Cepheids: Star Measures and Variables

by Jeanette Cain

Among the most interesting of variable stars are those known as the cepheids. They are giant stars with periods of variation generally from a few days to about five weeks, with the most common period about a week. Their change is brightness is not very great, but are quite regular and dependable in their changes. Like other variable stars, they get their name from a very bright and famous member of their type - a star of the constellation Cepheus known as Delta Cephei. Another well-known member of the cepheid family is Earth's North Star, Polaris.

The period-brightness relation of the cepheid variable star has become a valuable device for measuring extremely great distances in the universe. Distance scale determined by the period-luminosity relationship among cepheid variables depends on on estimates of the luminosity, the intrinsic brightness of the cephid stars. Several revisions of this scale have been made since its inception. These revisions resulted in new estimates of distances and populations in the areas of space reached by today's telescopes.