One of a class of variable star s whose period of variation is closely related to its luminosity and is therefore useful in measuring the distances to clusters of stars and galaxies.
Named for the prototype of this class found in the constellation Cepheus, classical Cepheids have periods from about 1.5 days to over 50 days and are Population I stars (see Populations I and II ). The longer the period of such a star, the greater its natural brightness; this relationship was discovered in 1912 by the American astronomer Henrietta Leavitt (b. 1868
d. 1921).