Q: Roughly how long will a star of 10 solar masses
   remain on the main sequence?


      Using the rough formula


                                                 -2.5
        lifetime           (     mass     )
      ---------------  =   ( ------------ )
      solar lifetime       (  solar mass  )


                                                          -2.5
                                              (    10    )
        lifetime       =  (solar lifetime) *  ( -------- )
                                              (     1    )


                       =  (10 billion years) * 0.003

                       =   30 million years


      In other words, much less time than the Sun.