Introduction
The Sun is a natural nuclear fusion reactor, powered by a proton–proton chain reaction which converts four hydrogen nuclei (protons) into helium, neutrinos, positrons and energy. The excess energy is released as gamma rays and as kinetic energy of the particles and as neutrinos — which travel from the Sun's core to Earth without any appreciable absorption by the Sun's outer layers.
As neutrino detectors became sensitive enough to measure the flow of neutrinos from the Sun, it became clear that the number detected was lower than that predicted by models of the solar interior. In various experiments, the number of detected neutrinos was between one third and one half of the predicted number. This came to be known as the solar neutrino problem.
Read more about this topic: Solar Neutrino Problem
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