Delta Cephei star
A Delta Cephei star is a Type I Cepheid variable, also known as a classical Cepheid, and named after the prototype, Delta Cephei. Such stars are massive (5 to 20 solar masses) Population I objects of high luminosity that undergo very regular pulsations, with periods of 1 to 135 days. Accompanying these pulsations are changes in brightness, with amplitudes of 0.1 to 2 magnitudes, and in spectral type, typically from F at maximum to G or K at minimum.
Delta Cephei stars occupy a region of the Hertzspung-Russell diagram known as the instability strip; having started life as hydrogen-burning B stars they are now in a post-main-sequence phase marked by instability to radial pulsation. A star may pass through such a phase several times after leaving the main sequence: once during shell hydrogen burning, twice more during core helium burning, and twice again during shell helium burning.