Q:  What is the range in periods of these Cepheids?

                    The periods range from 

                       log(P) = -0.1    -->   P =  0.8 days

                    to

                       log(P) = +1.9    -->   P = 79   days



              Q:  What is the range in magnitudes of these Cepheids?
                      Translate that to a range in luminosities (a factor
                      of 10 between brightest and faintest?  A factor of 20?)


                     The magnitudes range from 

                          M  = 16   to   M = 10       so   delta_M = 6 mag

                      which corresponds to 


                        I(brightest)          -0.4*(10 - 16)
                       -------------   =    10
                        I(faintest)

                                       =    250

                          
                      So the brightest Cepheids in that graph are about
                      250 times more luminous than the faintest ones.