color index

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.

A quantitative measure of a star's color. Such color is closely related to temperature. Stars are reddish at around 3000 K (5000°F), orange about 4500 K (7500°F), yellowish about 6000 K (10,300°F), white about 10,000 K (17,500°F), and bluish above 10,000 K. With proper calibration, color provides a means by which the temperature and spectral class can be evaluated. See also Spectral type; Star.

Color is defined through the star's magnitude m, which indicates brightness. The original magnitudes were visual, being estimated with the unaided eye, and depended on the eye's response, which is maximized for yellow light. Early photographic emulsions, however, were sensitive primarily to blue light. As a result, blue stars look relatively brighter to the photographic plate than they do the eye, and red stars considerably fainter. It was then necessary to establish a separate magnitude system for photography, called m_ptg. The original visual magnitudes were distinguished by calling them m_v. The two magnitudes were set equal for white stars with temperatures of 9300 K (16,280°F). Color can then be quantified by taking the difference between the two magnitudes, the color index becoming m_ptg − m_v. Blue stars have slightly negative color indices, red stars positive ones. See also Astronomical photography.

Photoelectric photometry allowed the establishment of the more precise UBV system, wherein V (yellow) and B (blue) filters respectively mimic the response of the human eye and the untreated photographic plate, and a U filter is added in the ultraviolet. The traditional color index is then replaced by B − V, and the addition of U allows a second color index, U − B. See also Hertzsprung-Russell diagram; Interstellar extinction.

The standard system has been expanded into the red and the infrared with other filters. Numerous color indices are then available to examine stars that radiate primarily in the infrared. Other photometric schemes, such as the Strömgren four-color system (uvby, for ultraviolet, violet, blue, and yellow), allow more sophisticated color indices that are responsive not only to temperature but to a variety of other parameters. See also Infrared astronomy.