The concept of variable stars It encompasses any star whose brightness, seen from Earth, is not constant.
They can be stars whose light emission really fluctuates (intrinsic), or stars whose light is interrupted in their path to Earth, by another star or a cloud of interstellar dust, called extrinsic variables.
Changes in light intensity in intrinsic variables are due to pulsations in the size of the star (pulsating variables) or to interactions between the components of a double star. Some other intrinsic variables do not fit into any of these two categories.
The only frequent type of extrinsic variable is the so-called "eclipsing binary". It is a double star formed by two nearby stars that periodically pass in front of each other. Algol is the best known example. The eclipsing binaries constitute almost 20% of the known variable stars.
Cepheids are stars that pulse radially producing brightness changes with a very stable and regular period. Cepheid variables are an important reference for measuring distances in space.
Their pulsation periods vary between one day and about four months, and their brightness variations can be between 50 and 600% between the maximum and the minimum. Its name comes from its prototype or representative star, Delta Cefei.
The relationship between its average luminosity and the pulsation period was discovered in 1912 by Henrietta S. Leavitt, and is known as the period-luminosity relationship. Leavitt found that the luminosity of a cepheid increases proportionally to its pulsation period.
Thus, astronomers can determine the intrinsic luminosity of a cepheid simply by measuring the pulsation period. The apparent luminosity of a star in the sky depends on its distance to the Earth; comparing this luminosity with its intrinsic luminosity the distance at which it is found can be determined. In this way, cepheids can be used as indicators of distances both inside and outside the Milky Way.
There are two types of cepheids. The most common are called classical cepheids and the others, older and weaker, are known as W Virginis stars. The two types have different period-luminosity relationships.
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