Q:  Astronomers can measure variations
      accurately in stars down to apparent 
      magnitude roughly m=27.  How far away
      can we use Cepheids to measure distance?


        If the most powerful Cepheids have
        absolute magnitude M=-6, then the
        limit is at

          (m - M)  =  (27 - (-6)) 
 
                   =  33 magnitudes

        We can convert this to a distance 
        using

           (m - M) =  5 log d  -  5

                         [(m-M) + 5] / 5
         --->   d  =  10

                        38/5 
                   =  10       

                   =   40 Mpc  approx