For the first time, Webb catches the before and after of a supernova

Astronomers say they've captured the clearest look yet of a star on the brink of supernova.

An international team used NASA’s James Webb Space Telescope to backtrack and find the original star that suffered a cataclysmic explosion on June 29. The discovery is an unprecedented accomplishment for the observatory.

Identifying the original star was harder to achieve than it sounds. Supernovas happen suddenly, and no one knows which star in the sky will explode next. Scientists must pore over older, pre-explosion photos of the same swath of space, hoping to spot the single bright point of light that has since vanished. Webb has only been operating for three years, so this is the first time one of its previous images has overlapped with the site of a new supernova. 

The result is a rare before-and-after look at a dying star. The supernova, called SN 2025pht, came from a red supergiant, one of the largest types of stars in the universe. It was roughly 40 million light-years away in the galaxy NGC 1637. 

"For multiple decades, we have been trying to determine exactly what the explosions of red supergiant stars look like," said Charlie Kilpatrick, a research assistant professor at Northwestern University, in a statement. "Only now, with [Webb], do we finally have the quality of data and infrared observations that allow us to say precisely the exact type of red supergiant that exploded and what its immediate environment looked like." 

A James Webb Space Telescope image of the galaxy NGC 1637, with a highlighted box around the location where the supernova occurred; the top four grayscale images on the right were taken by the Hubble Space Telescope, and the bottom four were taken by Webb. Credit: Charles D. Kilpatrick et al. / https://doi.org/10.3847/2041-8213/ae04de

What made the original star, about 15 times more massive than the sun, remarkable was the thick blanket of dust that surrounded it. That shell hid much of the star’s brightness and made it appear much fainter and redder than it really was. With Webb’s powerful infrared vision, astronomers were able to see through that dust.

The researchers matched the explosion’s location to a single red, bright source. They used Hubble Space Telescope and Webb images to confirm it. The star was roughly 100,000 times brighter than the sun, yet appeared more than 100 times less luminous than it should have because of the dust around it. 

This discovery may help solve a long-standing mystery: Astronomers have known many massive red supergiants should explode, but far fewer have been spotted doing so. The new finding suggests maybe they've been there all along, just hidden behind heavy dust clouds that blocked their light from previous telescopes. 

The James Webb Space Telescope has identified its first progenitor star of a recently discovered supernova explosion. Credit: NASA GSFC / CIL / Adriana Manrique Gutierrez illustration

"I’ve been arguing in favor of that interpretation, but even I didn’t expect to see such an extreme example as SN2025pht," said Kilpatrick, first author on the research published in The Astrophysical Journal Letters. "It would explain why these more massive supergiants are missing because they tend to be dustier."

The dust was keeping secrets about the star's chemistry, too. Most red supergiants are surrounded by dust made from oxygen-rich minerals, but this one’s dust was mostly graphite, a carbon-based chemical more often associated with other types of stars. That may mean some red supergiants go through unexpected changes late in life, perhaps shedding material or mixing elements in ways that alter the grains they produced.

Scientists look forward to using Webb to study more dusty environments around dying stars. Future telescopes, such as NASA’s upcoming Nancy Grace Roman Space Telescope, will help by monitoring dusty red supergiants and, with any luck, catching them in their final death throes.

"With the launch of JWST and upcoming Roman launch, this is an exciting time to study massive stars and supernova progenitors," Kilpatrick said. "The quality of data and new findings we will make will exceed anything observed in the past 30 years."