Hayashi track

The Hayashi track is a path taken by protostars in the Hertzsprung-Russell diagram after the protostellar cloud has reached approximate hydrostatic equilibrium. In 1961 Chushiro Hayashi showed that there is a minimum effective temperature (equivalently, a boundary on the right-hand side of the H-R diagram) cooler than which hydrostatic equilibrium cannot be maintained; this boundary corresponds to a temperature around 4000 K. Protostellar clouds cooler than this will contract and heat up until they reach the Hayashi boundary. Once at the boundary, a protostar will continue to contract on the Kelvin-Helmholtz timescale, but its effective temperature will no longer increase, as it will remain at the Hayashi boundary. Thus the Hayashi track is close to a vertical line on the H-R diagram. Stars at the Hayashi boundary are fully convective: this is because they are cool and highly opaque, so that radiative energy transport is not efficient, and consequently have large internal temperature gradients. Stars with masses <0.5 Solar mass remain on the Hayashi track (i.e. are fully convective) throughout their pre-main sequence stage, joining the main sequence at the bottom of the Hayashi track. For stars with masses > 0.5 Solar mass the Hayashi track ends, and the Henyey track begins, when the internal temperature of the star rises high enough that its central opacity drops and radiative energy transport becomes more efficient than convective transport: the lowest luminosity on the Hayashi track for a star of a given mass is thus the lowest luminosity at which it is still fully convective.
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The path on a Hertzsprung-Russell diagram which is followed by a protostar on its way to becoming a main sequence star.