A class of highly luminous yellow stars that vary periodically in brightness. Approximately one out of a thousand of the intrinsically brightest stars in any galaxy, like the Milky Way Galaxy, is a classical Cepheid variable. Among the 6000 apparently brightest stars visible to the unaided human eye in the night sky, more than a dozen are Cepheids (including Delta Cephei, the prototype; Beta Doradus in the south; and Polaris, the Pole Star). Cepheids have the same yellow color as the Sun (and hence, similar surface temperatures), but they have larger radii and therefore are intrinsically 1000 to over 100,000 times more luminous. Named for their naked-eye prototype Delta Cephei, the Cepheids are very well understood, and have repeatedly proved to be immensely important to many aspects of modern astronomy. Not only are Cepheids bright (supergiant) stars, so that they can be seen over large distances, but also they are at a particular stage in their evolution such that they are unstable to periodic oscillations. Motion of their surface manifests itself as regular changes in the apparent luminosity with time and so they can be easily identified.