A type of variable star which exhibits a regular pattern of changing brightness as a function of time. The period of the pulsation pattern is directly related to the star's intrinsic brightness. Thus, Cepheid variables are a powerful tool for determining distances in modern astronomy.
A Cepheid is a member of a class of very luminous variable stars. The strong direct relationship between a Cepheid variable's luminosity and pulsation period secures for Cepheids their status as important standard candles for establishing the Galactic and extragalactic distance scales.
Cepheid variables are divided into several subclasses which exhibit markedly different masses, ages, and evolutionary histories:Classical Cepheids, Type II Cepheids, Anomalous Cepheids, and Dwarf Cepheids.
The term cepheid originates from Delta Cephei in the constellation Cepheus, the first star of this type identified, by John Goodricke in 1784. Delta Cephei is also of particular importance as a calibrator of the Cepheid period-luminosity relation since its distance is among the most precisely established for a Cepheid, thanks in part to its membership in a star cluster and the availability of preciseHubble Space Telescope/Hipparcos parallaxes.
Source: wikipedia, NASA