Why does a star's spectral clssification depend on it's temperature?

A star's spectral classification is determined by its temperature because temperature affects the ionization and excitation of atoms in the star's atmosphere. Hotter stars emit more high-energy photons, which can ionize elements and produce distinct spectral lines. These lines, observed in the star's spectrum, reveal the presence of different elements and their ionization states, thereby allowing astronomers to classify the star into specific spectral types (like O, B, A, F, G, K, M). Consequently, the temperature directly influences the star's spectral characteristics, informing its classification.