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Hayashi limit

(limit on the temperature of a star dependent upon its mass and luminosity)

A Hayashi limit is a limit on the temperature (or radius) of a star (or any object that adheres approximately to the idea gas law) dependent on its mass and luminosity. A star of higher temperature (or equivalently, of small radius, given its luminosity) is necessarily internally unstable, i.e., hydrostatic equilibrium such that convection is inevitable until stability is reached. The region of the H-R diagram beyond the Hayashi limit is termed the forbidden zone. Protostars can form within this forbidden zone, i.e., unstable, but undergo the convection (and presumably other changes), leaving it. A star right at the Hayashi limit is fully convective. Hayashi tracks for different stellar masses track near the corresponding Hayashi limit. Metallicity also affects the Hayashi limit, and a magnetic field can affect a star's interior so as to leave a star in the forbidden zone longer than otherwise possible.

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