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combined magnitude

Combined magnitude is the total brightest of two or more celestial objects, such as the stars of a binary system that appear as a single point to the naked eye. Unfortunately, this can't be found simply by adding the separate magnitudes, m1 and m2, because the magnitude scale is logarithmic. Instead the combined magnitude, m, has to be calculated from the formula:

m = m1 - 2.5 log {1 + antilog [-0.4 (m2 - m1)]}.

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