A correlation between the periods and mean luminosities
of Cepheid variables. The period-luminosity
relation was discovered by Henrietta Leavitt
in 1912. The longer a Cepheid's pulsation period, the more luminous the
star. Since measuring a Cepheid's period is easy, the period-luminosity
relation allows astronomers to determine the Cepheid's intrinsic brightness
and hence its distance. If the Cepheid is in another galaxy, the Cepheid's
distance gives the distance to the entire galaxy.