Thus, the limiting reaction in the CNO cycle, proton capture by 7 N, has S (E 0) ~ S (0) = 3.5 keV·b, while the limiting reaction in the proton–proton chain reaction, the creation of deuterium from two protons, has a much lower S (E 0) ~ S (0) = 4×10 keV·b. Incidentally, since the former reaction has a much higher Gamow factor, and due to the relative abundance of elements in typical stars, the two reaction rates are equal at a temperature value that is within the core temperature ranges of main-sequence stars.