Q:  Given your relationship between L and M derived a moment ago,
          can you derive an equation relating the main-sequence lifetime 
          of a star to its initial mass?



          We earlier derived a relationship of roughly


                     L  ~  M^3


          So the lifetime on the main sequence must be


                             M            1
                     T  ~  ------   =   -----   =  M^(-2)
                             M^3         M^2


          More massive stars have SHORTER lifespans on the main sequence.