= Main-sequence turnoff
The main-sequence turnoff is the location in a coeval population's <Hertzsprung-Russell diagram> where stars are leaving the <main sequence> after exhausting their core hydrogen. More massive stars generally have larger <luminosity> per unit <mass> and shorter nuclear lifetimes, so the most massive stars still on the main sequence constrain the population's age. In a simple model, equating the <stellar nuclear timescale> $t\sim qM/L(M)$ to the common age determines a turnoff mass; detailed calibration requires composition and stellar models.
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