Portions of this entry contributed by Alfred Gautschy

A Cepheid variable is a young star of several solar masses and roughly 10⁴ solar luminosities whose luminosity changes periodically. As radiation streams out, some in the atmosphere of the star is ionized to , making the atmosphere more opaque. The decreased transparency of the stellar material blocks the energy flux and heats the gas, and the increased pressure pushes the envelope out, thus increasing the star's size and luminosity. As the star expands, it cools and gains an electron, converting back to . The enhanced transparency causes the atmosphere to shrink again.

The period of a Cepheid variable is related to its intrinsic luminosity. Hence, measuring the period of light fluctuations allows the object's absolute luminosity to be determined, and its distance then follows through comparison with the observed brightness. Therefore, the Cepheids play an important role as a standard candle in assessing the distance scale in the universe. Due to the wavelength dependence of interstellar absorption, the period-luminosity relationship is considerably tighter in the infrared than in the blue.

Type I Cepheids are population I stars which have brightnesses which fluctuate with a period between 1 to 100 days. W Virginis stars are sometimes called type II Cepheid variables.

Distance Scale, Dwarf Cepheid, RR Lyrae Variable, Standard Candle, Variable Star, W Virginis Star