Although the Harvard classification scheme had been created it still wasn’t clear why there was the variation in the spectra. It had been thought that the quantity atoms of a given element define the intensity of the lines of that element in the spectrum. Investigations show that the chemical composition of the stars is not very variable. The most common element is Hydrogen 70%, after that – Helium 28%, and all other elements are 2%.

In class O are the stars have the highest temperature - 30,000K. The stars from class B are blue – white with temperature 20,000K. The stars from class A are white and their average temperature is 10,000K. Class F – 7,000K. From Class G are yellow with spectrums like that on the Sun and temperature - around 5,500K. The stars from class K are orange and temperature is 4,000K; thos from class M are red with temperature of 3,000K. Every spectral class is divided into ten subclasses, marked with digits, from 0 to 9. For example: G1, G2 … G9.