Final answer:
Stars are classified on the H-R diagram based on temperature and absolute brightness.
Explanation:
The two characteristics used to classify stars on the Hertzsprung-Russell (H-R) diagram are temperature and absolute brightness. The H-R diagram plots stars based on their temperature (spectral class) against their luminosity. Temperature is associated with a star's spectral class, while luminosity (absolute brightness) determines the star's position on the diagram.
Learn more about Hertzsprung-Russell diagram here:
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