which piece of spectral data is necessary to determine the spectral class of a star

To determine the spectral class of a star, the most relevant piece of spectral data is **temperature**.

The spectral class is primarily based on the star's temperature, which influences the absorption lines present in its spectrum. The classification system (O, B, A, F, G, K, M) is ordered by decreasing temperature, with O-type stars being the hottest and M-type stars being the coolest.

While apparent magnitude, luminosity, and absolute magnitude provide valuable information about the star's brightness and distance, they do not directly define its spectral class.