Cepheid variable - Description, Use as a "standard candle", Examples

A class of variable star with a period of 1–50 days, characterized by precise regularity. There is a direct correlation between the period and luminosity (the longer the period, the more luminous the star). The observed brightness of a Cepheid indicates its distance from Earth, and is consequently of great importance in determining the distance scale of the universe.

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.

Because of this correlation (discovered by Henrietta Swan Leavitt in 1912), a Cepheid variable can be used as a standard candle to determine the distance to its host cluster or galaxy.

Description

A Cepheid is usually a population I giant yellow star, pulsing regularly by expanding and contracting, resulting in a regular oscillation of its luminosity. The luminosity of cepheid stars range from 10 times that of the Sun. Because Cepheids are from population I, they are sometimes called Type I Cepheids, while the similar (but belonging to population II) W Virginis variables are known as Type II Cepheids.

The exact mass of Cepheids with given brightness or oscillations is not known to any great precision, but astronomers hope to gather data for this from the newly-discovered third star of the Polaris system .

Use as a "standard candle"

The relationship between a Cepheid variable's luminosity and variability period is quite precise, and has been used as a standard candle for almost a century. This high luminosity, and the precision with which their distance can be estimated, makes Cepheid stars the ideal standard candle to measure the distance of clusters and external galaxies. Because of relatively high luminosity, Cepheid stars are visible from great distances. More recently, the Hubble Space Telescope succeeded in identifying some Cepheid stars in the Virgo cluster, at a distance of 60 million light years.

Period-luminosity relationship

The empirically derived relationship between a Cepheid variable's period, P (in days), and its absolute magnitude Mv is given by

This relationship is derived from data collected from Cepheids whose distances are determined by other means.

Examples

Some Cepheid variables with fairly bright apparent magnitudes and variations in brightness large enough to easily distinguish with the naked eye include Eta Aquilae, Zeta Geminorum, Beta Doradus, as well as the prototype Delta Cephei.