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Cepheid of stars class of variable brightness with regularity, for which is known the relationship between the period of variation and the absolute brightness. The prototype of this class is the Cephei, discovered in 1784. Note the absolute brightness, determined by the period of the change, and observed the apparent, it determines the distance of the Cepheid, which represents a distance indicator also storage of star or galaxy where it belongs. A Cepheid variable is a member of a particular class of variable stars, notable for a very tight correlation between their period of variability and absolute stellar luminosity. Thanks to this correlation, and the great precision with which the pulsational period, Cepheid variables can be used as standard candles is measured to determine the distance of globular clusters and galaxies in which they are contained. 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. The name of this class of stars is derived from δ Cephei, the first variable of this type observed in our galaxy. Subsequent observations have identified Cepheid stars in other galaxies, primarily in the two Magellanic clouds. A Cepheid is usually a young yellow giant star population I and intermediate mass that pulsates regularly expanding and contracting, thus changing its brightness in an extremely regular cycle. The brightness of Cepheid stars is generally between 1000 and 10,000 times that of the Sun and the oscillation period is from the agenda of the hundreds of days. The brightness profile of a Cepheid star during a pulsational cycle is typically not symmetrical, with the shorter and steeper ascending arm of the descending, and in addition to its main peak of luminosity curve often has a second peak, or “bump”, the position of which relative to the parent varies according to the period of oscillation of the pulsator itself.