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.
Electron capture always results in an emitted Neutrino,
a reason that supernovae produce neutrinos.