= Negative hydrogen ion opacity
{title2=$\kappa\propto\rho^{1/2}T^9$}
The <negative hydrogen ion> supplies important continuum <opacity> in cool hydrogen-rich stellar atmospheres. A useful local approximate law is $\kappa=\kappa_0\rho^{1/2}T^9$; it is not valid over arbitrary temperatures and compositions. In an <ideal gas> <power-law opacity radiative envelope> matched to $\nabla_{\rm phot}=1/8$, it gives $\nabla_{\rm rad}=-1/3+(11/24)(T/T_{\rm eff})^{9/2}$, rising rapidly toward <convective instability>.
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