The CNO cycle (for carbon-nitrogen-oxygen), or sometimes Bethe-Weizsäcker-cycle, is one of two sets of fusion reactions by which stars convert hydrogen to helium, the other being the proton-proton chain. Theoretical models show that the CNO cycle is the dominant source of energy in stars more massive than than about 1.5 times the mass of the sun. The proton-proton chain is more important in stars the mass of the sun or less. This difference stems from temperature dependency differences between the two reactions; pp-chain reactions start occurring at temperatures around ×10 , making it the dominant force in smaller stars. The CNO chain starts occurring at approximately ×10 [citation needed], but its energy output rises much faster with increasing temperatures. At approximately ×10 , the CNO