The stars seem blue, white, yellow or red, depending on their surface temperature: the first of this color scale are the hottest, and the last the least.
Our Sun, for example, is a yellow star (average surface temperature of 6,000 degrees Celsius).
is defined as color index of a star the difference between its visual and photographic magnitude. In general, the two measures do not correspond because, at parity of magnitude, the different colors of the stars impress the photographic emulsion differently.
For the blue ones, the color index is negative, because the human eye values its magnitude as lower with respect to the measurement in the photographic emulsion. For the red ones, the color index is positive, because the human eye gives a measure of its magnitude greater than that of the emulsion.
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