Electron capture is a nuclear process consisting of an electron combining with the nucleus, reducing the nucleus's positive charge by one, changing one proton to a neutron. It can happen spontaneously in certain radioactive elements, and also can happen to individual protons (i.e., hydrogen nuclei) if the electrons have sufficient kinetic energy to make up the difference between the proton's and the resulting neutron's mass. The latter can happen at extreme temperatures, e.g., the center of large stars, or during supernovae. It can (or perhaps always) is assisted by quantum tunneling, and under the right conditions, can capture an electron from an inner electron shell. Electron capture always results in an emitted neutrino, a mechanism by which supernovae produce neutrinos.