Blue stars have the hottest surface temperatures. A star’s visible color is closely related to the wavelength of light it emits most strongly, and hotter stars radiate more toward the blue and ultraviolet parts of the spectrum.
Blue stars are generally classified among the hottest ordinary stars, with surface temperatures often above 10,000 kelvin and the hottest blue-white O-type stars reaching roughly 30,000–50,000 kelvin or more. By comparison, the Sun’s visible surface is about 5,800 kelvin and appears yellow-white, while cool red stars can have surface temperatures below 4,000 kelvin.
This is the opposite of the everyday association between red and heat. In astronomy, the color describes the temperature of the star’s photosphere, or visible surface—not the temperature of its core. A blue star is not necessarily larger than every red star, although many blue stars are massive, luminous, and relatively young.
Astronomers use stellar spectra and color indexes to estimate temperature. Dust, distance, and Earth’s atmosphere can alter a star’s apparent color, so professional measurements are more precise than visual inspection alone.