Burning regimes in the T − ρ plane according to Woosley et al. (1973). Below a certain temperature the burning of silicon is incomplete. At high temperatures the state of nuclear statistical equilibrium is achieved and silicon is transformed into iron peak elements. This region is subdivided in the regime of normal freeze-out and α-rich freeze-out. The shaded area covers values of χ between 1 and 10 according to Eqs. (3) and (4).