Tags: Star.
B-V colour is one of the two colour indices in the UBV photometric system.Astronomers measure the amount of light a star gives off in different colours. As a result of Wien’s displacement law hot stars give off more blue light than red; cool stars give off more red light than blue. Coloured filters are used to measure different wavelengths of light from stars. The magnitude of the star is measured first through a standardized B-band (blue) filter.