UBV system

The UBV system is a system of stellar magnitudes, devised by Harold Johnson (1921–1980) and William Morgan at Yerkes Observatory, which consists of measuring an object's apparent magnitude through three color filters: the ultraviolet (U) at 3600 Å; the blue (B) at 4200 Å; and the visual (V) in the green-yellow spectral region at 5400 Å. It is defined so that, for A0 stars, B - V = U - B = 0; it is negative for hotter stars and positive for cooler stars. The Stebbins-Whitford-Kron six-color system (U, V, B, G, R, I) is defined so that B + G + R = 0. The difference, B - V is a good measure of a star's actual color. For example, Betelgeuse has B - V = 1.85, indicating that it is quite red. On the other hand, Rigel has B - V = -0.03, indicating that it is bluish. Most stars fall between these extremes, except for a few redder-than-red stars (mostly carbon stars) and a few bluer-than-blue stars (mostly young, high-mass stars).