Protostar is an early stage in the evolution of a star that usually grows to the point of beginning nuclear fusion and becoming a star by gathering mass. It is made of a contracting cloud of cold and dark interstellar medium (mostly hydrogen gas). Protostars has lower temperature than an ordinary star. For a sun-like star, deuterium fusion occurs in the protostar stage, while proton-proton fusion, which sustains stars for billions of years, starts gradually millions of years later. The modern picture of protostars was first suggested by Chushiro Hayashi in 1966.