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.
A star's color index is like its cosmic fingerprint for temperature! The Sun has a B-V of +0.65, making it officially yellow-white, while the hottest stars have negative color indices - they're so blue-hot they emit more blue light than visual light.
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