Cepheid variable

Cepheid variable. (a) The light curve, temperature variation and change in radius represented schematically for the cycle of a typical Cepheid variable. Cepheid variable. (b) The period-luminosity relationship for classical Cepheids and W Virgins stars. A type of pulsating variable star, named after the group's prototype, Delta Cephei, which varies between magnitudes 3.6 and 4.3 in a period of 5.4 days. Cepheid variables have an unstable structure that causes them to pulse in and out. Their size may change by as much as 10 per cent during a cycle and the temperature varies too. As pressure builds up inside, the star expands until the pressure is released, rather like it might be through a valve. The star then contracts and the cycle starts again (see illustration (a)). Cepheids are luminous yellow giant stars that radiate ten thousand times as much energy as the Sun, so they can be seen at very great distances. In 1912, Henrietta Leavitt, working at Harvard College Observatory, noted a number of Cepheids in the Small Magellanic Cloud and plotted their light curves. It became clear to her that there was a relationship between the periods, typically between 3 and 50 days, and the average apparent brightness: the brighter the star, the longer its period. This is called the period-luminosity relation (see illustration (b)). The importance of this discovery lies in the fact that Cepheids can be used as distance indicators. All the stars in the Small Magellanic Cloud can be considered to be at roughly the same distance (certainly in relation to the distance of the SMC itself), so the apparent magnitudes will differ from the absolute magnitudes by a constant factor. Once the distance to a single Cepheid variable was found by an independent method, the distances to all others could be deduced simply from measuring their periods. Two distinct varieties of Cepheid variable have been identified: the so-called classical Cepheids and "Population II" Cepheids, also commonly known as W Virginis stars. Their period-luminosity relations differ: for a given period, classical Cepheids are about two magnitudes brighter than W Virginis stars. This is a result of differences in mass and chemical composition. The lower mass of W Virginis stars results in a lower luminosity, but the effect is partially offset by the low abundance of elements heavier than helium ("metals") in the stars of the old Population II. It is essential to distinguish whether a variable star is a classical Cepheid or a W Virginis star before its distance can be deduced. This is best achieved by determining the metal content from analysis of its spectrum.