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From the second half of the 19th century the Italian Jesuit astronomer Angelo Secchi (1818-1878), observing the Spectra of the stars (that is, those stripes with the colors of the rainbow that are obtained by passing the light through a prism) , he noted that they had different characteristics according to the surface temperatures of the stars themselves. The classification of stars according to their spectrum is called, since then, spectral classes.
Temperatures, in turn, are closely related to the color of the stars: the hottest emit a white-blue light and the colder a dark-red light.
Our Sun, which has an intermediate temperature between these two extremes, emits, as is known, a predominantly yellow light. Therefore, Secchi pointed out the basis of the spectral classification that, in its essential lines, is still followed.
The stars are divided into 10 spectral classes, each of which is assigned a letter of the alphabet in this sequence: O, B, A, F, G, K, M, R, N, S. To the first letters correspond the hottest stars, characterized by the simplest spectra; to the last, the coldest, spectra of increasing complexity. Superheating stars, called Wolf Rayet type by the name of the astronomers who studied them, are indicated by the letter W and sometimes associated with the O, at the head of the succession.
As in each spectral class, that is, in each letter, there are different varieties of stars, a subsequent division into 10 spectral types has also been created for each letter.
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