Temperature is on the horizontal axis and it decreases to the right, so hot blue stars are on the left. Luminosity (the total power the star gives out, compared with the Sun) is on the vertical axis. Both axes use a log scale, so each step is ten times (or double) the one before.

Wien's law: hotter stars have their peak at shorter wavelengths.

λ_max = b / T,   b = 2.898 × 10^-3 m K

Stefan-Boltzmann law: each square metre of surface gives out σT⁴, so the luminosity depends on both temperature and size. This is how the radius tile is worked out.

L = 4πR²σT⁴   so   R / R_Sun = √(L / L_Sun) / (T / T_Sun)²

Stars on a line of equal radius are all the same size. Giants are cool but very luminous, so they must be huge. White dwarfs are hot but dim, so they must be tiny. The dots get bigger with radius, but they are not to scale. Values are typical published estimates and some (for example, Betelgeuse's luminosity) are uncertain. The temperature bands for the spectral classes are approximate.