Q: Where do Cepheid stars of period P = 10 days fall on the 
      HR diagram?


            abs mag  M  =  -1  -  2.8*log(10)

                        =  -1  -  2.8*1

                        =  -3.8

            So these stars lie above the main sequence, 
            roughly in the middle of the diagram horizontally.


   Q: Where do Cepheid stars of period P = 100 days fall on the 
      HR diagram?

            abs mag  M  =  -1  -  2.8*log(100)

                        =  -1  -  2.8*2

                        =  -6.6

            So these stars lie FAR above the main sequence, 
            roughly in the middle of the diagram horizontally.
            They are at the top-middle of the diagram.


   
   Q: Recall that RR Lyrae stars have (B-V) colors between +0.20 and +0.50,
      and mean absolute magnitudes MV = +0.6.  Where do RR Lyrae
      stars lie on the HR diagram?

            RR Lyrae stars lie close to the upper main sequence,
            just a bit above and to the right of the main sequence.