Last modified on 8 August 2000.
Encyclopædia Galactica
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Cepheid variable star
A type of pulsating variable star, named after the group's
ptototype, Delta Cephei, which varies between
magnitudes 3.6 and 4.3 in a period of 5.4 days. Cepheid
variables have an unstable structure that causes them to pulse in
and out. Their size may change by as much as 10% during a cycle and
the temperature varies too. As pressure builds up inside, the star
expands until the pressure is released. The star then contracts and
the cycle starts again. Cepheids are luminous yellow
giant stars that radiate ten thousand times
as much energy as the Sun, so they can be seen at very great distances.
There is a relationship between the period, typically between 3 and 50
days, and the average apparent brightness: the brighter the star, the
longer its period. This is called the period-luminosity relation. The
importance of this lies in the fact that Cepheids can be used as
distance indicators. Two distinct
varieties of Cepheid variables have been identified: the so-called
classical Cepheids and Population II Cepheids. Their period-luminosity
relations differ: for a given period, classical Cepheids are about two
magnitudes brighter than Population II Cepheids. This is a result of
differences in mass and chemical composition. It is essential to
distinguish whether a variable star is a classical Cepheid or a
Population II Cepheid before its distance can be deduced. This is best
achieved by determining the metal content from analysis of its spectrum.
(Mitton, 1993, Dictionary of Astronomy, London: Penguin Books)
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Created by
Rami T. F. Rekola