period-luminosity(P-L) relation A relation showing graphically how the period of the light variation of a Cepheid variable depends on its mean luminosity: the longer the period the greater the luminosity and hence mean absolute magnitude of the star. The relation was discovered by Henrietta Leavitt in 1912 and much subsequent work was done in calibrating the graph. It was found, in the 1950s, that there are two types of Cepheid – classical Cepheids and the less luminous type II Cepheids – whose P-L relations are approximately parallel, as shown on the graph. The relation results from the dependence of both luminosity and period of light variation, i.e. of pulsation, on stellar radius (see pulsating variables).
The relation is an invaluable means of determining the distances of Cepheids and hence of their surroundings. To calibrate the two curves, the mean absolute magnitude of a small number of Cepheids is determined from the measured value of their periods, and the distances must then be found from an independent measure. The distance to other Cepheids can then be calculated from their periods and mean apparent magnitudes (see distance modulus). Cepheids fluctuate more markedly at blue and near-UV wavelengths than at longer red and near-IR wavelengths. The mean brightness is thus more easily and accurately measured at longer wavelengths. Cepheids occur in many star clusters within our own Galaxy and because of their great luminosity can be observed in nearby galaxies. They can therefore be used for measuring an immense range of distances. See distance determination.