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A red dwarf is a cool, low-mass main-sequence star supported by sustained core hydrogen burning. Its small radiating area and low effective temperature make it faint compared with the Sun. A brown dwarf may have a similar radius but does not sustain that same long-lived hydrogen-burning state.

The lowest-mass red dwarfs are approximately fully convective stars. Mixing can bring fresh hydrogen into the burning region instead of leaving most fuel isolated in an envelope. Their low luminosity relative to available fuel therefore permits long main-sequence lifetimes. A small stellar radius also makes a given exoplanet block a larger fraction of the disc, improving the geometric signal in exoplanet transit photometry.