The Southern Binary Gamma Centauri

Mr Andrew James

John Herschel first suspected that Gamma Centauri was a close double star on 2nd April 1835 and confirmed this one month later. Herschel was unable to achieve any useful measures, due to poor seeing conditions and the stars’ close proximity, during his first few observations of this binary system, but by October 1835, he had successfully resolved and measured the pair, finding the mean separation as 0.8 arcsec and the position angle of 173 degrees. By 1860 it was established as a true binary star, but in reality the initial 1835 position angle had caused much consternation to later observers and orbital mathematicians.

The interpretation of these results affected the derivation of the seven orbital elements, specifically, the duration of the orbital period ‘P’ and the size of the semi-major axis ‘a’. Worst these early measures suggested the stars were moving contrary to the retrograde motion of the binary that we see today.

This poster discusses the troublesome past observational history of the system - both as a known binary and as a possible triple system. Also presented is the current knowledge of the system as it now approaches its second periastron.

A short observational summary for amateurs explains how observers are able to resolve these main components into the immediate future.