A Cepheid variable star one of a class of variable stars whose periods (i.e., the time for one cycle) of variation tend to be proportional to their luminosity and that are therefore useful in measuring interstellar and intergalactic distances. Most are spectral type F (moderately hot) at maximum luminosity and type G (cooler, Sun-like) at minimum. The prototype star is Delta Cephei, the variability of which was discovered by John Goodricke in 1784. In 1912 Henrietta Leavitt of Harvard Observatory discovered the aforementioned period-luminosity relationship of the Cepheids.
Cepheids are now considered to fall into two distinct classes. The classical Cepheids, which are dependable in their period-luminosity relationship, all have periods from about 1.5 days to more than 50 days and belong to the class of relatively young stars found largely in the spiral arms of galaxies and called Population I. Short-period Cepheids, also called cluster-type variables, or RR Lyrae variables, all have periods of less than one day and show no dependable relationship between period and luminosity; this last fact caused considerable confusion among astronomers before it was recognized. RR Lyrae variables can still be employed as distance indicators because their absolute magnitudes tend to be alike. Short-period Cepheids belong to Population II, a class of older stars found in the core and in the halo of the Milky Way Galaxy.
Classical Cepheids exhibit a relation between period and luminosity in the sense that the longer the period of the star, the greater its intrinsic brightness; this period-luminosity relationship has been used to establish the distance of remote stellar systems. The absolute magnitude of a classical Cepheid can be estimated from its period. Once this is known, the distance of the star can be deduced from a comparison of absolute and apparent (measured) magnitudes.
Excerpt from the Encyclopedia Britannica without permission.