'We were amazed': Scientists using James Webb telescope may have discovered the earliest supernova in the known universe

Astronomers using the James Webb Space Telescope (JWST) may have discovered the most distant supernova in the universe. This stellar explosion, hosted by a very faint galaxy, occurred when the universe was only 730 million years old.

Besides adding a new potential record to JWST's already-impressive list, this detection provides insight into the origin of a superbright gamma-ray burst observed in March. These sudden, short-lived outbursts of gamma-rays are among the most powerful explosions in the universe.

Dubbed GRB 250314A, the burst of energy was discovered by the Space Variable Objects Monitor, a small X-ray telescope developed by China and France. Within a couple of days of the initial alert, scientists estimated that the intense flash originated from a very distant object that existed just 730 million years after the Big Bang.

Because not many of these high-energy events have been discovered within the first billion years of the universe, this was a rare chance for astronomers to understand how early-universe stars and galaxies evolve.

When two research teams examined the properties of this gamma-ray burst, they found evidence that it may have been produced by an exploding star at the edge of the universe — confirming one of the team's predictions.

"We were amazed that our predictions worked so well, and that we had been able to demonstrate that JWST could see individual exploding stars at such extreme distances," A.J. Levan, lead author of one of the two papers and a professor at Radboud University in the Netherlands and the University of Warwick in the United Kingdom, told Live Science in an email.

Both new studies were published Dec. 9 in the journal Astronomy & Astrophysics.

A hunt for clues

Short gamma-ray bursts, which last less than two seconds, are thought to arise from mergers of neutron stars, the ultradense remnants of dead stars. Long gamma-ray bursts, by contrast, are produced when massive stars collapse to form a neutron star or a black hole.

The initial burst from GRB 250314A lasted around 10 seconds, placing it comfortably in the long-duration category. Therefore, researchers were curious to know if the gamma-ray burst was produced by a supernova — the catastrophic death of a massive star.

Although gamma-ray bursts last only a few seconds to minutes, they leave behind an afterglow — smoothly fading light with energy lower than gamma-rays (X-rays, optical light, radio and infrared) that lasts several days. Because gamma-ray bursts are so brief, most of the information about them is revealed by their longer-lasting afterglows.

To confirm their predictions, the researchers had to separate the light from the afterglow, the supernova and the host galaxy. GRB 250314A produced a detectable infrared and X-ray afterglow, but luckily, it faded by the time JWST observed the site months later. Hence, this glow was expected to be too faint to explain the observed light, indicating that another source contributed to it.

"This leaves us to disentangle the [light from the] galaxy and the supernova," Levan said. If most of the light was produced by the host galaxy, then the galaxy should have been a very compact and unusually old galaxy with stars that formed at close to 200 million years after the Big Bang.

"This would be an interesting result in its own right because we don't see many galaxies like this, and in particular, this isn't the sort of galaxy you'd expect to find a gamma-ray burst in," he added.

Therefore, the gamma-ray burst's properties could be explained only by a supernova, the team concluded.

The distant twin

The brightness of a supernova depends on how much radioactive material is expelled during the explosion. This, in turn, is determined by the mass of the star's core when it explodes.

For several reasons, astronomers think stars in the early universe may have had more massive cores than those seen today. The supernova associated with GRB 250314A, therefore, offered a rare opportunity to study the nature of early-universe stars. Because GRB 250314A was possibly the earliest supernova ever observed, the researchers compared it with supernovas seen in the nearby universe. Surprisingly, it turned out to be remarkably similar to modern stellar explosions.

"This may be a chance; after all, it is only one object," Levan said. "However, it could also suggest that the exploding stars [in the early universe] — and thus the overall stellar population — aren't as different as we think.."

To confirm that it is a supernova, researchers still need to reestimate how much of the observed light comes from the supernova itself and how much originates from the afterglow or the host galaxy. They plan to carry out follow-up observations next year, after the supernova has faded, which will make it much easier to separate the contributions from these different sources.