The Star Color Index

B - V

Apparent Magnitude

Photometry is the measurement of the brightness and color of stars. The visual or apparent magnitude of a star approximates to the brightness of the star as seen by the unaided eye. This magnitude may be designated as V for visual.

Photographic Magnitude

Currently photometry is carried out with electronic instruments, but in earlier years photometry was carried out by measuring the exposure produced by the image of a star on a photographic plate. In comparison with the response of the human eye, a photographic emulsion is relatively more sensitive to short wavelength, blue light than it is to red. Thus a blue star would produce more exposure on a photographic emulsion than would be expected from its Apparent Magnitude, while a red star would produce less. Therefore the unfiltered photographic image of a star gives a measure of a stars magnitude weighted toward the blue end of the spectrum. This is the B (for blue) or photographic magnitude. (Optical and electronic filtering allow modern electronic detectors to approximate the color response of the eye or of a photographic emulsion.)

The Color Index

The difference B - V between the two magnitude estimates is known as the "B- V color index of the star" ( or just the "color index" for short). It gives a numerical measurement of the color of a star. For blue stars will be negative, while for very red stars, it will be a positive number.

The color index correlates with both the spectral type and perceived color of the star, as shown in the following table (See Kaler, p. 80).

Color Index Spectral Type Approximate Color
1.41 M0 Red
0.82 K0 Orange
0.59 G0 Yellow
0.31 F0 Yellowish
0.00 A0 White
-0.29 B0 Blue


Copyright © 1998 - 2010 by Arnold V. Lesikar,
Professor Emeritus
Dept. of Physics, Astronomy, and Engineering Science,
St. Cloud State University,St. Cloud, MN 56301-4498

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