A chain of nuclear reactions in stars that converts hydrogen into helium, with the associated release of nuclear energy. Carbon, nitrogen, and oxygen act as catalysts to speed the six-stage reaction. It is also known variously as the carbon cycle, the carbon–nitrogen–oxygen cycle (CNO cycle), and the Bethe–Weizsäcker cycle. The cycle becomes increasingly important with rising temperature and dominates the proton–proton reaction at temperatures above 18 million K, as in the cores of stars of over 2 solar masses.