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.
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.
The lower main sequence consists of relatively cool, low-mass hydrogen-burning stars whose luminosity is supplied mainly by the proton–proton chain.
The zero-age main sequence is the locus where stars begin stable core hydrogen burning with nearly their initial composition.
