A pre-main-sequence star is a young star that is in the process of forming and has not yet reached the stable state of hydrogen fusion that characterizes main-sequence stars. This phase occurs after a star has formed from a collapsing cloud of gas and dust (a protostar) but before it begins hydrogen burning in its core. During the pre-main-sequence stage, the star is typically still gaining mass as material from the surrounding accretion disk falls onto it.
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A pre-main-sequence star contracts toward the main sequence before sustained core hydrogen burning supplies its luminosity. Convection, accretion, and deuterium burning shape its track across the Hertzsprung-Russell diagram.