Delta Cephei, the orange-colored star in the center of this image, is the original classical Cepheid variable star. It is located in the constellation of Cepheus. Its luminosity varies from magnitude 3.48 to 4.37 over a period of 5 days, 8 hours, 47 minutes, and 32 seconds because of pulsations in the outer layers of the star. Its variability was discovered by John Goodricke in 1784. In 1912 Henrietta Swan Leavitt discovered the relationship between the period of a Cepheid variable and its brightness. The brighter the Cepheid, the longer its period. This period-luminosity relationship can be used to determine a Cepheid's absolute magnitude. The inverse-square law can then be used to determine distance based on the star's apparent brightness. The inverse square law says that a star that is twice as far away will be one-quarter as bright. The relationship between Delta Cephei's period and luminosity was very important in establishing a standard for measuring distances in the universe.