page_title: CNO cycle
section_title: CNO-IV
hierarchical_section_title: CNO cycle / Cold CNO cycles / CNO-IV
context_page_description: The CNO cycle 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 hypothesized to be dominant in stars that are more than 1.3 times as massive as the Sun.
  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:
  4 ¹
  ₁H
  + 2
  e⁻
  → ⁴
  ₂He
  + 2
  e⁺
  + 2
  e⁻
  + 2
  ν
  ₑ + 3
  γ
  + 24.7 MeV → ⁴
  ₂He
  + 2
  ν
  ₑ + 7
  γ
  + 26.7 MeV
  The positrons will almost instantly annihilate with electrons, releasing energy in the form of gamma rays. The neutrinos escape from the star carrying away some energy. One nucleus goes on to become carbon, nitrogen, and oxygen isotopes through a number of transformations in an endless loop.
  The proton–proton chain is more prominent in stars the mass of the Sun or less.
context_section_description: Like the CNO-III, this branch is also only significant in massive stars. The reactions are started when one of the reactions in CNO-III results in fluorine-19 and gamma instead of nitrogen-15 and alpha, and continues ¹⁸
  ₈O
  →¹⁹
  ₉F
  →¹⁶
  ₈O
  →¹⁷
  ₉F
  →¹⁷
  ₈O
  →¹⁸
  ₉F
  →¹⁸
  ₈O
  :
  In some instances ¹⁸
  ₉F
   can combine with a helium nucleus to start a sodium-neon cycle
caption: A proton reacts with a nucleus causing release of an alpha particle.