How Hot Is Hot?

Stars are too distant to stick a thermometer under their tongue. We can’t even do that with our own star, the sun. But you can get a pretty good feel for a star’s temperature simply by looking at its color.
The temperature of a distant object is generally measured by evaluating its apparent brightness at several frequencies in terms of a blackbody curve. The wavelength of the peak intensity of the radiation emitted by the object can be used to measure the object’s temperature. For example, a hot star (with a surface temperature of about 20,000 K) will peak near the ultraviolet end of the spectrum and will produce a blue visible light. At about 7,000 K, a star will look yellowish-white. A star with a surface temperature of about 6,000 K—such as our sun—appears yellow. At temperatures as low as 4,000 K, orange predominates, and at 3,000 K, red.
So simply looking at a star’s color can tell you about its relative temperature. A star that looks blue or white has a much higher surface temperature than a star that looks red or yellow.