main sequence star is a star that generates energy by fusing hydrogen into helium; low-mass stars use the proton-proton chain, while higher-mass stars use the CNO cycle. Main sequence stars are very stable; while they may become gradually larger and brighter over time, over shorter timescales the internal pressure from fusion counteracts the force of gravity. They form the primary diagonal stripe on an H-R diagram, visible from top left (bright and hot) to bottom right (dim and cool).

Stars spend most of their lives on the main sequence, generally between a few million years for the most massive Class O stars and a few trillion years for the least massive Class M stars. After exhausting their hydrogen fuel, low-mass M dwarfs generally die off, while others evolve into subgiants and from there go to later stages of stellar evolution.

Stellar Luminosity Classes
-1 · 0 · I · II · III · IV · V · VI · VII
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