Electron fraction in stellar matter ($Y_e={\sum }_i{X}_i{Z}_i/A_i$)

For electrically neutral fully ionized matter, $Y_e$ is the number of electrons, or total protons, per baryon. With isotope mass fractions $X_i$, charge numbers $Z_i$ and mass numbers $A_i$, the displayed sum neglects small nuclear mass-defect corrections. Strong and electromagnetic reactions preserve it, while electron capture and beta-plus decay reduce it. The proton-to-neutron number ratio is $Y_e/(1-Y_e)$.

The neutron excess is the number of neutrons minus protons per baryon. A mixture of carbon-12 and oxygen-16 has zero neutron excess, while neon-22 contributes $2/22$ per baryon in that isotope. Thus a neon-22 mass fraction $X_{22}$ gives $\eta =X_{22}/11$. A nonzero excess shifts iron-group ashes from symmetric nickel-56 toward neutron-rich isotopes, even before additional weak reactions occur.