<P> 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 . </P>
<P> 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 . </P>
in the pre-main sequence stage protostars shine by
in the pre-main sequence stage protostars shine by
gravitational contraction
gravitational contraction