UBV photometric system

UBV system, even Johnson system or Johnson -Morgan system called, is a broad-band photometric system.

The letters U, B and V stand for ultraviolet, blue and visual magnitudes, which are measured in order to classify him in the UBV system.

The choice of the blue end of the spectrum was due to limitations of the photographic film. The system was introduced in the 1950s by the astronomer Harold Lester Johnson and William Wilson Morgan. The filters were then selected so that the principal wavelength of the response functions at 364 nm of U, 442 nm and 540 nm for B. V.. The zero point of the BV and UB color index ' was selected for A0 V stars, which are not affected by interstellar reddening.

The UBV system has a disadvantage: the short wavelength limit the U- filter are much more limited by the earth's atmosphere than the filter itself Therefore, the observed size classes in the level and composition of the atmosphere can change. However, many stars have been measured with this system, including many of the bright stars.