The spectral stellar classification scale, also known as the Harvard spectral classification, is a system used to categorize stars based on their spectra—the light they emit. This system classifies stars according to their temperature and the characteristics of their spectra, which are primarily determined by the star’s surface temperature.

The spectral types are designated by the letters O, B, A, F, G, K, and M, arranged from the hottest to the coolest stars. Each spectral type is further subdivided into ten categories using the digits 0 through 9, with 0 being the hottest and 9 the coolest within each type. For example, a G2 star is hotter than a G9 star but cooler than an F0 star.

In addition to spectral types, stars are also classified by their luminosity, which is related to their size and stage of evolution. This is indicated by Roman numerals in the Morgan-Keenan (MK) system