Hubble Space Telescope image of the galaxy M100 showing a number of Cepheid variables. The Cepheid star in the center of the box is located in a star-forming region of the spiral arm of the galaxy.
A Cepheid variable is a yellow giant or supergiant pulsating variable whose period of pulsation is directly related to its luminosity: the longer the period, the greater the mean intrinsic brightness. This strict period-luminosity relationship makes Cepheids important cosmic distance indicators, since, by measuring a Cepheid's period and comparing the intrinsic brightness, which this yields, with the apparent brightness, the star's distance can be worked out. In addition, since Cepheids are so bright that they can be seen in other galaxies, they help establish a distance scale well beyond the Milky Way (see cosmic distance ladder).
Cepheids fall into two distinct categories. Type I, also known as Delta Cephei stars or classical Cepheids, are extreme Population I objects, found in the spiral arms of galaxies. They typically have periods of 5 to 10 days, and are more metal rich and about 4 times more luminous than Type II Cepheids. The Type IIs, or W Virginis stars, are Population II objects with characteristic periods of 10 to 30 days, found primarily in globular clusters, galactic halos, and elliptical galaxies.
Although the luminosities of all Cepheids are proportional to their periods, there is a different relationship for each type. The light curves of Cepheid variables have a characteristic shark-fin shape, with a rapid rise to maximum, a brief stay at peak brightness, and a smooth, slow decline to minimum. In evolutionary terms,
Cepheids have left the main sequence and lie on the instability strip of the Hertzsprung-Russell diagram. Over 1,000 have been cataloged, of which the first to be found was Eta Aquilae and the best known is Polaris, the North Star.