A pulsating variable star. This type of star undergoes a rhythmic pulsation as indicated by its regular pattern of changing brightness as a function of time. The period of pulsation has been demonstrated to be directly related to a Cepheid's intrinsic brightness making observations of these stars one of the most powerful tools for determining distance known to modern day astronomy. The existence of of this period-luminosity relationship was a point of contention during the 1920 Curtis-Shapley debate.

a type of pulsating variable star with a luminosity that can be determined from the period of its variation: Cepheids with long pulsation periods are bigger and thus more luminous than short period Cepheids.

A type of variable star used to determine distance. Shapley used a relation between the period of variation of this type of star and its absolute magnitude to calibrate distances to globular clusters during the `Great Debate.' Curtis disputed these distances. Cepheids are used today as reliable distance estimators.

Variable star with a period of 60 days whose period of variation is related to its luminosity. Collectively, all such stars with similar spectra and period/luminosity realtionships.