 Electron fraction in stellar matter

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 $Y_e/(1-Y_e)$.