A Cepheid variable is a member of a particular class of variable stars, notable for a fairly tight correlation between their period of variability and absolute stellar luminosity.

Because of this correlation (discovered by Henrietta Swan Leavitt in 1912), Cepheid variables can be used as a standard candle to determine the distance to their host globular clusters or galaxies. Since the period-luminosity relation can be calibrated with great precision using the nearest Cepheid stars, the distances found with this method are among the most accurate available.

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 10³ to 10⁴ times that of the Sun.

The reason for the pulsing is a certain abundance of He⁺ in their atmosphere. The powerful radiation generated by the star ionizes a small fraction of the He⁺ to He⁺², which is much more opaque to radiation. The atmosphere starts to block some of the outgoing radiation, becomes hotter and starts expanding. The hotter and bigger atmosphere causes the observed increase in the star's luminosity.

The expanding atmosphere soon starts to cool down, and He⁺² recombines into He⁺. Now the atmosphere is relatively transparent again, loses heat and shrinks again. The whole process is then repeated.

This expansion-contraction sequence is repeated every few days in a highly predictable way.

Use as a standard candle

The relationship between a Cepheid variable's luminosity and its variability period is quite precise, and has been used a standard candle for more than a century. A three-day period Cepheid has a luminosity of about 800 times the Sun. A thirty-day period Cepheid is 10,000 times as bright as the Sun. The scale has been calibrated using nearby Cepheid stars, for which the distance was already known.

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 globular clusters and external galaxies. Of course, a small error will be present because we do not know the precise location of the Cepheid variable within the cluster or galaxy. This error is typically small enough to be irrelevant in these kinds of measurements.

Cepheid stars are visible from great distances. Edwin Hubble first identified some Cepheids in the Andromeda galaxy, thus proving its extragalactic nature (not known at that time). More recently, the Hubble Space Telescope succeeded in identifying some Cepheid stars in the Virgo cluster, at a distance of 60 million light years.

Notes

• Some Cepheid stars (for example Polaris), have shown a decrease in their oscillation over a period of few tens of years, and now are virtually constant.

• Cepheid stars are sometimes divided into two types, type I Cepheid and type II Cepheid. The latter type, composed of Population II stars, are now usually called W Virginis variables, and show a similar behaviour.

See also: RR Lyrae variable, Hubble Constant