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

To determine the spectral class of a star, the most critical piece of spectral data is the absorption line spectrum. This spectrum reveals the presence and characteristics of various absorption lines, which correspond to different elements and ionization states in the star's atmosphere. By analyzing these absorption lines, astronomers can classify the star according to the Harvard spectral classification system (which includes classes O, B, A, F, G, K, and M) based on temperature and the prominence of specific spectral features such as hydrogen, calcium, sodium, and other elements. The strength and presence of these lines, along with the overall continuum of the spectrum, provide vital information about the star's temperature, composition, and physical properties.