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.

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?
measurementAstronomerstarmassbinary star system