= Fully mixed hydrogen-burning evolutionary track
{title2=$\left.d\log L/d\log T_e\right|_{X=1}=256/53$}
With negligible heavy-element contribution to the <mean molecular weight>, $\mu=4/(3+5X)$ and $\kappa\propto1+X$. At fixed <mass>, <composition-dependent CNO homology> gives $L\propto(1+X)^{-1}(3+5X)^{-4}$ and $R\propto[X(1+X)]^{1/16}(3+5X)^{-9/16}$. The <Stefan–Boltzmann law> then gives $T_e\propto X^{-1/32}(1+X)^{-9/32}(3+5X)^{-23/32}$. From the formal $X=1$ reference, declining $X$ initially increases <luminosity> and <effective temperature>, unlike the usual redward main-sequence turn-off. A trace CNO catalyst abundance is implicit in the burning coefficient; an exactly metal-free pure-hydrogen star cannot burn through that cycle.
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