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.

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Color Index 

in astronomy, the difference between the stellar magnitudes obtained for two wavelength bands. It describes the main features of the energy distribution in the spectrum of a celestial body, that is, its color. The concept of color index was introduced by K. Schwarzschild in the early 1900’s. The international color index, expressed as the difference between the international photographic and photovisual magnitudes, was the basic index until the 1950’s.

In contemporary astronomy, the most widespread photometric system (UBV) usually uses the U—B and B—V color indexes, which correspond to the difference of stellar magnitudes in the ultraviolet (U), blue (B), and yellow (V) regions of the spectrum. By extending the UBV system into the red and infrared regions (to obtain R and I magnitudes), other color indexes, such as V —R and V—I can be obtained. The zero point of a color index is fixed in such a way that all color indexes register zero for a series of selected similar dwarfs of spectral class AO. The B—V and U—B color indexes are negative for stars of earlier spectral classes (“bluer” stars) than for AO and are positive for later spectral classes (“redder” stars). The zero point of a color index may be different in other photometric systems.

Color indexes are determined either photographically or photoelectrically and are used to study interstellar absorbtion of light and the nature and evolution of stars, stellar systems, and other objects.

A. S. SHAROV