The magnitude of a star in blue light on the Johnson photometry system. The B filter is centred on a wavelength of 436 nm and has a bandwidth of 89 nm. It is the photoelectric equivalent of the older photographic magnitude, but modified to be free of ultraviolet light. There are also B filters in Geneva photometry, Walraven photometry, and the six-colour system, but the context should always make it clear when these are intended.
Subjects: Astronomy and Astrophysics.