The effective temperature of a star is the temperature of its visible surface, as opposed to the core at which it generates its energy through thermonuclear reactions or the rarefied corona of great heat where electrons meet ionized gases with the radiation of heat but in so sparse a gas that it is invisible. The color of a star indicates its temperature along a spectrum from the very cool (by stellar standards, that is) red M stars that radiate heavily in the infrared to the very blue O stars that radiate largely in the ultraviolet. The effective temperature of a star indicates the amount of heat that the star radiates per unit of surface area. From the warmest surfaces to the coolest is the sequence of star types known as O, B, A, F, G, K, and M. The effective temperature of a star's surface is identical with the ironically-named black-body temperature of the visible surface.

A red star could be a tiny red dwarf, a star of feeble energy production and a small surface or a bloated giant or even supergiant star such as Antares or Betelgeuse, either of which generates far greater energy but passes it through a surface so large that the star radiates little per unit of surface area. A star near the middle of the spectrum, such as the modest Sun or the giant Capella radiates more heat per unit of surface area than the feeble red dwarf stars or the bloated supergiants, but much less than such a white or blue star as Vega or Rigel.