carbon cycle

carbon cycle

(Bethe–Weizsächer cycle) A chain of nuclear fusion reactions by which energy may be generated in stars. The overall effect of the cycle is the transformation of hydrogen nuclei into helium nuclei with emission of gamma-ray photons (γ), positrons (e⁺), and neutrinos (ν). The major sequence of reactions is as follows:

¹²C + ¹H → ¹³N + γ

¹³N → ¹³C + e⁺ + ν

¹H + ¹³C → ¹⁴N + γ

¹H + ¹⁴N → ¹⁵O + γ

¹⁵O → ¹⁵N + e⁺ + ν

¹H + ¹⁵N → ¹²C + ⁴He

The carbon nucleus, ¹²C, reappears at the end of the cycle and can be regarded as a catalyst for the reaction:

¹H → ⁴He + 2e⁺ + 2ν + 3γ

Because nitrogen (N) and oxygen (O) intermediates are involved, the cycle is often termed the carbon-nitrogen-oxygen cycle or CNO cycle.

The carbon cycle is very strongly temperature dependent and becomes the dominant energy-producing mechanism at core temperatures exceeding about 20 million K. It is therefore thought to be the major source of energy in hot massive stars of spectral types O, B, and A. The carbon cycle was proposed by Hans Bethe and independently by Carl von Weizsächer in 1938. See also proton-proton chain reaction.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006

carbon cycle

[′kär·bən ‚sī·kəl]

(geochemistry)

The cycle of carbon in the biosphere, in which plants convert carbon dioxide to organic compounds that are consumed by plants and animals, and the carbon is returned to the biosphere in the form of inorganic compounds by processes of respiration and decay.

(nuclear physics)

carbon-nitrogen cycle

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.