hydrogen burning

In astronomy, any of several processes by which hydrogen is converted to helium by nuclear fusion in the core of a star. In the Sun, the main process is the proton-proton chain, while in heavier stars the carbon cycle is more important. In both processes, four protons are converted to a helium nucleus with the emission of positrons, neutrinos, and gamma rays. The temperature must exceed several million kelvins for hydrogen burning to start and the least massive stars (brown dwarfs) never become hot enough.