color index

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.

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color index [′kəl·ər ‚in‚deks]

Abbreviated CI.

(astronomy)

Of a star, the numerical difference between the apparent photographic magnitude and the apparent photovisual magnitude.

More generally, the difference in apparent magnitudes between two specified spectral regions.

(pathology)

The amount of hemoglobin per erythrocyte relative to normal, equal to the percent normal hemoglobin concentration divided by percent normal erythrocyte count.