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The main sequence is the band in the Hertzsprung-Russell diagram occupied by stars whose main energy source is sustained core hydrogen burning. Different positions along it primarily reflect different masses: low-mass stars are cooler and fainter, while higher-mass stars are generally hotter and more luminous. Composition and age broaden the band, so it is not one exact universal curve.
A star spends a large fraction of its luminous lifetime in this comparatively long-lived stage. Its track begins near the zero-age main sequence and moves as its core composition changes, ending at the terminal-age main sequence. In a coeval population, the main-sequence turnoff identifies which stars have exhausted core fuel and therefore provides an age diagnostic.
