A quantitative measure of a star's color. Even a casual look at the night sky reveals the stars to be colored. From spectral analysis and the laws of radiation, it has been known since the 1920s that such color is closely related to temperature. Stars are reddish at about 3000 K (4940°F), orange at about 4500 K (7640°F), yellowish-white at about 6000 K (10,340°F), white at about 10,000 K (17,540°F), and bluish-white above 10,000 K. With proper calibration, color provides a means by which the temperature and spectral class can be evaluated. Visual examination is too crude, however. Instead, color must be properly defined and quantified.