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CNO: THE POWERHOUSE OF THE STARS
The sun and stars generate heat and light energy from nuclear reactions - the fusion of light atomic nuclei to heavier ones, with lost mass being converted to kinetic energy obeying Einstein's equation E = mc2. In the sun several reactions happen, but basically four protons combine to form one helium nucleus and two positrons that annihilate into gamma-rays.
In more massive stars with a higher central temperature a different process dominates called the CNO-cycle, using nuclei of carbon, nitrogen and oxygen as catalysts in a remarkable process first calculated by von Weizsäcker and Bethe in the 1930s. Those nuclei were not present after the Big Bang or in the first stars, which produced them and flung them into space in spectacular supernova explosions, to be condensed into new "second generation" stars.
The diagram shows the reactions in which carbon participates, is converted to nitrogen and then to oxygen and back to carbon in the process. The end result is again that four protons are brought together to form a helium nucleus. In the process two positrons are emitted when protons become neutrons, together with neutrinos that leave the star, and gamma rays that heat it up. It can take a million years for the whole cycle, slow because it is very difficult for a proton to find and penetrate a carbon nucleus - which has a high electric charge and so repels the proton. Both very high density and very high temperatures are needed.
Appreciate the beautiful symmetries in the diagram - the hexagon, the rectangle of the four H (hydrogen nuclei being protons), the triangle formed by the three nitrogen isotopes* 13N, 14N, and 15N, the inverted triangle of 12C, 13C, and 150, and the pair of radioactive decays on opposite corners emitting positrons and neutrinos. The products are positioned outside the hexagon. Isn't it interesting that carbon, the one element essential for life, is the one element essential for starlight? And carbon naturally forms hexagonal structures - graphite, buckyballs, etc. This diagram inspired Shanthi's piece.
*The superscripts are the total number of protons + neutrons in the nucleus, the mass number |