As early as the 1840s Sir James South bemoaned the fact that F.G.W. Struve had swept the sky clear of new double stars and there was little left for him to do. Twenty or so years later when Burnham began to find many new pairs using a 6-inch telescope even T.W. Webb expressed the view that he could not hope to keep up this rate of discovery. In fact this was just the start of a golden period for visual discovery which lasted in essence until the middle of the last century. After that it is fair to say that minds were concentrated on getting more observations of the existing systems in order to accumulate stellar masses and dynamical parallaxes. Even so the work of Paul Couteau and Paul
Muller in France and Wulff Heintz in the USA indicated that there was no shortage of new pairs for those prepared to look for them with suitable apertures. The Hipparcos satellite which operated between 1989 and 1993 found about 15,000 new systems, some of which would have been too difficult for visual observers but some of the pairs can be resolved visually and the widest discoveries have been seen with very small telescopes. Hipparcos, and the associated Tycho mission which looked at other observations made by the satellite to a fainter magnitude but with less accuracy than the main mission, was by no means a complete survey.
In short there are still new double stars to be found either by lunar occultation or by visual examination in a concerted manner of, say, POSS films. As already mentioned, Schmidt survey films or prints can show stars down to 5'' separation. In his study of the pairs on POSS prints originally found on astrographic plates by Pourteau, Domenico Gellera noted a number of closer components in these systems. These pairs have not been confirmed so far but at typical magnitudes of 12-16 and separations of about 5'', these could be recorded with a 10-inch Schmidt-Cassegrain with a CCD camera (see Figure 16.1). The power of modern telescopes and CCD cameras is such that even pointing at a random area of sky, one is likely to record pairs which are not catalogued.
Direct visual discovery is another matter. New pairs still turn up and the French observer Jean-Claude Thorel using the 50-cm refractor at Nice has discovered four to date but these are by-products of a measurement programme rather than a deliberate attempt to survey for new discoveries. Sky conditions, particularly seeing, would need to be very good so that stars surveyed show sharp round disks and any close companion (within range of the telescope) would be relatively easily visible. It is one thing to measure a known pair whose separation is below the Rayleigh limit but it is quite another to discover one at the same distance.
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