By Aldo Serenelli
Energy is generated in the interior of the Sun through sequences of nuclear reactions in which four protons fuse together to form a helium-4 nucleus. These sequences are accompanied by the release of two particles known as electron neutrinos. Models suggest that 99% of the nuclear energy released by the Sun originates from three reaction sequences — collectively known as the proton–proton (pp) chain — that are initiated by the fusion of two protons. In a paper in Nature, the Borexino Collaboration1 reports the first complete measurement of neutrino fluxes that originate from these three sequences, based on an analysis of more than 2,000 days of data collection. The results help us to understand the details of how and why the Sun shines.
Neutrinos interact weakly with matter, and therefore escape almost unhindered from the Sun’s interior, to reach Earth about eight minutes later. Solar neutrinos therefore provide a direct view into the nuclear furnace in the Sun’s core. The Borexino experiment (Fig. 1) detects such neutrinos and determines how much energy they have by measuring the amount of light produced when the particles interact with the detecting agent (an organic liquid, called the scintillator, which is kept underground to minimize the amount of background radiation that can interfere with the neutrino signals). In contrast to all other solar-neutrino experiments, Borexino can measure the energies of both high- and low-energy neutrinos, which makes it possible to study the structure of the solar core using a technique known as neutrino spectroscopy.
Electron neutrinos can change into two other types (or flavours) of neutrino, known as tau and muon neutrinos, as they travel to Earth, a phenomenon known as flavour oscillation. The Borexino experiment is more sensitive to electron neutrinos than to tau or muon neutrinos, and so flavour oscillation needs to be accounted for when the measured neutrino fluxes are used to calculate the fluxes produced in the Sun. Taking this into consideration, the Borexino collaborators used the measured neutrino flux to work out the total power generated by nuclear reactions in the Sun’s core, with an uncertainty of about 10%, and found that this is the same as the measured photon output — thus showing that nuclear fusion is indeed the source of energy in the Sun. This value, calculated for the amount of energy produced through nuclear reactions, is comparable with previous2 results obtained by combining data from several neutrino-detection experiments, and places the most robust and model-independent constraints on the source of solar energy.
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