Spectral types

(or spectral classes) is a system in which stars are classified according to thier spectra.

At the beggining, the astronomers classified the stars according to the strength of hydrogen absorption lines. In this system the catagories goes from A to B,.. according to the decreasing strength of their hydrogyn lines.

Now, given that the different spectral types correspond to different surface temperatures, and that the sequence A, B, . . . is not a temperature-ordered sequence. The Spectral Classes are, in order of decreasing temperature, O, B, A, F, G, K, M. Each is divided into 10 subclasses, identified by a number from zero to nine. For some of the hotter spectral types, there is even use for half subclasses.

It was originally thought that stars became cooler as they evolved, so that the temperature sequence was really an evolutionary sequence. Therefore, the hotter spectral types were called early and the cooler spectral types were called late.

Even though that these evolutionary ideas are not correct. However, the nomenclature still remains, and stars like a B0 or B1 star still discribed as an ‘early B’ and a B8 or B9 as ‘late B’.