color index, in astronomy, difference in an object's brightness as recorded between any two well-defined bands of the electromagnetic spectrum by using optical filters of different colors. If blue and red filters are used, then the color index B-R would be zero for a white star (spectral class A0). The color index is positive for stars redder than a white star and negative for stars bluer than a white star. In effect, measuring the color index is equivalent to measuring the difference between the amount of blue light and red light that the star radiates. Although a star's true color might be changed as its light travels through clouds in interstellar space, the color index is an important indicator of the temperature of a star, gas cloud, or galaxy.
"color index." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (April 23, 2019). https://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/color-index
"color index." The Columbia Encyclopedia, 6th ed.. . Retrieved April 23, 2019 from Encyclopedia.com: https://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/color-index