The spectral type of a star measures itsGroup of answer choicesmass.size.chemical composition.temperature.luminosity.

The spectral type of a star is primarily based on its temperature. It categorizes stars into different classes that correspond to various temperature ranges, which in turn affect their color and spectral characteristics. Stars are classified into spectral types using a system denoted by classes O, B, A, F, G, K, and M, with O-type stars being the hottest and M-type stars being the coolest.

In summary, while other factors like mass, chemical composition, and luminosity are relevant in understanding stars, the spectral type specifically measures and indicates a star's temperature. Hence, the correct answer from the choices provided is temperature.