Cepheid

Cepheid [′sē·fē·əd]

(astronomy)

One of a subgroup of periodic variable stars whose brightness does not remain constant with time and whose period of variation is a function of intrinsic mean brightness.

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Cepheid 

a type of variable star with a periodic variation of brightness (with an amplitude ranging from 0.1 to 2 stellar magnitudes) caused by the pulsation of the star’s outer layers. The name was derived from the prototype variable star—δ Cephei.

Classical, or ordinary, cepheids (type I cepheids) include su-pergiants of spectral classes F and G; the periods of variation of their brightness range from one to 50 days, occasionally reaching 218 days. Both the mass and luminosity of classical cepheids increase with the period, and the more massive cepheids are also younger. The distinct relationship between the periods of light variation and luminosity makes it possible to determine, for the observed period and apparent stellar magnitude, the distances to cepheids, as well as to the star clusters and galaxies in which they are located, up to 3–4 megaparsecs. Cepheids thus serve as indicators of intergalactic distances. The period-age relation is used to investigate the modes of star formation in galaxies. Given the same period of light variation, W Virginis variables, which are classified as cepheids (type II cepheids), are two stellar magnitudes fainter than classical cepheids. Short-period cepheids are sometimes called RR Lyrae variables.

REFERENCES

Pul’siruiushchie zvezdy. Moscow, 1970.
Iavleniia nestatsionarnosti izvezdnaia evoliutsiia. Moscow, 1974.

IU. N. EFREMOV