A match flares from both ends. First the head ignites, and a moment later the flame engulfs the wooden stick. Something similar may happen in some Type Ia supernovae — except the match is a white dwarf (a star the size of Earth), and the flame is a thermonuclear explosion.

In 2022, astronomers observed supernova 2022an — a dim, rapidly fading flash similar to the 91bg subtype events. But in its spectrum, in the near-infrared, they spotted an unusual detail: a narrow absorption line at a wavelength of about 1.037 microns. It persisted from day 30 to day 90 after maximum brightness. This line is the "fingerprint" of helium atoms flying toward us at 13,000 kilometers per second. For comparison: at that speed, you could travel from Earth to the Moon in half a minute.

Where did this gas come from? In such systems, helium is usually "stripped" from the surface of a companion star. But then its velocity would be much lower, like a breeze, not a hurricane. Here, the speeds are enormous, and the helium line is narrow, as if the material were ejected in a single dense clump. So the helium is part of the explosion itself — the unburned remnants of the white dwarf's outer shell. This is the first direct evidence that 91bg supernovae can retain helium layers, as predicted by double detonation models: first, a thin helium shroud on the star explodes, and the shock wave ignites the carbon core.

The discovery exposes a gap in theory: current calculations of helium double detonations do not model spectra at such late times as 2022an. Observations have outpaced theory. But it is infrared spectroscopy, capable of peering through dust and gas, that has become the tool to "read" the composition of the ejected material. It seems that to understand how stars die, we need to look at them in light invisible to the eye.