Cno Cycle

Cno Cycle

The CNO cycle (Carbon Nitrogen Oxygen cycle) is a process carried out in the cores of stars in which four hydrogen nuclei are fused into one helium nucleus with the production of nitrogen and oxygen as intermediate products of the reaction. 12 C acts as an initiating catalyst for the reaction. Within a star, the CNO cycle becomes the dominant energy source once the temperature of the core exceeds about 16 million degrees Kelvin and is the main source of energy for stars with masses greater than about 1.1 solar masses where such temperatures are common. Stars the size of the Sun or smaller are unable to generate the core temperatures required to drive the CNO cycle and at these lower temperatures the proton-proton (p-p) fusion chain predominates. In the Sun, about 10% of the energy comes from the CNO cycle. In both processes 0.7% of the total mass is converted into energy, with a yield of 4.1 x 10-12 ergs per each helium nucleus created. The CNO cycle produces slightly less energy that can be used by the star than the p-p chain, since it produces more neutrinos. Neutrinos interact only extremely weakly with matter, and essentially all escape the stellar interior without causing any heating.

The rate at which the CNO cycle progresses is extremely temperature dependent. A temperature increase of 1% in the Sun's core would increase the p-p chain rate by 46%, but the CNO cycle rate would increase at 350%. This temperature sensitivity greatly changes the structure of the star's interior and affects the star's later evolution. In massive stars, 50% of energy production takes place within the innermost (by mass) 2% of the star, as opposed to solar mass stars, in which 50% of energy is produced in the innermost 10% of the star. As a result of this huge energy production within a relatively small volume, the energy is not able to escape by radiating away, and convection currents begin throughout the stellar interior. As a result, material in the core is mixed with the outer regions of the star, and a degenerate helium core never forms. In late stellar evolution helium fusion begins without the helium flash seen in low mass stars. Mixing also results in core material enriched by fusion with carbon, nitrogen, and oxygen being brought to the surface of the star. In medium mass stars this material is expelled as a planetary nebula where the elemental abundances can be measured. Thus, CNO enrichment serves as a benchmark for theories of stellar evolution.

there are six fundamental steps in the CNO cycle. In the initiating step in the CNO cycle, a 12 C captures a proton and becomes a radioactive 13 N nucleus, with the emission of gamma rays. The second step in the cycle takes place, with the 13 N atom emission of a positron and a neutrino during a transformation to the 13 C isotope of carbon. The 13 C atom then captures a proton, becoming an 14 N atom. The fourth step is the bottleneck of the CNO cycle and ultimately determines the rate of the reaction process. The likelihood of an 14 N atom for capturing a proton (termed the cross section) is rather low in solar mass stars. As a result, the CNO causes significant enrichment in 14 N in the star. Once the proton capture occurs, however, the resulting 15 O atom quickly decays into 15 N. In the final step, 15 N captures a proton, and decays into 12 C and an alpha particle (a helium nucleus). The net effect is to combine four protons into a helium nucleus, with carbon acting as a catalyst. In all of the CNO reaction steps there is a release of radiation and/or position and neutrino emissions

In the last step, one time in 2500 the 15 N captures another proton before decaying to become 16 O. Eventually, however, a 14 N nucleus and an alpha particle are produced. In this branch of the cycle, the net effect is to turn six protons and a 12 C nucleus into an alpha particle and an 14 N nucleus, with an enrichment in fluorine also a consequence.