In astronomy, the color index (FI ) is an originally introduced by Karl Schwarzschild metric for the color of stars.
He is now generally defined as the difference between the measured apparent brightness (magnitude ) of a star in the short-wavelength region ( blue ) and the brightness in the long wavelength region ( red ):
From the definition of astronomical magnitude scale results then that a star appears more reddish, the greater the color index.
The different photometric systems are different on the respective reference values for the long-wave and short-wave region. The most common use today is the UBV system ( Dreifarbenphotometrie ) by Harold L. Johnson ( 1921-1980 ) and William Wilson Morgan. U stands for the brightness in the ultraviolet light with the center wavelength of 365 nm, B is the brightness at 440 nm (blue ) and V is the brightness at 550 nm ( yellow). V stands for visual, because the human eye perceives the most stars in the yellowish area. In the UBV system three color indices are based on these reference values formed: UB, UV and BV, where BV visual observer has the greater significance and is expressed eg in star catalogs often.
A (BV ) color index of 0.00 ± corresponds to the color bluish white ( Rigel ), 0.09 is white (star Deneb ), our sun has a color index of 0.65 (yellowish), examples of extremes are 119 Tauri with a color index of 2.06 ( deep red ) and Spica with -0.23 (blue)