Final answer:
The Hertzsprung-Russell diagram is a plot of stellar luminosity against surface temperature. Stars of the same temperature can fall into different luminosity classes. High-mass stars are found above the main sequence as giants or supergiants, while low-mass stars are found below the main sequence as white dwarfs.
Step-by-step explanation:
The Hertzsprung-Russell (H-R) diagram is a plot of stellar luminosity against surface temperature.
The main sequence is a diagonal line that extends from high temperature and high luminosity to low temperature and low luminosity.
Stars of the same temperature can fall into different luminosity classes on the H-R diagram, which are determined by studying the details of their spectra.
High-mass stars, which emit more energy and are hotter, are found above the main sequence as giants or supergiants. Low-mass stars are found below the main sequence as white dwarfs.