(Note that the above equation does not even include reactions such as X + p  ®  Y + n, X + n  ®  Y + a, etc.) Much of the work in setting up the network of equations is in deciding which reactions are important, and which can be neglected.)

At oxygen burning temperatures, nuclei such as §32, ³¹ P, and ³⁰ Si are all destroyed in favor of ²⁸ Si, which is the most tightly bound nucleus in the intermediate mass range. Once the temperature becomes high enough to photodisintegrate ²⁸ Si (T₉   ~  3), a host of (a,g), (p,g), (n,g) reactions (and their inverses) begin simultaneously. This is the Silicon Burning phase. Some of the fastest reactions run to equilibrium, but most reactions will not reach this stage. Thus, the results of silicon burning depend on how far towards completion the reactions run. During this time, there is also a slow leakage of nuclei from the intermediate-mass region to the iron group (which are the most tightly bound nuclei).

As silicon burning nears completion, the nuclei come closer and closer to establishing nuclear statistical equilibrium. In this case, the nuclear Saha equation defines the abundance of each species. This has a very simple form. Consider that, from the Saha equation, the abundance ratio of species \Am1,Z to A,Z is

Unfortunately, the analysis above is not completely accurate, since it fails to account for reactions that involve the weak nuclear force, i.e., beta decays. Since the neutrinos from these decays leave the star, there is no inverse reaction associated with beta-decays, and no equilibrium condition. To first order, however, the effect of beta-decays can be modeled through their impact on [`Z] / [`N]. Thus,

The question of beta-decays and the value of [`Z] / [`N] has important consequences for supernova calculations and for cosmic abundance determinations. For example, consider silicon burning, where ²⁸ Si is converted to iron peak elements. Let's examine the case where the two dominant resultant species are ⁵⁴ Fe and ⁵⁶ Ni. These nuclei are in statistical equilibrium via the reactions ® ¬

This process of nucleosynthesis is call the e-process, for equilibrium process.

In general, the peak iron element will have approximately the same proton to neutron ratio as given by [`Z] / [`N]. Thus,

In nature, the most abundant isotope in the iron group is ⁵⁶ Fe, with 26 protons and 30 neutrons. This suggests that the e-process usually either occurs very rapidly at low temperatures, so that ⁵⁶ Fe is produced via the decay of ⁵⁶ Ni, or proceeds very slowly so that beta-decays make [`Z] / [`N] » 0.87.