Cepheid and RR Lyrae Timescales

We can use the reasoning of the left frame to understand why RR Lyrae stars have shorter periods than Cepheid variables. Since the pulsation period is inversely proportional to the square root of the average density, increasing the density decreases the pulsation period. RR Lyrae stars are giants, so they have higher average densities than supergiant Cepheid variables. Thus, Cepheids have longer periods (1-100 days) than RR Lyrae stars (typically a day or less).