= Stellar nuclear timescale
{title2=$t_{\rm nuc}\sim E_{\rm fuel}/L$}
The time for <stellar nuclear fusion> to consume the accessible fuel at characteristic <luminosity> $L$. If a fraction $f$ of a star's <mass> $M$ supplies fuel abundance $X$ and released <energy> per unit fuel <mass> $E_H$, <hydrogen burning> gives $t_{\rm nuc}\sim fXME_H/L$. Accessible core fuel, rather than the entire star, sets the lifetime unless mixing is complete. This is longer than a structural <Kelvin-Helmholtz cooling time> when the nuclear reservoir greatly exceeds the gravitational contraction reservoir.
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