with that material.
The host galaxy was found to have about 6 billion solar masses of stars, roughly 0.3 times the sun’s metal content and a star formation rate of 14 to 21 solar masses a year, typical for its era. However, it differs sharply from the host of SN Eos, the only spectroscopically confirmed supernova at this distance.
There is no spectrum of the transient. The team’s analysis rests on broadband photometry, meaning measurements of its brightness in a few filters, from which they reconstructed its spectral energy distribution. The redshift comes from the host galaxy’s spectrum. Researchers say a definitive classification needs deep NIRSpec spectroscopy and that they can’t fully rule out different scenarios.
It could also be a superluminous supernova caught on the rise. “Prompt JWST follow-up observations of AT 2026abck will test these different scenarios,” they write. A Type II should fade quickly in ultraviolet, while a superluminous one would keep brightening.
Written for you by our author Shreejaya Karantha, edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive.
If this reporting matters to you, please consider a donation (especially monthly). You’ll get an ad-free account as a thank-you.
Publication details
Ming-Yang Zhuang et al., NEXUS: Discovery of a Luminous Young Transient at z=5, arXiv (2026). DOI: 10.48550/arxiv.2609.38373
Journal information:
arXiv
© 2026 Science X Network
Citation:
Webb spots possible early-universe supernova just days after it happened (2026, October 13)
retrieved 13 October 2026
from https://phys.org/news/2026-10-webb-early-universe-supernova-days.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no
part may be reproduced without the written permission. The content is provided for information purposes only.