The overwhelming majority of stars have surface temperatures between about 2,500 and 40,000°C. At the low end of the temperature range are two types of stars, red dwarfs and red giants, that could hardly differ more in size and luminosity. A similar disparity exists between blue supergiants and the central stars of planetary nebulae, which together account for some of the hottest stars known.

Rising temperature implies changing color, from red, through orange, yellow, and white, to bluish-white. Some stars, however, emit a large portion of their total energy output beyond the visible range of the spectrum. Protostars and brown dwarfs, for example, give off mostly infrared radiation, whereas Wolf-Rayet stars, O stars and the central stars of planetary nebulae are powerful emitters of ultraviolet radiation.