Figure 13-5 Period and Luminosity for Cepheid and RR Lyrae Variables This graph shows the relationship between period and luminosity for Cepheid variables and RR Lyrae variables. Cepheids come in a broad range of luminosities: The more luminous the Cepheid, the longer its pulsation period. By contrast, RR Lyrae variables are horizontal-branch stars that all have roughly the same average luminosity of about 100 L_⊙