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The Cepheids They are a special type of variable stars that change their luminosity cyclically, in times between 1 and 50 days. Its name derives from "delta Cefei", which is the first star of this type, discovered in 1784 by the English amateur astronomer John Goodricke.

Its physical structure is that of giant stars, up to 10 times the Sun, yellow. These are found both in our Galaxy, where they are preferably located in the arms of the spiral, as in others.

The importance of the Cepheids is enormous for the determination of stellar distances: they have been baptized as the military stones of the Universe.

Indeed, there is a very precise relationship between the cyclic variation of the luminosity of a Cepheid and its intrinsic luminosity, or absolute Magnitude, and, the longer this cycle is, the brighter the star is.

On the other hand, astronomers, measuring the apparent or visual magnitude of a star and knowing the absolute one, can determine its distance. In this way, each Cepheid represents a true distance indicator.

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