Carbon cycle (astronomy)

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In astronomy the carbon cycle (or Bethe-Weizsack cycle) is one of two primary fusion reactions where stars convert hydrogen to helium, with the foremost option the proton-proton chain reaction. Theoretical models show that the carbon cycle is the dominant source of energy in stars that is greater than the sun and that the proton-proton chain reaction dominates in stars of the size of the sun or less. The difference is from how the temperature-dependent reactions are: the proton-proton chain reactions occur at temperatures around 4 × 106 K, which is the dominant of smaller stars. The carbon cycle starts at around 13 × 106 K, but its efficiency rises much faster with increasing temperatures. At ~ 17 × 106 K, the carbon cycle begins to become the dominant energy source.

The sun has a temperature of around 15.7 × 106 K and only 1.7% of the helium-4 cores (alpha particles) formed in the sun come from the carbon cycle. The process was proposed by Carl von Weizsäcker and Hans Bethe independently of each other in 1938 and 1939, with the cycle as proposed by Bethe:

C12+H=N13, N13=C13+, C13+H=N14, N14+H=O15, O15=N15+, N15+H=C12 +He4[1]

The carbon cycle is also known by another name: the carbon-nitrogen-oxygen cycle or CNO cycle. As the cycle is run, they use four protons of carbon, nitrogen and oxygen isotopes as catalysts to form an alpha particle, two positrons and two neutrinos. The positrons will almost immediately be ejected with electrons, which liberates energy in the form of gamma rays. Neutrinos interact very poorly with matter and move freely out of the star and exert some energy. The carbon nitrogen and oxygen isotopes are essentially one and the same core that undergoes a series of transformations in a constantly repeating cycle.


  1. Bethe, H. A., 1939. Energy Production in Stars. Physical Review, 55, 434. Online