Cepheids are unusual because they vary in brightness is a regular manner, and the length of the brightening and dimming cycle is directly related to their brightness or absolute magnitude. The longer the time between brightness peaks, the brighter the star is. Scientists have observed many of these stars, and a plot of their absolute magnitude or luminosity versus the period of their cycle yields the following graph.
To learn more about cepheids and other standard candles, take about five minutes to
read
this Space Science Short from NASA.