system of classifying stars by spectral type, based on photometric measurements of the ultraviolet (U), blue (B), and visual (V 【yellow】) magnitudes. These magnitudes are measured through filters sensitive to light at wavelengths of 360, 420, and 540 nanometres, respectively. This system was introduced in the early 1950s by the American astronomers Harold Lester Johnson and William Wilson Morgan (Morgan, William Wilson) and has largely superseded the less accurate system using the north polar sequence.