The low-mass main-sequence: the comparison between theory and observation.

Summary. A comparison between theoretical zero-age main-sequence stellar model calculations and the observational data derived from nearby stars is undertaken in the mass-luminosity and mass-radius planes for stars of less than solar mass. It is found that, although theory succeeds in representing the empirical low-mass mass-luminosity relation, there is a discordancy between theory and observation in the mass- radius plane for stars of mass less than 0.5 M0. This discordancy, which is comparable to the estimated observational uncertainty, is such that the theoretical radii are 30 percent or more smaller than the observationally derived radii. Although the large empirical uncertainty renders the significance of this discordancy questionable, it is speculated that a systematic underestimation of effective temperatures along the very late main-sequence may be partially responsible. Key words: low-mass stars - luminosities - radii - stellar structure