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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 $t_{\mathrm{nuc}}\sim fXM{E}_H/L$. 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.

Stellar nucleosynthesis encompasses nuclear production during stellar evolution and explosions, including hydrogen burning, core helium burning, advanced burning and neutron-capture processes. Stellar mass, composition, mixing and mass loss govern which nuclei form and which eventually escape. Slow neutron-capture process and rapid neutron-capture process build many nuclei beyond the iron group.

The terminal-age main sequence denotes the end of central hydrogen burning. It does not mark complete exhaustion of hydrogen throughout the star; a hydrogen-burning shell can subsequently supply its luminosity.

Let a fixed-mass, fully mixed stellar model evolve by hydrogen burning, with $L=C{M}^6{Z}^{-7/6}H(X)$ and $H(X)=\mu (X{)}^{26/3}{X}^{-1/3}(1+X{)}^{-7/6}$. Energy conservation gives $\dot{X}=-L/(M{E}_H)$. The time to reach $X=f{X}_0$ is therefore $t=(E_H/C)M^{-5}{Z}^{7/6}{\int }_{f{X}_0}^{X_0}H(X{)}^{-1}dX$. If $X_0,f$ and the function $\mu (X)$ are common to the population, the integral is constant and $M_{\mathrm{to}}\propto Z^{7/30}{t}^{-1/5}$. This includes the change of luminosity during fuel depletion rather than replacing the luminosity by its initial value.