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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.
From: B magnitude in A Dictionary of Astronomy »
Subjects: Astronomy and Astrophysics.
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