Cepheid variables

Article By:

Madore, Barry F. Observatories of the Carnegie Institution for Science, Pasadena, California.

Freedman, Wendy L. Observatories of the Carnegie Institution for Science, Pasadena, California.

Last reviewed:November 2019

DOI:https://doi.org/10.1036/1097-8542.120900

A class of high-luminosity yellow (supergiant) stars. Individual Cepheids vary periodically in brightness. Those Cepheids with the longest periods on average have the highest luminosities. Approximately one out of a thousand of the intrinsically brightest stars in any galaxy, like the Milky Way Galaxy, is a classical Cepheid variable. Among the 6000 apparently brightest stars visible to the unaided human eye in the night sky, more than a dozen are Cepheids (including their naked-eye prototype, Delta Cephei; Beta Doradus in the south; and Polaris, the northern Pole Star). Cepheids have the same yellow color as the Sun (because they both have similar surface temperatures), but Cepheids have significantly larger radii than the Sun, and therefore they are intrinsically 1000 to over 100,000 times more luminous. Cepheids are very well understood observationally and theoretically; moreover, they have repeatedly proved to be immensely important to many aspects of modern astronomy. Not only are Cepheids intrinsically bright (supergiant) stars (so that they can be seen over large distances) but also they are at a particular stage in their structure and evolution such that they are unstable to periodic oscillations. Motions of a Cepheid's surface and the induced changes in their surface temperature and surface brightness manifest themselves as regular changes in the apparent luminosity with time and so they can be easily identified. See also: Polaris; Supergiant star

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