Cepheid variable
From Wikipedia, the free encyclopedia
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.
Contents |
[edit] 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. 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].
The variation in luminosity is caused by a cycle of ionization of helium in the star's atmosphere, followed by expansion and deionization. While ionized, the atmosphere is more opaque to light. This cycle has a period roughly equal to the star's dynamic time scale, therefore giving information on the mean density of the body as well as its luminosity.
[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 almost a century. This connection was discovered in 1912 by Henrietta Swan Leavitt. She measured the brightness of hundreds of Cepheid variables and discovered a distinct period-luminosity relationship. 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 relationship between a Type I Cepheid's period P (in days), and its absolute magnitude Mv has been empirically derived by many astronomers throughout the Twentieth Century. The relationship is calibrated using data collected from Cepheids whose distances are determined by other means. Ejnar Hertzsprung attempted the first calibration in 1913,[1] but, due to his ignorance of interstellar absorption, his results were highly inaccurate. The accuracy of the period-luminosity relationship has remained essentially unchanged since the calibration given in 1968 by Allan Sandage and Gustav Tammann. [2]
A more recent notable calibration was published by Michael Feast and Robin Catchpole in 1997. Using data from the Hipparcos satellite, Feast and Catchpole calculated the distances to many Galactic Cepheids via trigonometric parallax. The resultant period-luminosity relationship was:
with P measured in days. Since the amount of interstellar absorption varies from galaxy to galaxy, the above equation is only valid for Cepheid variables in the Milky Way.[3][4]
[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.
[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] See also
[edit] References
- SEDS: Variable Stars
- Leavitt, H 1908, Annals of Harvard College Observatory, vol. LX, no. 4, p. 107
- ^ Hertzsprung, Ejnar. "Uber die raumliche Verteilung der Verlandlichen vom Delta Cephei – typus". Astronomische Nachrichten. 196 (1913) 201-210.
- ^ Sandage, Allan & Gustav A. Tammann. " A composite period-luminosity relation for Cepheids at mean and maximum light". Astrophysical Journal. 151 (1968) 531-545.
- ^ Feast, Michael W. & Robin M. Catchpole. "The Cepheid period-luminosity zero-point from Hipparcos trigonometrical parallaxes". Monthly Notices of the Royal Astronomical Society. 286 (1997) L 1-5.
- ^ Allen, Nick. "The Cepheid Distance Scale: A History". Aug 2005. http://www.institute-of-brilliant-failures.com.
