Nuclear fusion makes the stars shine: during this process, atomic nuclei combine at enormously high temperatures inside the stars and hydrogen is converted to helium. In stars that are even more massive than the Sun, the so-called CNO cycle, a process consisting of several steps involving carbon (C), nitrogen (N) and oxygen (O), is thought to be primarily responsible for this. This process was first predicted by Hans Bethe and Carl Friedrich von Weizsäcker in the 1930s. However, the CNO cycle had never been observed directly until now. In 2020, the Borexino collaboration succeeded for the first time in detecting neutrinos from the Sun’s CNO cycle. The Borexino team includes the Jülich Nuclear Physics Institute’s research group led by Prof. Livia Ludhova, who was one of the coordinators of the CNO cycle analysis.

Neutrinos originate from our Sun as one of the products of nuclear fusion. Unlike light, however, which takes several hundreds of thousands of years to travel through the hot and dense stellar matter, neutrinos penetrate to the outside without hindrance. As they interact only weakly with matter, these particles are almost impossible to detect. Physicists are trying to do so with the help of the Borexino experiment, which is being conducted near Rome in the world’s largest underground experimental laboratory. “More than ten years ago, we first detected neutrinos from inside the Sun,” says Ludhova. Bor­exino can reliably determine not only the quantity of neutrinos, but also their energy. “With its energy, each neutrino carries with it a fingerprint of the reaction from which it originated,” Ludhova explains.

In 2020, the researchers were able to identify neutrinos from the CNO process in the Sun using data obtained from Borexino since July 2016. There, this process plays a minor role, because the sun, being a so-called low-mass star, only derives about one per cent of its energy from this process, in contrast to high-mass stars. Nevertheless, the discovery is a great scientific success as it proves the existence of the CNO cycle: the journal Physics World lists it among the ten most important breakthroughs in physics in 2020.

The detection of the CNO process in the Sun can reveal even more about the interior of the Sun, such as the proportion of elements that are heavier than hydrogen and helium. Livia Ludhova is confident: “Through the analyses of the measurements, we will improve our understanding of the Sun and also of other stars.”

Jens Kube