Cepheid variables (sē´fēĬd), class of variable stars that brighten and dim in an extremely regular fashion. The periods of the fluctuations (the time to complete one cycle from bright to dim and back to bright) last several days, although they range from 1 to 50 days. These stars are important because the period of a Cepheid depends on its intrinsic brightness, or absolute magnitude, in a known way: the brighter the star, the longer its period. All Cepheid variables with the same period have nearly the same intrinsic brightness, but their apparent brightnesses differ because they are at different distances. By observing a Cepheid's period, one can determine how bright it actually is. By comparing this intrinsic brightness to how bright it appears to be, one can determine the star's distance. Thus Cepheids are important indicators of interstellar and intergalactic distances, and they have been called the "yardsticks of the universe." The Cepheid class takes its name from Delta Cephei, the first such star discovered in 1784. Cepheids are yellow supergiant stars, and their fluctuations in luminosity result from an actual physical pulsation, with attendant changes in surface temperature and size. The stars are hottest and brightest when expanding at maximum rate midway between their largest and smallest size. The period-luminosity relation was discovered by Henrietta Leavitt and Harlow Shapley by studying the many Cepheids in the Magellanic Clouds, the two closest galaxies; these stars are all almost equally distant. It was found that the brighter variables had the longer periods. The absolute magnitude of a few Cepheids is required to infer absolute, rather than merely relative, distances. These absolute magnitudes were measured by a statistical study of the proper motions of Cepheids within our own galaxy. In the 1950s a second class of Cepheids with different period-luminosity relations was found, leading to a dramatic doubling of estimated cosmological distances. The Hubble Space Telescope will permit the observation of Cepheids in more distant galaxies, giving a more accurate picture of the size and age of the universe.