colour index kleurindex The difference between the apparent magnitude of a star at two different wavelengths (always the shorter-wavelength magnitude minus the longer-wavelength magnitude) to give a quantification of the star's colour. The magnitude of an object at different wavelengths are measured by using different filters before the detector. Often the Johnson system with UBV passbands are used. Other passbands may also be used (for instance g-r). quantity 量 magnitude magnitude