A group of nuclear reactions that involve the interaction of protons (nuclei of hydrogen atoms, designated by ¹H) with carbon, nitrogen, and oxygen nuclei. The cycle involving only isotopes of carbon and nitrogen is well known as the carbon-nitrogen (CN) cycle. These cycles are thought to be the main source of energy in main-sequence stars with mass 40% or more in excess of that of the Sun. Completion of any one of the cycles results in consumption of four protons (4 ¹H) and the production of a helium (⁴He) nucleus plus two positrons (e⁺) and two neutrinos (ν). The two positrons are annihilated with two electrons (e^-), and the total energy release is 26.73 MeV. Approximately 1.7 MeV is released as neutrino energy and is not available as thermal energy in the star. The energy E = 26.73 MeV arises from the mass difference between four hydrogen atoms and the helium atom, and is calculated from the Einstein mass-energy equation E = Δmc², where Δm is the mass difference and c² is the square of the velocity of light. Completion of a chain can be thought of as conversion of four hydrogen atoms into a helium atom. Because the nuclear fuel that is consumed in these processes is hydrogen, they are referred to as hydrogen-burning processes by means of the carbon-nitrogen-oxygen (CNO) cycles.