What four properties does the HR diagram use to classify stars?

The HR diagram classifies stars based on four key properties: luminosity (or absolute magnitude), surface temperature (measured in Kelvin), spectral classification, and color. Luminosity indicates the total energy output of a star, while surface temperature affects its color and spectral type. Spectral classification categorizes stars based on their absorption lines and temperatures, leading to the identification of different stellar types. Together, these properties help illustrate the relationships between different types of stars and their evolutionary stages.