This is the prototype of Cepheid variable stars that are used for finding distances in space. The brightness of Delta Cephei rises and falls every 5 days as it pulsates in size; at its peak, the star is more than twice as bright as when at its faintest. Changes in its brightness can be followed with the naked eye by comparing it with nearby stars of known magnitude. Delta Cephei is also an attractive double star, having a fainter companion that is visible with the smallest telescopes.
Stars of this type are believed to form with masses of 3–12 times that of the Sun, and then have passed through the main sequence as B-type stars. With the hydrogen consumed in their core region, these unstable stars are now passing through later stages of nuclear burning. The estimated mass of Delta Cephei, as derived from the color index, is 4.5 ± 0.3 times the mass of the Sun. By comparison, the mass derived from evolutionary models is 5.0 - 5.25 times the Sun's mass. At this stage of its evolution, the outer layers of the star have expanded to an average of 44.5 times the girth of the Sun.