High Quality Content by WIKIPEDIA articles! The energy source of these objects is gravitational contraction (as opposed to hydrogen burning in main sequence stars). On the HR diagram, the pre-main sequence stage of stars with masses>0.5 Solar mass translates into a move along Hayashi tracks (almost vertically down) and later along Henyey tracks (almost horizontally to the left, towards the main sequence). PMS stars can be differentiated from main sequence dwarf stars by using stellar spectra to measure the correlation between gravity and temperature. A PMS star will have a larger radius than a main sequence star, and thus be less dense and have lower surface gravity.

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