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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.
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