- noun the circulation of carbon, by which carbon atoms from carbon dioxide are incorporated into organic compounds in plants during photosynthesis. They are then oxidised into carbon dioxide during respiration by plants or herbivores which eat them and by carnivores which eat the herbivores, thus releasing carbon to go round the cycle again.
- Process occurring in the cores of hot stars whereby hydrogen is fused into helium via carbon – and not to be confused with the terrestrial carbon cycle found in ecology. In the astrophysical version, a hydrogen nucleus fuses with one of carbon as the start of a complex process whereby nitrogen and oxygen nuclei are also produced en route. The final products are a helium nucleus and a carbon nucleus, so that the carbon is a catalyst in the reaction and is left over for recycling in another fusion.