A protostar is a forming star whose energy comes chiefly from gravitational contraction and accretion, before sustained core hydrogen burning supplies its luminosity. A fixed-mass contraction model approximates the stage after most accretion has ended; an actively accreting protostar need not have fixed mass or the same Hayashi track.
During continued compression of a first hydrostatic core, molecular hydrogen dissociation absorbs energy and softens the effective equation of state. This can trigger a second rapid contraction before a compact stellar core becomes pressure-supported. It is distinct from the initial nearly isothermal collapse and from later Kelvin-Helmholtz contraction at roughly fixed mass.
An accelerated contraction phase associated with molecular hydrogen dissociation absorbing compression energy and reducing effective pressure support. A compact stellar core forms when the gas can again oppose contraction. Its timing and geometry depend on the accretion and thermodynamic history.
A transient pressure-supported stage of a collapsing cloud after rising optical depth inhibits cooling. Continued accretion and changes of the molecular equation of state can lead to further collapse. It is distinct from the later compact stellar core.
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A protostar is an early stage in the formation of a star. It forms from a cloud of gas and dust in space, known as a nebula, which undergoes gravitational collapse. As the material in the nebula comes together, it begins to clump and compact, leading to an increase in temperature and pressure at the center of the forming star.