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Volume 644 Issue 8077, 21 August 2025
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Volume 644 Issue 8077, 21 August 2025

Inside out

Massive stars are predicted to have a well-defined, layered structure composed of the progressively heavier elements formed by the fusion reactions in their core. This begins with lighter elements such as helium in the outer layer, progressing through carbon, oxygen, sulfur and silicon to iron at the core. But observing these layers is nearly impossible because the only time a star’s core is exposed is when it explodes as a supernova, which tends to result in the layers being mixed. In this week’s issue, Steve Schulze and colleagues present observations of supernova 2021yfj that capture signals from the star’s inner-most layer. The researchers observed a thick shell, rich in silicon and sulfur, which formed deep in the star’s interior and was expelled just before the explosion. Although this observation might serve to affirm predictions about the star’s structure, it also raises questions about how a massive star can be stripped of its outer layers so readily to expose its very inner secrets. The moment is captured in the artist’s impression on the cover, which illustrates the massive star that formed supernova 2021yfj shortly before it exploded, stripped of its outer layers to reveal the inner shell rich in silicon (grey), sulfur (yellow) and argon (purple).

Cover image: W. M. Keck Observatory/Adam Makarenko

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