= Cepheid variable
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A <Cepheid variable> is a pulsating evolved <star> whose regular changes in <stellar radius> and <temperature> produce a repeating <light curve>. The <kappa mechanism> supplies driving in a suitable part of the <instability strip>. Large <stellar radius> gives a long dynamical time, connecting the pulsation period to the <star>'s mean <mass density>.
The practical importance is the calibrated <Cepheid period-luminosity relation>. Measuring a period predicts an <absolute magnitude> within a specified population and spectral band; comparison with <apparent magnitude> then constrains distance. <Extinction>, composition and unresolved companions affect that comparison. Thus the variable brightness acts as a physical ruler only after calibration, rather than every pulsating <star> having one universal <luminosity> at a given period.
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