The light that stars emit is always the result of nuclear fusion, the process by which two or more atomic nuclei are fused together under extreme high-energy conditions to produce a single, larger nucleus. In the fusion reaction that takes place most often within stars, two hydrogen nuclei are joined together to form a single helium nucleus. In the process, energy is released, which the star emits as waves of light across the entire electromagnetic spectrum.
The more massive a star, the greater its gravitational pull - and this in turn, affects the rare at which it consumes its nuclear fuel. A super-massive star, by virtue of its enormous gravitational pull, experiences such high-energy conditions in its core that it consumes its nuclear fuel quite rapidly. A medium-sized star, such as our own Sun, might take ten billion years to fuse all of the hydrogen in its core, but a super-massive star (between 40 and 120 times the size of the Sun) might run out of nuclear fuel in just ten million years.
Stars are primarily classified by spectral type (a measure of their temperature), which is determined by the visible light they emit. The most intense light emitted by the hottest stars tends toward the blue end of the visible spectrum, while the light reaching us from cooler stars tends toward the red end. Stars in the middle of the range, such as the Sun, emit a yellow light. Arranged from hottest to coolest, the spectral types are O, B, A, F, G, K, and M. (A convenient mnemonic is "Oh, be a fine girl, kiss me!") The Sun is a G-class star. (‘The Bedside Baccalaureate’, edited by David Rubel)
