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UBV photometric system

(Johnson System, Johnson-Morgan System)
(commonly-cited photometric system for describing stars' colors)

The UBV photometric system, also called the Johnson system or Johnson-Morgan system, is an early well-accepted photometric system and is a system generally assumed if no other system is specified or implied. It specifies U, B, and V passbands, in turn, specifying specific B-V and U-B color indices. Magnitudes are calibrated as zero for A0 V stars unaffected by reddening.

The UBVRI photometric system, i.e., Johnson-Cousins system is an extension with red and infrared passbands, R band and I band.

Further reading:

Referenced by pages:
color index
I band
north polar sequence (NPS)
photometric system
rare designator prefixes
Strömgren photometric system
Vega system