onion-shell burning

Slice an onion and you find layer inside layer, each one distinct. A massive star near the end of its life has exactly this structure, but each layer is a different nuclear furnace burning a different fuel. From the surface inward the star burns hydrogen, then helium, then carbon, then heavier elements, each in its own shell, with the hottest, densest, most advanced burning at the very center. This nested arrangement is onion-shell burning, and it is the dramatic final architecture of a star many times the Sun's mass.

It builds up step by step. When a massive star exhausts the hydrogen in its core, the core contracts and heats until it can fuse helium; when the helium runs out, it contracts and heats again to fuse carbon, then neon, then oxygen, then silicon. Each new fuel ignites at a higher temperature in a smaller, denser region, while the previous fuels keep burning in shells farther out. The pace accelerates frighteningly: a 20-solar-mass star burns hydrogen for millions of years but silicon for only about a day, because the later fuels release far less energy and the core is leaking it away furiously.

Onion-shell burning ends at iron, and that ending is fatal. Iron is the most tightly bound nucleus, so fusing it neither releases energy nor can be coaxed to — it is nuclear ash. Once the core is iron, the furnace at the center goes silent; there is no new fuel to ignite, no fresh energy to hold the core up against gravity. The iron core grows until it can no longer support itself and collapses in a fraction of a second, triggering a core-collapse supernova. The onion structure is thus the unmistakable signature of a massive star about to explode.