How is a luminosity class assigned to a star?

A luminosity class is assigned to a star based on its luminosity, size, and temperature as determined from its spectrum and position on the Hertzsprung-Russell diagram. Astronomers analyze the star's spectral lines to ascertain its surface gravity, which indicates whether it's a main sequence star, giant, supergiant, or another class. By comparing these characteristics with theoretical models of stellar structure and evolution, the luminosity class is classified accordingly. This classification helps in understanding the star's stage in its life cycle and its intrinsic brightness.