During the late 19th century measuring visual double stars was one of the most popular branches of astronomy among both amateur and professional astronomers. Today only a small number of professionals remain active in the field. With more than 78,100 entries in the Washington Visual Double Star Catalog (WDS), 1996, the task of keeping tabs on these binaries is a monumental one indeed! The work, however, is wide open to amateur participation and is one of the areas where the dedicated amateur has the potential of doing professional-level work — and having a lot of fun to boot.

Why Measure Double Stars?

The main reason for measuring binary stars is to determine the total mass of a double-star system. Why? This information is of crucial importance to theorists working on stellar evolution. Indeed, our understanding has benefited greatly from thousands of measurements made by double-star observers since the time of Wilhelm Struve in the early 19th century.

Remarkably, only two basic pieces of information are necessary to determine the orbit of a double star: the position angle and separation of the pair. The position angle defines the location of the companion (the fainter star in a pair) relative to the primary star. The apparent separation of the primary and secondary is measured in arcseconds.