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 to the common age determines a turnoff mass; detailed calibration requires composition and stellar models.
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The main sequence turnoff (MSTO) is an important concept in astrophysics, particularly in the study of stellar populations and the evolution of stars. It refers to the point on the Hertzsprung-Russell (H-R) diagram where a cluster of stars begins to evolve off the main sequence phase of their lifecycle.