The quantification of a star's color. Color index is determined by subtracting the V (visual) magnitude from the B (blue) magnitude. A white star with a temperature of 9200 K will have a color index (B-V) of 0.0. Blue stars have slightly negative color indexes. Yellow, orange and red stars have increasingly positive color indexes. The full range runs from about -0.4 (blue) to +2.0 (red carbon stars).
In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature. To measure the index, one observes the magnitude of an object successively through two different filters, such as U and B, or B and V, where U is sensitive to ultraviolet rays, B is sensitive to blue light, and V is sensitive to visible (green-yellow) light (see also: UBV system). The set of passbands or filters is called a photometric system.