<P> The CNO cycle (for carbon--nitrogen--oxygen) is one of the two known sets of fusion reactions by which stars convert hydrogen to helium, the other being the proton--proton chain reaction . Unlike the latter, the CNO cycle is a catalytic cycle . It is dominant in stars that are more than 1.3 times as massive as the Sun . </P> <P> In the CNO cycle, four protons fuse, using carbon, nitrogen and oxygen isotopes as catalysts, to produce one alpha particle, two positrons and two electron neutrinos . Although there are various paths and catalysts involved in the CNO cycles, all these cycles have the same net result: </P> <Dl> <Dd> 4 1 + 2 e − → 4 2He + 2 e + + 2 e − + 2 ν e + 3 γ + 24.7 MeV → 4 2He + 2 ν e + 3 γ + 26.7 MeV </Dd> </Dl>

In the cno cycle which of the following acts as a catalyst