The classes of stars, represented by letters at the bottom of the diagram, correspond to stellar temperatures. Their order, from hotter to cooler, is O, B, A, F, G, K, M.

(Some have suggested remembering this by the following phrase: "Only Boring Astronomers Find Gratification Knowing Mnemonics.")

Stars on the left-hand side of the Hertzsprung-Russell Diagram are hotter and bluer than stars on the right-hand (cooler) side. Stars on the upper part of the diagram have a greater luminosity, while those on the lower part of the diagram are less luminous.

Lines of constant stellar radius (size) run at an angle across the entire diagram, from the upper-left down to the lower-right. A star at a given temperature falling on the upper (brighter) part of the diagram will be larger than a star at the same given temperature falling on the bottom (dimmer) side. In order to know a star's size, it is necessary to know something about both its temperature and its luminosity.