The correct statement is:
The Hertzsprung-Russell diagram plots stars according to their luminosity and temperature, which is also associated with spectral class.
This accurately describes the purpose of the Hertzsprung-Russell diagram, as it illustrates the relationship between a star's luminosity (brightness) and its temperature (which is often indicated by color and spectral class).
To clarify the incorrect statements:
- Stars with the highest temperatures actually appear towards the left side of the Hertzsprung-Russell diagram, not the right.
- The diagram does not plot stars based on their size and shape.
- The Sun is classified as a G-type star and is located in the middle of the diagram, not the far left.