A pre-main-sequence star contracts toward the main sequence before sustained core hydrogen burning supplies its luminosity. Convection, accretion, and deuterium burning shape its track across the Hertzsprung-Russell diagram.
A Hayashi track is the nearly vertical path of a fully convective pre-main-sequence star on the Hertzsprung-Russell diagram. Photospheric opacity fixes a weak relation between luminosity and effective temperature.
Matter accreting at rate onto a star of mass and radius releases gravitational power of orderup to an efficiency and an inner-boundary correction.
At the temperatures relevant to pre-main-sequence stars, a useful steep power-law approximation to the specific deuterium-burning rate is . Its sensitivity to central temperature makes deuterium burning act as a temporary thermostat.
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A pre-main-sequence star is a young star that is in the process of forming and has not yet reached the stable state of hydrogen fusion that characterizes main-sequence stars. This phase occurs after a star has formed from a collapsing cloud of gas and dust (a protostar) but before it begins hydrogen burning in its core. During the pre-main-sequence stage, the star is typically still gaining mass as material from the surrounding accretion disk falls onto it.