An Etymological Dictionary of Astronomy and Astrophysics

A → correlation between the periods and luminosities of → Cepheid variable stars: Cepheids with longer periods are intrinsically more luminous than those with shorter periods. The relation was discovered by Henrietta Leavitt in 1912 when studying Cepheids in the → Small Magellanic Cloud.

Once the period of a Cepheid variable is determined from observations, the period-luminosity relation can be used to derive its luminosity. Since
luminosity is a function of → distance, the distance can then be calculated with the luminosity. The period-luminosity relation is an invaluable tool for the measurements of distances out to the nearest galaxies and thus for studying the structure of our own Galaxy and of the Universe.

See also: → period; → luminosity; → relation.