Q:  Suppose we measure the metallicity of star which is identical
           to the Sun in every way.  What would its value of
           [M/H] be?



         In both stars, the fraction

      
             number of atoms of metals
            ---------------------------
              number of atoms of H
            

         would be the same -- for example, 0.002.  So, the 
         metallicity would be

                       [  0.002  ]
               log_10  [ ------- ]   =   log_10 (1)  =   0
                          0.002