The Sun produces 4 x 10²⁶ joules per second of electromagnetic radiation, a fraction of which is intercepted by Earth. The source of this energy is a series of reactions that converts four protons into one helium nucleus plus 26.7 MeV of energy that appears as energy in the reaction products. Since 1 MeV is equivalent to 1.6x10^-13 J there must be

= 0.94 x 10³⁸ reactions/s

occurring in the sun to maintain its energy flow. The basic reaction chain (86% of the time) is the fusion sequence:

¹H + ¹H ƲH + e + + n e,

²H + ¹H ƳHe + g,

³He + ³He Æ⁴He + ¹H + ¹H.

These fusion reactions occur only at the center of the Sun where the high temperature (~10⁷ K) gives the hydrogen and helium isotopes enough kinetic energy to overcome the long-range repulsive Coulomb force and come within the short-range of the attractive strong nuclear force. The reaction energy slowly percolates to the surface of the Sun where it is radiated mainly in the visible region of the electromagnetic spectrum. Only the neutrinos escape from the Sun without giving up their energy.

A detailed mathematical model of the temperature and density profile of the Sun powered by nuclear reactions also serves as a model of other stars. Since we cannot observe the nuclear reactions directly for confirmation of the nuclear processes, astrophysicists look to the neutrinos produced in the fusion of two protons to form deuterium and in the less common (14%) branch of the reaction chain where the fusion of ³He with ⁴He leads to isotopes of beryllium and boron that emit neutrinos. Massive underground neutrino detectors have found fewer neutrinos than expected from the model calculations. One speculation on the missing neutrinos is that they convert from neutrinos associated with electron processes to those associated with muon or tau processes as they transit from the interior of the Sun to Earth. Such a conversion can only occur if at least one of the neutrino species has a non-zero mass. This is a topic of much current research interest.