2.3.4 Cosmic onion
Nuclear fusion can continue in a massive star, with succeeding generations of nuclei combining to produce heavier and heavier elements.
When a star’s core reaches a temperature of about 500 million K, carbon fuses to neon plus helium. Once the carbon runs out, the core contracts, raising the temperature to about 1,500 million K, and neon combines with leftover helium to form magnesium.
This process continues, until the core temperature reaches 7,000 million K, by which time the star is like a ‘cosmic onion’, with an iron core, surrounded by concentric shells of silicon and sulphur, magnesium and neon, oxygen and carbon, helium and an outer shell of hydrogen. Each layer is cooler than the one below.