Sun model completely confirmed for the first time

This second cycle was postulated as another source of the sun’s energy in the 1930s by physicists Hans Bethe and Carl Friedrich von Weizsäcker independently of one another, 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.

Several years ago the Borexino Experiment team presented for the first time an overall investigation of the fusion processes of the pp chain using its neutrinos. Scientists from the physics department of the Technical University of Munich (TUM) were centrally involved both measurement processes.

Because of their energy distribution, the neutrinos of the CNO cycle were difficult to distinguish from those generated by the radioactive decay of tiny traces of other elements. Primarily bismuth-210 from trace impurities on the surface of the detector wall were responsible for concealing the signals of the CNO cycle.

Due to convection movements, these contaminants got into the detector liquid. In order to eliminate the disturbance, the convection inside the Borexino detector had to be brought to a standstill, which was technically extremely elaborate.

“For a long time I thought it would never be possible to successfully make this measurement,” says Stefan Schönert, Professor for Experimental Astroparticle Physics at TU Munich. “But six years of hard work have paid off and now we’ve proven the presence of the CNO neutrino signal for the first time.”

These results confirm not only the theoretical predictions on the sun’s two fusion processes, they also provide evidence regarding the metallicity of the sun, i.e. the concentration levels of nuclei which are heavier than hydrogen and helium.

Different astrophysical investigative methods have generated differing results in past years. “The new Borexino results now support observations with higher metallicity values,” says Prof. Lothar Oberauer of TUM.

This is particularly important in the context of the fundamental properties of stars such as their size, temperature, brightness and lifetime, which are determined by the degree of metallicity. Understanding the chemical composition of the sun is therefore essential to understanding the properties of all stars.