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A Hertzsprung-Russell diagram plots stellar luminosity against effective temperature or a calibrated spectral classification. Conventionally, high luminosity is upward and high temperature is to the left. The Stefan–Boltzmann law, $L=4\pi R_{\star }^2\sigma T_{\mathrm{eff}}^4$, explains why stars of equal temperature but different luminosities must have different stellar radii.

The main sequence crosses the diagram, giant stars occupy a luminous region, and white dwarfs lie at low luminosity despite their high temperatures. An evolutionary track follows one model star through this space; a population diagram instead combines many stars at once. A colour-magnitude diagram is the observational counterpart, requiring distance, extinction and spectral calibration for the conversion.

TODO: the plotted location of Procyon B is missing from the damaged source diagram. The Stefan–Boltzmann law would determine the stellar radius from its effective temperature and bolometric luminosity:A visual luminosity alone must first be converted with the appropriate bolometric correction. Procyon B is a white dwarf, so its small stellar radius and hot surface place it below the main sequence, rather than among giant stars or supergiants. This classification does not supply the missing numerical coordinates. A source-based numerical stellar radius cannot be claimed from the surviving image.^[ref]