Editing CNO cycle (section)

== Use in astronomy ==
While the total number of "catalytic" nuclei are conserved in the cycle, in [[stellar evolution]] the relative proportions of the nuclei are altered.  When the cycle is run to equilibrium, the ratio of the carbon-12/carbon-13 nuclei is driven to 3.5, and nitrogen-14 becomes the most numerous nucleus, regardless of initial composition.  During a star's evolution, convective mixing episodes moves material, within which the CNO cycle has operated, from the star's interior to the surface, altering the observed composition of the star. [[Red giant]] stars are observed to have lower carbon-12/carbon-13 and carbon-12/nitrogen-14 ratios than do [[main sequence]] stars, which is considered to be convincing evidence for the operation of the CNO cycle.<ref name="Mar98">{{cite journal |last1=Marks and Sarna |title=The chemical evolution of the secondary stars in close binaries, arising from common-envelope evolution and nova outbursts |journal=Monthly Notices of the Royal Astronomical Society |date=December 1998 |volume=301 |issue=3 |pages=699–720 |doi=10.1046/j.1365-8711.1998.02039.x |doi-access=free |bibcode=1998MNRAS.301..699M }}</ref>