The James Webb Space Telescope followed the same supernova from its peak brightness until the point when its inner layers became directly observable.
The object, named SN 2025rbs, is located about 14.5 megaparsecs away, or nearly 47 million light-years. It belongs to the Type Ia category. These explosions occur when a white dwarf, the very dense remnant of a star, undergoes an uncontrolled nuclear reaction that destroys it.
Artist's impression of a Type Ia supernova: in a binary system, a white dwarf explodes after accreting matter from its companion star.
Credit: University of Warwick/Mark Garlick
The researchers observed SN 2025rbs three times with James Webb: 1, 23, and 84 days after its visible-light peak. They combined these data with ground-based observations. Together, the observations cover wavelengths ranging from visible light to the mid-infrared, which is invisible to our eyes.
This coverage provides a particular advantage. At first, the ejected matter remains dense enough to conceal part of the interior. As it expands, it gradually becomes more transparent. Astronomers can then observe increasingly deeper regions.
Starting at peak brightness, James Webb obtained an unprecedented mid-infrared spectrum for a Type Ia supernova at this stage. A spectrum breaks light down according to its wavelengths. The resulting signatures make it possible to identify chemical elements and estimate the speeds of the matter containing them.
After 23 days, the mid-infrared already mainly revealed emissions produced by matter that had become more transparent. By the 84th day, this part of the spectrum had reached what astronomers call the nebular phase. Visible light and the near-infrared, however, still showed an ongoing transition.
The data also reveal a layered structure. Stable nickel appears mainly at the lowest velocities, meaning near the center of the explosion. Radioactive cobalt occupies intermediate velocities. Argon lies farther outward. This distribution preserves a trace of how the white dwarf exploded.
Another detail still challenges existing models. The calculations approximately reproduce a magnesium signature in the near-infrared, but strongly underestimate certain signatures of the same element farther into the infrared. Future simulations will need to explain this discrepancy to better connect the observed spectra with the physical conditions of the explosion.