What crucial information is required to convert the angular separation measured for a visual binary into the physical semi-major axis ($a$) needed for the mass calculation?
Answer
The distance to the binary system, typically determined through stellar parallax measurements.
Angular separation measured in arcseconds must be converted into a physical distance in astronomical units (AU) before being used in the mass calculation, a conversion that necessitates knowing the system's distance, usually found via stellar parallax.

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