Earlier in its life, the object is a protostar that grows by acquiring mass from its surrounding envelope of interstellar dust and gas.
The star continues to contract, its internal temperature rising until it begins hydrogen burning on the zero age main sequence.
The energy source of PMS objects is gravitational contraction, as opposed to hydrogen burning in main-sequence stars.
A PMS object has a larger radius than a main-sequence star with the same stellar mass and thus has a lower surface gravity.
Although they are optically visible, PMS objects are rare relative to those on the main sequence, because their contraction lasts for only 1 percent of the time required for hydrogen fusion.