In this paper several physical processes are examined which contribute to the opacity of a protostar in the temperature range K. At low temperatures the opacity is produced by solid grains. Above 150 the volatile ices evaporate, but the residual refractory compounds in the grains still provide the major absorption. When the grains are completely evaporated (above 1300 ) only hydrogen molecules and H- contribute to the opacity. It is shown that solid carbon is unlikely to form, and that absorption by molecules, both in vibration-rotation bands and in the vibrational continuum, is probably unimportant. Application is made to star formation, neglecting angular momentum and magnetic field considerations. For protostars greater than one tenth of a solar mass the opacity never becomes high enough to raise the internal temperature sufficiently to establish hydrostatic equilibrium. Hence no llelmholtz contraction phase exists; such a protostar collapses in free fall all the way to stellar densities.