UBV photometric system


The photometric system called UBV, also called the Johnson system, is wide band, and usually employed for classifying stars according to their colors.
It has been the first known standardized photometric system. The letters U, B, and V stand for ultraviolet, blue, and visual magnitudes, which are measured for a star; two subtractions are then performed in a specific order to classify it in the system.
The choice of colors on the blue end of the spectrum is because of the bias that photographic film has for those colors. It was introduced in the 1950s by American astronomers Harold Lester Johnson and William Wilson Morgan. A telescope and the telescope at McDonald Observatory were used to define the system.
The filters are selected so that the mean wavelengths of response functions are 364 nm for U, 442 nm for B, 540 nm for V. Zero points were calibrated in the B−V and U−B color indices selecting such A0 main sequence stars which are not affected by interstellar reddening. These stars correspond with a mean effective temperature of between 9727 and 9790 Kelvin, the latter being stars with class A0V.
The UBV system has some disadvantages. The short wavelength cutoff that is the U filter is defined mainly by the terrestrial atmosphere rather than the filter itself; thus, it can vary with altitude and atmospheric conditions. However, many measurements have been made in this system, including many of the bright stars.

Extensions

The Johnson-Cousins UBVRI photometric system is a common extension of Johnson's original system that provides redder passbands.