A hydrostatic sphere of gas in the process of becoming a star. A protostar forms by the gravitational collapse of a dense core within a giant cloud of dust and molecular gas (mostly H₂). As the core collapses from the inside out, it surrounds a central protostar with a cocoon of accreting dust and gas that hides it from view at optical wavelengths. Observations at longer wavelengths penetrate this material and reveal that the protostar is radiating due to the impact of infalling gas and dust. Most of the envelope material accretes first onto a circumstellar disk, from which it is then conveyed to the protostellar surface. As the envelope dissipates around a low-mass star such as the Sun, the protostar becomes visible as a pre-main-sequence T Tauri star that continues for a time to add to its mass from the accretion disk. It becomes a full-fledged star when the core temperature reaches the level required for nuclear fusion of hydrogen (temperature T ~ 10⁷ K). For stars at least 10 times more massive than the Sun, nuclear burning begins before infalling material is dissipated, and the resulting high-intensity radiation quickly clears away the remaining envelope.