Stellar evolution, the theory of how stars evolve, relies on observations of many stars of different masses, colors, ages, and chemical composition. The energy of stars is provided by nuclear fusion reactions in their core, and their evolution is strongly dependent upon their mass. The Sun, through the Standard Solar Model, is the only star for which the stellar evolution theory can be deeply tested through neutrinos emitted from various thermonuclear processes. The experimental study of solar neutrinos has made a fundamental contribution both to astroparticle and to elementary particle physics, offering an ideal test of solar models and providing, at the same time, fundamental indications concerning the physics of the neutrino sector. The solar neutrino experiments (with atmospheric neutrinos) have given compelling evidence for the existence of neutrino oscillations caused by nonzero neutrino masses and neutrino mixing. This has a huge impact on particle physics. It also has consequences on the prediction of the neutrino flavor composition from high-energy neutrino sources. Neutrinos do not only play a key role during the life of a star. When a massive star has exhausted its hydrogen, it evolves by producing energy through the fusion of heavier elements up to iron. Neutrinos produced during such reactions escape unimpeded from the stellar material and more and more intense nuclear burning is needed to replace the huge amount of energy carried away. Once the inner region of a star becomes primarily iron, further compression of the core no longer ignites nuclear fusion; the star collapses to form a compact object such as a neutron star or a black hole. A prominent prediction from theoretical models of the core-collapse of a massive star is that 99% of the gravitational binding energy of the resulting remnant is converted to neutrinos with energies of a few tens of MeV over a timescale of 10 s. Neutrinos were observed from the celebrated 1987A supernova in the Large Magellanic Cloud, the first event of multimessenger astrophysics.
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