What criterion is used by astronomers to systematically categorize stars based on the Harvard spectral classification system?

Answer

Patterns of light absorbed in their spectra

Astronomers rely on the patterns of light absorbed in a star's spectra to assign letters in the stellar classification system, which serves as a proxy for temperature.

What criterion is used by astronomers to systematically categorize stars based on the Harvard spectral classification system?
physicsastronomytemperaturestarclass