As a star forms, the in falling gas gives up its kinetic energy by colliding with its neighbors. This heats up the gas in the core of the forming star. Once the temperatures reach about 10 million degrees, thermonuclear fusion between hydrogen atoms (actually protons) can start. Believe it or not, a particular proton will wait millions of years before it experiences a single fusion event to become a deuterium nucleus. The 'hydrogen-fusion reaction' releases enough heat energy (pressure) to keep the star from collapsing further, and as a byproduct a lot of light is also produced. The light, in the form of powerful gamma rays, makes its way to the star's surface, heating the gas, and producing a brilliant surface. The Sun emits nearly 400 trillion trillion watts of power. To do this, the fusion reactions have to convert 15 billion tons of matter into pure energy every hour. It's been doing this for about 4.5 billion years, and has lost only 0.02 % of its total mass. When a star gets to be as small as 80 times the mass of Jupiter, it can no longer sustain hydrogen fusion in its core, and objects at about 20 times the mass of Jupiter cannot even sustain deuterium fusion either. Objects between 20 and 80 Jupiter masses are called Brown Dwarfs.
