The sun generates its energy through the fusion of hydrogen to helium. This occurs in two ways: the majority of the energy, approximately 99 percent, comes from a process of fusion and decay that begins with two hydrogen nuclei and ends with one helium nucleus. The process is referred to as the PP (proton-proton) chain.
The rest of the energy is a result of a cycle in which four hydrogen nuclei finally combine to form a helium nucleus containing carbon, nitrogen and oxygen as catalysts and intermediate products. In stars larger than our sun the majority of energy generated is generated by the second process, referred to as the CNO process because of the involvement of carbon, nitrogen and oxygen.
The certificate for the fusion cycle posted in the 1930s
The second cycle was postulated as another source of solar energy in the 1930s by physicists Hans Bethe and Carl Friedrich von Weizsäcker independently of each other, but could not be experimentally confirmed until now.
Physicists working on the Borexino experiment in the underground laboratory deep below the Italian Gran Sasso Massif have now succeeded for the first time in proving the presence of this cycle based on the neutrinos it produces.
With several years ago, the Borexino experiment team for the first time presented an overall investigation of the fusion processes of the PP chain using its neutrinos. Scientists from the Department of Physics at the Technical University of Munich (TUM) are centrally involved in both measurement processes.
Interference obscured the signal until now
Because of their energy distribution, the neutrinos of the CNO cycle are difficult to distinguish from those generated by the radioactive decay of tiny traces of other elements. Mainly bismuth-210 of trace impurities on the surface of the detector wall are responsible for hiding the signals from the CNO cycle.
Due to convection movements, the contaminants got into the detector liquid. In order to eliminate the disturbance, the convection had to be brought into the Berexina detector, which was technically completely worked out.
“For a long time I thought it would never be possible to successfully make this measurement,” says Stefan Schönert, professor of experimental astroparticle physics at Tu Munich. “But six years of hard work have paid off and now we have proven the presence of the CNO neutrino signal for the first time.”
New evidence on the metallicity of the sun
The results confirm not only the theoretical predictions on the two fusion processes of the sun, but also evidence about the metallicity of the sun, i.e. the concentration levels of nuclei that are heavier than hydrogen and helium.
Different astrophysical investigative methods have generated different results in recent years. “The new Borexino results now support observations with higher metallicity values,” says Professor Lothar Oberauer of TUM.
This is particularly important in the context of the fundamental properties of stars such as their size, temperature, brightness and life, which are determined by the degree of metallicity. Understanding the chemical composition of the sun is therefore essential to understand the properties of all stars.