Cepheid variable

From Free net encyclopedia

Revision as of 08:06, 4 April 2006; view current revision
←Older revision | Newer revision→

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.

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. 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.

1 Description
2 Use as a "standard candle"
- 2.1 Period-Luminosity Relationship
3 Notes
4 Examples
5 See also
6 References

[edit]

Description

Image:HRDiagram InsStrip.png

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. 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 [1].

[edit]

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 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 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. Because of relatively high luminosity, 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.

[edit]

Period-Luminosity Relationship

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

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

[edit]

Notes

Some Cepheid stars (for example Polaris), have shown a decrease in their oscillation over a period of a 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.

[edit]

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.

[edit]