= Main-sequence lifetime
{title2=$t_{\rm MS}\sim fMc^2\epsilon/L$}
The time a <star> spends with core hydrogen <nuclear fusion> as its principal energy source. If the usable fuel fraction $f$, conversion efficiency $\epsilon$, and composition are comparable, the <main-sequence lifetime> scales as $M/L$. With a local <mass-luminosity relation> $L\propto M^\alpha$, the lifetime scales as $M^{1-\alpha}$. This is a scaling estimate: convective core size and changing <luminosity> affect detailed ages.
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