Neutrinos produced in nuclear reactions inside the Sun. Neutrinos are produced as well in laboratory nuclear reactions. The first direct tests of how the Sun produces its luminosity (observed most conspicuously on Earth as sunlight) have been carried out by observing solar neutrinos. The results of these experiments confirm the theory of how the Sun shines and stars evolve. Moreover, the results show that neutrinos behave differently than is predicted by the standard model of particle physics.