period-luminosity relation for Cepheid-type variable stars—the curve
showing how their brightness varies over time - as established by Harlow
Shapley in 1918. An astronomer henceforth could observe the period, or
time from one maximum brightness to the next maximum, for any other Cepheid
variable star, and then read off the graph the star's absolute magnitude.
Comparison of this estimated absolute ("real") magnitude with
the observed apparent magnitude yields the distance, since brightness
diminishes with the square of the distance.
uncertain, see this note.