color index

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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 classspectral class,
in astronomy, a classification of the stars by their spectrum and luminosity. In 1885, E. C. Pickering began the first extensive attempt to classify the stars spectroscopically.
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 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

The difference between the apparent magnitude of a star measured at one standard wavelength and the apparent magnitude at another (always longer) standard wavelength. Its value depends on the spectral distribution of the starlight, i.e. whether it is predominantly blue, red, etc., and it is therefore an indication of the color (i.e. temperature) of the star. It is independent of distance. Prior to the UBV system the international color index was mainly used; this is the difference between photographic and photovisual magnitudes (m pgm pv ). In the now widely used UBV system (see magnitude) color index is usually expressed as the difference B V , where B and V are the magnitudes measured with blue starlight (at a wavelength of 440 nanometers) and greenish-yellow starlight (550 nm), respectively. The color index U B is also used, where U is the apparent magnitude measured with ultraviolet radiation (365 nm) from a star.

Stars are classified into spectral types, which are further subdivided into luminosity classes; each has a characteristic intrinsic color index, given as (B V )0 and (U B )0. These two indices are defined as zero for unreddened A0 main-sequence stars, such as Vega, and are therefore negative for hotter stars, i.e. those emitting more ultraviolet (O and B stars), and positive for cooler ones (A1 to M stars). Since color index is easily measured, it is usually used on graphs in preference to spectral type or temperature. Any excess of the measured value of color index of a star over the expected intrinsic value indicates that the starlight has become reddened by passage through interstellar dust (see extinction). The difference between the values is the color excess, E , of the star:

E = (B V) – (B V )0

The value of E gives the amount of reddening.

There are also color indices relating to magnitudes measured at red and infrared wavelengths. For example, in the indices V R and V I , I and R are the magnitudes measured at 0.7 μm and 0.9 μm.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

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.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

color index

[′kəl·ər ‚in‚deks]
Abbreviated CI.
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.
The amount of hemoglobin per erythrocyte relative to normal, equal to the percent normal hemoglobin concentration divided by percent normal erythrocyte count.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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