Hayashi limit

A Hayashi limit is a limit on the temperature (and radius) of a star (or any stable object that adheres to the ideal gas law), the limit being dependent on the star's mass and luminosity. Given a particular luminosity, there is a relation between stars' temperatures and radii, and for a star consisting of an ideal gas, given also a specific mass, the ideal gas law determines whether such a star would be stable. A star of higher temperature than the limit, or equivalently, of smaller radius, is necessarily internally unstable, i.e., convection is inevitable until stability (hydrostatic equilibrium) is reached. A Hayashi limit forms a line on the H-R diagram (HRD), marking a corner of the HRD termed the forbidden zone. Protostars can form within this forbidden zone, i.e., they are unstable, but after convection (and presumably other changes), they can leave it. The term Hayashi phase is used for this period in the star's life. A star right at its Hayashi limit is fully convective. Hayashi tracks for different stellar masses fall 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.