Parameter defining how a star’s brightness varies with color. It is defined by taking the difference in magnitude (which is related logarithmically to intensity) at different wavelengths. For example, the B-V color index is defined by taking the difference between the magnitudes in the blue and visual regions of the spectrum and the U-B color index is the analogous difference between the UV and blue regions of the spectrum. The color index is directly related to the temperature of the Self-luminous object held together by its own self-gravity. Often refers to those objects which generate energy from nuclear reactions occurring at their cores, but may also be applied to stellar remnants such as neutron stars..
Some or all content above used with permission from J. H. Wittke.