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Cepheids are a type of radial pulsating variable stars of high luminosity with periods ranging from 1 to 135 days and whose magnitudes vary between hundredths to 2 units.
They are the pulsating variable stars that present the least irregularities in the duration of their period. Unlike non-radial variables, they retain their spherical shape during pulsation.
One of the main characteristics that allows them to be distinguished from other variable stars is that the width of the light curve varies according to the band of the visual spectrum in which they are observed. In particular, the modulations appear more pronounced at lower wavelengths, such as blue and ultraviolet. It has been found that the spectral type is more advanced the longer the period is. The spectral type at the maximum is F, and at the minimum they can range between G and K.
But of all the characteristics that these stars present, the most important is the relationship between their period and their luminosity.
This feature has been used to measure cosmic distances, since these stars are so bright that they have been seen in other galaxies.

The most important consequence of the period-luminosity relationship is that it provides a method for evaluating the absolute magnitude of a Cepheid. Once this is known, it is possible to know the distance by calculating the difference with respect to the apparent magnitude (distance module). For this reason, Cepheids have the important role of distance indicators and have been used for this purpose continuously. The relationship stated by H. Leavitt is of the form:

M = a log P + b

where M is the mean value of the absolute magnitude and P the period. Since the coefficient a is negative, the more luminous the Cepheid, the longer its period. Initially, we can classify

Cepheid stars into two large groups:

- The typical Cepheids whose type star is delta cephei, called population I.

- The W Virginis type Cepheids to which the population II stars belong.

In Aquarius we can find to Cepheids: