A protostar is a massive object that forms from the contraction of gas from a giant molecular cloud. This protostellar phase is the early stage of star formation. For a protostar with the mass of the Sun it will last about 100,000 years. It starts with a core of increased density in a molecular cloud and ends with the formation of a T Tauri star, which then develops into a main sequence star. This is heralded by the T Tauri wind, a type of super solar wind that marks the change from the star accreting mass into radiating energy. A T Tauri star is similar to those of main sequence stars of the same mass, but they're significantly more luminous because their radii are larger. See related links for more information.