Caption: "This diagram shows a simplified, not-to-scale cross section of a massive, evolved post-main-sequence star (with stellar mass >∼ 8 M_☉). Where the pressure and temperature permit, concentric shells of hydrogen (H), helium (He), carbon (C), neon (Ne), magnesium (Mg), oxygen (O), silicon (Si) plasma are nuclearly burning deep inside the star below a massive concentric envelope non-nuclear-burning primordial hydrogen (H), helium (He), and metals with roughly solar composition though with large variation in the abundance of metals from roughly solar to 10**(-6) solar or less. The star has developed an onion-layered structure.
          The resulting nuclear burning products settle on the next lower layer, building up the shell below. As a result of silicon burning, a non-nuclear-burning plasma iron core builds up at the center. Once this iron core reaches about the Chandrasekhar mass ∼ 1.4 M_☉ (see Type II supernova: Core collapse), the iron core can NO longer sustain its own mass and it undergoes a collapse. What happens is that degenerate electrons combine with the protons in the iron nuclei to create neutrons and the degenerate electron gas pressure vanishes causing the collapse. The implosion turns into an explosion because the creation of the neutrons releases neutrinos (which in this dense environment do interact significantly with matter) which push outward the outer layers of the star. The explosion is a core collapse supernova." (Somewhat edited.)