A Population I Cepheid variable, sometimes known as a Delta Cephei star; abbr. DCEP. In contrast to the superficially similar W Virginis stars of Population II, classical Cepheids are massive (5–15 solar masses), young bright giants or supergiants found exclusively in the disk population of galaxies, where they are often members of open clusters. The periods are from 1 to over 100 days, with amplitudes of 0.5–2 mag. The stars exhibit well-defined period–luminosity and period–luminosity–colour relations that are the basis of a widely used method of determining extragalactic distances.