A star behind the galaxy cluster MACS J0308.9+2645 ran out of fuel, collapsed, and threw most of itself into space. The light from that explosion was detected in James Webb Space Telescope images taken on August 23, 2025 — a faint spot hidden behind a bright foreground star.
The unusual part: that flash was only the first. The same explosion is expected to appear twice more. And the astronomers who catch it will not be the ones who found it.
The setup
The cluster's gravity splits the light of the galaxy behind it into three separate images, scattered across the sky. Each image is light that took a different route through the cluster. The routes are not the same length and pass through different gravitational fields, so the three images do not arrive at the same time.
Connor Larison, a researcher at the Space Telescope Science Institute, and colleagues published the first full analysis on October 23. It is a preprint submitted to *The Astrophysical Journal*, not yet peer-reviewed. They named it SN Ares. It was discovered under the VENUS project, which uses Webb to study rich galaxy clusters, and announced on January 7.
Three models, three answers
To determine when each image appears, you need a map of the cluster's mass. The team independently built three models using three different software packages: Lenstool, glafic, and AstroLensPy. Each was fitted to dozens of background galaxies that the cluster shows more than once.
All three models agree that the already-visible image appears first. The order of appearance is determined by the geometry of the system, not by any particular model.
The waiting time is where they diverge. Lenstool indicates a second appearance 70 years after the first. glafic says 72. AstroLensPy says 76. Each carries an error of a year or two. Those formal errors are smaller than the differences between the models, which suggests mass-model uncertainty matters more than numerical precision of any single fit.
The last image appears several years after the second in all three models. All assume the same expansion rate of the Universe. The paper notes that changing that rate by 3 km/s/Mpc shifts the delays by about five years. The forecast states a minimum date, not a specific one: at least about 70 years.
The record, by a wide margin
This is by far the largest predicted delay for any lensed supernova discovered. Most repeat within days or years. The previous record holder, supernova Requiem, will get its next image about two decades after the first. Ares's predicted minimum delay is roughly three and a half times larger.
The gap between images depends on the scale of the Universe, so a measured delay constrains the expansion rate — the quantity where the two best measurement methods still disagree. If the delay could be measured precisely, it could help constrain that rate. That is why the authors describe Ares as a chance for a high-precision cosmological measurement.
The spectrum
Webb's NIRSpec instrument took the spectrum on September 23, 2025, a month after the discovery images. It shows helium in two strong lines along with hydrogen. That combination is a signature of a Type IIb supernova — a massive star stripped of most of its hydrogen envelope, but not all of it.
The authors call SN Ares the most distant Type IIb supernova identified by spectrum — a claim they describe narrowly: this class and identification by spectrum. The redshift is 1.276. It comes from emission lines of the host galaxy, not from the supernova's own light.
What cannot be checked yet
The number everything rests on cannot currently be verified independently. The lensing methods underlying all three models are described in papers the collaboration has not yet published. No one outside it can reproduce the prediction. The authors expect it to change: Webb plans multi-object spectroscopy of the cluster in early 2027, measuring more redshifts across the field and potentially narrowing the predicted delays.
Catching the return is someone else's problem. "Such a long delay between images of a strongly lensed supernova has never been observed before and could be a chance for a predictive experiment that would allow incredibly precise constraints on cosmological evolution," Larison said in the January 7 discovery announcement.
A monitoring campaign of that scale requires a telescope no one has built yet. The paper notes that even the Habitable Worlds Observatory, NASA's next flagship, may need in-orbit servicing to last long enough.
