The Mass-Luminosity Relation is an equation relating a star's mass with its Luminosity.
L M ————— = ( ————— )a LSun MSun
a = 3.5 is often used for Main Sequence Stars. Stars close to the Eddington Luminosity have a close to 1. a depends upon whether the star is the right size for significant convection.
eclipsing binaries and Parallax measurements help confirm the relation.