The detection of neutrinos produced by the Sun’s secondary solar fusion cycle is reported in Nature this week. Measurements of these neutrinos could provide new insights into the structure of the Sun and the abundance of elements within its core.
Stars are fuelled by the nuclear fusion of hydrogen into helium, which occurs via two processes: the proton–proton (pp) chain, involving only isotopes of hydrogen and helium, and the carbon–nitrogen–oxygen (CNO) cycle, in which fusion is catalysed by carbon, nitrogen and oxygen. The pp chain dominates energy production in stars similar in size to our Sun, producing around 99% of the energy, and has been studied extensively. Studying the CNO cycle has been more challenging, as the neutrinos produced by this mechanism exceed background signals by only a few counts per day.
The Borexino Collaboration report the detection, with high statistical significance, of neutrinos emitted during the CNO fusion cycle within the Sun. This is achieved using the highly sensitive Borexino detector at the Gran Sasso Laboratory, Italy, which is capable of blocking out or accounting for most sources of background noise. These results represent the first known direct experimental evidence for the CNO cycle, the authors note, and demonstrate that the CNO cycle contributes around 1% of the Sun’s energy (as theory has predicted).
Measurements of neutrinos produced by CNO fusion can be used to determine the abundance of carbon, nitrogen and oxygen in stars, the authors propose. The CNO cycle is thought to have a larger contribution to energy production in stars more massive than our Sun. Knowing the abundance of elements heavier than helium in stars (known as the metallicity) can help us to understand the dominant power source for different stars. In an accompanying News & Views, Gabriel Orebi Gann suggests that the Borexino Collaboration’s achievement moves us closer to a complete understanding of our Sun and of the formation of massive stars, and is likely to define the goals in this field for years to come.
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