= Helium-3 freeze-out estimate with Gamow factors
{title2=$\tau(T)/\tau(T_0)=(T/T_0)^{2/3}\exp\{\bar\eta[(T_0/T)^{1/3}-1]\}$}
In the pp-I limit, <helium-3 relaxation time> is $\tau^{-1}=n_1\sqrt{2\lambda_{11}\lambda_{33}}$. At fixed proton <number density>, each <thermonuclear reaction rate> coefficient obeys $\lambda(T)/\lambda(T_0)=(T/T_0)^{-2/3}\exp[-\eta(T_0)((T_0/T)^{1/3}-1)]$. Taking their geometric mean gives the displayed relaxation ratio with $\bar\eta=[\eta_{11}(T_0)+\eta_{33}(T_0)]/2$. A local power-law approximation instead gives $\tau\propto T^{-(\alpha+\beta)/2}$. These are estimates rather than a complete radial model: the proton <number density>, composition and pp-II branch change along a real <stellar composition profile>.
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