Which characteristics are used to classify stars on the Hertzsprung-Russell diagram? Check all that apply.

size

density

composition

temperature

absolute brightness

Answer is: temperature and absolute brightness.

The Hertzsprung-Russell diagram shows the relationship between the star's absolute luminosities (the total amount of energy emitted per unit of time by a star) versus their effective temperatures (the temperature of a black body that would emit the same total amount of electromagnetic radiation).

The Hertzsprung-Russell diagram puts each star on a graph according the star's brightness against its temperature.

temperature and absolute brightness for sure. I know its not the composition because they use spectographs for that but not sure about size and density.