THE PERIOD-LUMINOSITY RELATION OF THE CEPHEIDS

Abstract

Around the turn of the century two investigations were started at the Harvard Observatory that had a far-reaching influence on the further development of astronomy: Henrietta S. Leavitt's investigation of the variable stars in the two Magellanic Clouds and Solon I. Bailey's investigation of the variable stars in globular clusters. The Magellanic Clouds turned out to be veritable mines of cepheids with periods longer than one day. In contrast, the overwhelming majority of the variables in globular clusters were cepheids with periods shorter than one day ; they were so characteristic of globular clusters that Bailey proposed for them the name cluster -type variables. Further investigations of these variables led to two very remarkable results. The most striking was Miss Leavitt's discovery that the brightness of the cepheids in the Magellanic Clouds is a function of the period of the lightvariation (the observed brightness increases with the period). Since there was every reason to believe that the objects in each Cloud are at practically the same distance from us, the observed period-brightness relation clearly implied a period-luminosity relation for the cepheids with periods longer than one day. Once established, this period-luminosity relation obviously would provide a simple and very powerful tool for determining the distance of any cepheid of known apparent brightness and period.