A color index is a quantitative measure of the color, and thus the spectral type, of a star. It is derived from the difference
in the apparent stellar
magnitude as measured using two photometric systems of different and accurately known color sensitivity. A commonly used set of three bands, defined by filters and by the color sensitivity of the detectors, is the UBV (ultraviolet blue visual) system. Each of the UBV bands is about 1000 angstroms () wide, and they are centered at 3500, 4300, and 5500 , respectively. In this system the color index is found by subtracting the visual magnitude from the blue (B-V) or from the ultraviolet (U-V) magnitude. Other multicolor systems extending into the infrared have also been found useful (see Hertzsprung-Russell diagram).
The interest of measuring the color index lies in the connection between the color of a star and its photospheric
temperature. Since a star radiates only approximately as an idealized blackbody (see blackbody radiation), for which a temperature-color relation is known, a color index does not yield a good estimate of the absolute photospheric temperature. Hotter stars, however, tend to radiate relatively more of their total output at shorter wavelengths. Consequently, color indices allow the best measure of the difference in spectral type and relative temperature in a set of stars, to a precision of about 1 percent. Careful corrections must be made for the starlight's passage through the Earth's atmosphere and interstellar dust, which affect various wavelengths differently.
Because the magnitude of a hot star will be greater in the blue region of the spectrum than in the visual, blue, or hot, stars have more negative color index values than cooler, redder stars. The actual values of the B-V color index range from about -0.3 for the hottest young supergiants at 50,000 K, to about +2.0 for the coolest observable main sequence dwarf stars. The B-V color index of the Sun is +0.62.