Weak neutrino spectrum depends on temperature, density and abundances of nuclei involved. The latter is large set of varying quantities and no simple description exist. Therefore, such a problem cannot be studied in general conditions. Particularly, abundances depend on history of the matter.

Well known exception is Nuclear Statistical Equilibrium (NSE), thermodynamic state defined using only three parameters: temperature, density and electron fraction. Electron fraction can be defined as number of electrons divided by number of nucleons (protons and neutrons). Using NSE approximation we are able to compute all abundances and add weighted contribution of all nuclei.

See viewgraphs for NSE , NSE neutrinos and related paper .