= Accretion-disk thermal S-curve
{title2=$T_{\rm eff}(\Sigma)$}
At fixed disk radius, balancing local heating and cooling gives thermal equilibria parameterized by <surface density> and <temperature>. In the <hydrogen-ionization disk instability>, this locus has cold and hot stable branches joined by unstable equilibria. Crossing the turning points drives the state changes of a <dwarf nova>. <Thermal stability of an accretion disk> tests the local heating and cooling response, while <viscous evolution of an accretion disk> changes the surface density.
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