A type of luminous giant star whose luminosity varies in a periodic fashion. Cepheids are characterized by a rapid rise in luminosity followed by a slow decline. The period of the cycle is related to the luminosity of the Cepheid by the Period-Luminosity relationship. The more luminous the Cepheid, the longer the period. This property makes Cepheids useful for obtaining distances. One determines the pulsation period and uses the relationship to get the luminosity. The apparent brightness of the star then gives you the distance. Cepheids come in two types, Type I which are metal rich and Type II which are metal poor. Type I Cepheids are more luminous than Type II.