Caption: "This diagram shows a simplified, not-to-scale cross section of the CORE a massive, evolved post-main-sequence star (with stellar mass >∼ 8 M_☉). There is usually a much, much larger extended non-nuclearly burning hydrogen (H,Z=1)) concentric envelope (with typically cosmic composition (meaning inside modern galaxies: fiducial values by mass fraction: 0.73 H, 0.25 He-4, ∼ 0.02 metals)) which is NOT shown. Note, the envelope can be largely helium (He,Z=2) in some cases. (Somewhat edited.)

Features:

1. In the CORE, where the pressure and temperature so determine, there are concentric shells of hydrogen (H,Z=1), helium (He,Z=2), carbon (C,Z=6), neon (Ne,Z=10), magnesium (Mg,Z=12), oxygen (O,Z=8), and silicon (Si,Z=14) plasma that are undergoing nuclearly burning.

2. The star has developed the shown onion-layered structure as the resulting nuclear burning products settle on the next lower layer, building up the shell below.

3. As a result of silicon burning, a non-nuclear-burning plasma iron core builds up at the center. nuclearly burning

4. Once the iron core reaches about the Chandrasekhar mass ∼ 1.4 M_☉ (see Wikipedia: 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 (of the Fermi gas (AKA degenerate) of electrons) combine with the protons in the iron nuclei to create neutrons and the degenerate electron gas pressure vanishes causing the collapse. (Note, under ordinary density conditions, quantum mechanics forbids electrons and protons to simply combine and mutually neutralize.) 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.)

5. EOF