The Star Family
T Tauri stars represent a very early stage in the history of a low-mass star, when the protostar (S 1) emerges from the surrounding molecular cloud (S 25), but before sustained core hydrogen burning on the main sequence of the Hertzsprung-Russell diagram (S 4). The energy is derived from gravitational collapse (as well as some deuterium burning beginning at about one million degrees Kelvin), since the star is not yet hot enough for the proton-proton reaction converting hydrogen to helium, for which a minimum temperature of four million K is required. T Tauri stars are therefore termed “pre-main sequence” objects, and are always young (one to ten million years) and less than two solar masses. Higher mass pre-main sequence stars are known as Herbig Ae/Be stars (S 3).