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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.
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 *. 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. 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. Period-Luminosity Relationship The empirically derived relationship between a Cepheid variable's period, (in days), and its absolute magnitude is given by This relationship is derived from data collected from Cepheids whose distances are determined by other means. Notes 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. See also | ||||||||
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