The difference between a star's brightness measured through two different color filters, typically B (blue) and V (visual/yellow), expressed as B-V. It provides a quantitative measure of a star's color and temperature, with negative values indicating hot blue stars and positive values indicating cool red stars.
Developed in the early 20th century from photographic astronomy, combining 'color' from Latin 'color' and 'index' from Latin 'indicare' (to point out). The B-V system was standardized by Johnson and Morgan in the 1950s, revolutionizing stellar classification by providing precise temperature measurements.
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