pre-main-sequence star

A star in the process of formation, which has developed beyond the protostar stage but has not reached the point where the steady fusion of hydrogen into helium in its core places it on the main sequence of the Hertzsprung-Russell diagram. A protostar becomes a pre-main-sequence star when its core becomes hot and dense enough to produce conditions of hydrostatic equilibrium. At the this stage, the main mechanism by which energy is transported in the stellar interior changes from radiation to convection. The chief source of energy is gravitational collapse, until the temperature and density are high enough for hydrogen fusion to dominate, at which point contraction stops. Stars in the process of forming are surrounded by thick shells of obscuring dust and can be detected only by their infrared or millimetre-wave emission. Eventually, the dust is blown away by stellar winds or radiation pressure, to be destroyed, reincorporated in the interstellar medium or form planetary systems.