Which are more common: luminous stars or feeble stars?

        Clearly, there are more low-luminosity stars than high-luminosity
        stars in this sample.  And, since high-luminosity stars
        should be easier to see, we may conclude that low-luminosity
        stars really are more common.



   Why is there a connection between color and luminosity?

        Blackbody emission increases very rapidly with increasing
        temperature:
 
                               2                4
                Lum  =  (4 pi R  ) *  (sigma * T  )



   What's that small clump of stars at lower left?

        Very hot, but low in luminosity.  The only way that 
        a hot object can emit very little light is if it has
        a small surface area.  We could call these "white dwarfs."