effective temperature
/ ef-EK-tiv /
Heat a piece of iron and it glows: first dull red, then orange, then yellow-white, and if you could push it hotter, blue-white. The colour tells you the temperature. Stars do the same thing on a grand scale — a star's colour is a thermometer reading of its surface. The effective temperature is that reading: a single number that captures how hot the glowing outer layer of a star is.
More precisely, a star is not a perfect uniform surface, so astronomers define its effective temperature as the temperature of an ideal radiator (a blackbody) that would give off the same total energy per unit area. By the Stefan-Boltzmann law, energy radiated per unit area grows as the fourth power of temperature, and by Wien's displacement law, the colour at which a star is brightest shifts to shorter (bluer) wavelengths as it heats up. The Sun's effective temperature is about 5,800 kelvin and it peaks in yellow-green light; a red dwarf may be around 3,000 K, a hot blue star above 20,000 K.
Effective temperature is the horizontal axis of the Hertzsprung-Russell diagram (usually drawn with hot on the left), and it is the physical meaning behind a star's spectral type and colour index. It does not describe the searing core, only the photosphere — the visible surface from which light escapes. Two stars of identical temperature can differ wildly in luminosity if one is far larger, which is exactly how the HR diagram separates giants from dwarfs.
Betelgeuse glows distinctly red-orange because its surface is only about 3,500 K, while Rigel a few degrees away glows blue-white at around 12,000 K. Same constellation, opposite ends of the temperature scale — you can almost read it with your naked eye.
Colour is a thermometer: redder is cooler, bluer is hotter.
Counterintuitively, blue stars are the hot ones and red stars are the cool ones — the opposite of how we label faucet taps. Effective temperature also refers only to the surface; a star's core is millions of degrees hotter.