Cepheid variable stars

A Cepheid variable or Cepheid is a member of a particular class of variable stars, notable for a fairly tight correlation between their period of variability and absolute luminosity. The namesake and prototype of these variables is the star Delta Cephei, discovered to be variable by John Goodricke in 1784.
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Stars which have left the main sequence and have begun to pulsate because they are unstable. During the course of these pulsations, the radius, temperature and luminosity of the star varies. All of these properties vary within a regular, repeating period of between 3 and 50 days. The length of this period is related to the average absolute magnitude of the star. This is something known as the period-luminosity relationship. Temperature can vary by about 1,500 Kelvin whilst radius varies by about 10-30 per cent. Luminosity changes are roughly about one magnitude. Cephieds are named after the prototype star, Delta Cephei and are generally yellow giant stars which are on their way to becoming red giant stars. They occupy a region on the Hertzsprung-Russell diagram known as the instability strip because of its elongated shape. There are two types of Cepheids variable star because of the two distinct stellar populations. Population I cepheids are known as classical cepheids are are, on average, two magnitudes brighter than W Virginis stars, which are Cepheids of population II.