Stellar nucleosynthesis begins with hydrogen fusion ('burning') to produce helium-4 in the stellar core at a temperature of 10 Million Celsius, by way of the hydrogen-burning proton-proton (p-p) reaction and the CNO cycle (carbon nitrogen oxygen cycle), releasing a great deal of energy (resulting from the difference in binding energy between hydrogen and helium according to the formula E=mc2). The stars' lifetime depends critically upon its size (and hence attainable core temperature). The p-p reaction rate is proportional to T4, and that of the CNO cycle to T17. The star exists in quasi-equilibrium, balancing the inward pulling gravitation pressure against the outward pushing radiation pressure. This phase lasts about 1010 years for a star like our sun.

For large stars, when the helium fuel is exhausted, the radiation pressure decreases allowing the core to contract under gravity until its temperature reaches the fusion ignition temperature of helium, at 100M C, when the helium-4 is fused to produce carbon-12. The hotter core expands the outer layers of the star, which cool down. A quasi-equilibrium state then exists until the helium fuel is exhausted, and the core of the star again contracts until it reaches 300M degrees Celsius when carbon burning commences. The shell again expands and cools. This process continues through concentric shells of oxygen-16, neon-20, silicon-28, and sulphur-32 burning, each stage lasting a shorter and shorter time and yielding less and less energy. For each stage the core gets smaller and hotter whilst the shell gets bigger and cooler. The star is becoming a red giant. The process is halted when the core is turned into nickel-56, from which energy cannot be extracted by fusion. Fusion of nickel consumes energy and this signals the explosive demise of the star in a supernova, but only if the star was originally heavier than three solar masses.

The carbon-burning processes (>5×108K):

C-12 + C-12 Mg-24 +

C-12 + C-12 Na-23 + p

C-12 + C-12 Ne-20 + He-4

The oxygen-burning processes (>109K):

0-16 + O-16 S-32 +

0-16 + O-16 P-31 + p

0-16 + O-16 S-31 + n

0-16 + O-16 Si-28 + He-4

The silicon-burning processes (>2×109K):
Si-28 + 's 7 He-4
Si-28 + 7 He-4 Ni-56

A summary of the various stages (shells) of nuclear burning in heavy stars is shown below. Note that the starting fuel for each stage is supplied by and dependant upon the ashes from the previous burning stage. The burning temperature is shown for each stage, as well as the energy (in ergs per gram of fuel) that the reaction provides. Photons provide the cooling for the hydrogen and helium-burning stages, but neutrinos provide the cooling for all subsequent stages. Note that nickel-56 'burning' consumes energy. The very last stage represents the supernova stage.

Nuclear Fuel Temperature (K) Main Ashes Erg/gram of fuel Cooling
H-1 20 M He-4, N-14 5000-8000 × 1015 photons
He-4 200 M C-12, O-16, Ne-22 700 × 1015 photons
C-12 800 M Ne20, Mg-24, O-16, Na-23, Mg25, Mg-26 500 × 1015 neutrinos
Ne-20 1500 M O-16, Mg-24, Si-28 110 × 1015 neutrinos
O-16 2000 M S-28, S-32 500 × 1015 neutrinos
Si-28 3500 M Ni-56, A=56 approx nuclei 0 - 300 × 1015 neutrinos
Ni-56 6000 - 10,000 M n, He-4, H-1 -8000 × 1015 neutrinos
A=56 approx depends upon density photo-disintegration and neutronization
See Neutrinos.