The stars Sirius, Capella, and Algol, which I used as examples in yesterday's lecture , are binary systems which are both apparently bright and intrinsically bright. The mass of a binary system can be deduced from Kepler's Third Law. For a visual binary, measure the time taken for the stars to trace a complete ellipse on the sky. For a spectroscopic binary, measure the time between maxima in the observed radial velocity. Computed masses range from 0.08 solar masses to 50 solar masses.
Masses vary systematically along the main sequence.
Thus, just as temperature, luminosity, and radius vary systematically along the main sequence, so does mass. In the supergiant and giant regions of the H-R diagrams, stars of different masses are scrambled together. In other words, if you change the mass of a star by only a little bit, you change its luminosity by a lot. A star twice as massive as the Sun will be more than 10 times as luminous as the Sun.