standard candle

In astronomy, a standard candle is a class of objects whose distances can be computed by comparing their observed brightness with their known luminosity. Cepheid variable stars are useful as standard candles because their pulsation period is related to their luminosity in a known way. To use a Cepheid variable star to make a distance measurement, an astronomer would measure the apparent brightness of the star and its pulsation period, then calculate the luminosity of the star from its period, and finally compute the distance by comparing the apparent brightness to the calculated luminosity.