Question

how are stars classified on the hertzsprung - russell (h - r) diagram?
by their size and distance from earth
by their color and the number of planets orbiting them
by their brightness (luminosity) and surface temperature
by their age and location in the galaxy

Explanation:

The Hertzsprung - Russell (H - R) diagram plots stars based on their brightness (luminosity) on the vertical axis and surface temperature (which is related to color) on the horizontal axis.

Answer:

C. By their brightness (luminosity) and surface temperature