The energy source of PMS objects is gravitational contraction, as opposed to hydrogen burning in main - sequence stars. In the Hertzsprung -- Russell diagram, pre-main - sequence stars with more than 0.5 M first move vertically downward along Hayashi tracks, then leftward and horizontally along Henyey tracks, until they finally halt at the main sequence. Pre-main - sequence stars with less than 0.5 M contract vertically along the Hayashi track for their entire evolution.
in the pre-main sequence stage protostars shine by
gravitational contraction