Cepheid variable stars

A class of luminous, yellow supergiants that are pulsating variables and whose period of variation is a function of their luminosity. These stars expand and contract at extremely regular periods, in the range 1-50 days. Their highest brightness and surface temperature occur when their expansion velocity is greatest. Similarly, their minima in brightness and temperature occur when they are in the contraction phase. The longer the period, the more luminous the star. The process that drives the pulsation of Cepheid variables is the kappa mechanism. In fact, Cepheids provide one of the most powerful tools for measuring distances to other galaxies (period-luminosity relation). However, this method is limited to the distance of the Virgo cluster of galaxies (15-20 Mpc) even with the HST or the largest ground-based telescopes. One particularly special Cepheid is the North Star, Polaris.