Double Stars

Abstract

SOME stars when looked at in a telescope are seen really to consist of two stars so near together that the naked eye is not able to distinguish them, but sees them as a single star.1 The knowledge of some of these objects may be presumed to be almost as old as the telescope. In fact Hevel remarked some of them in the middle of the seventeenth century, but no attention was paid to them, as it was thought that they were really far asunder in space, and merely appeared close together in the heavens, because they were nearly in the same visual ray. It thus escaped notice that one star frequently moves round the other, and Lambert, as late as 1761, founded his opinion that those fixed stars that appear near others, were in no physical connection with them, upon this absence of relative motion, because, as he says in his “Cosmologische Briefe,” if they do not move round each other, and still gravitate towards each other, they must long ago have collapsed. But a few years after the Rev. John Michell applied the rules of the calculation of probabilities to the stars in the Pleiades, and showed that it was exceedingly improbable that these stars could appear so near together, if their proximity was the result of a random scattering of the stars over the heavens, and he showed that among 40,000 stars, one could expect to find only one pair within twelve seconds of are of each other, and none nearer. These speculations were, however wholly conjectural, as long as no proper observations were available, and it was therefore to the purpose when the highly merited Jesuit, Christian Mayer, of the observatory at Mannheim, founded by the Elector of Pfalz, commenced to search for, and systematically to observe, double stars. But he met with no support from his contemporaries, and had to defend his opinions in several polemical pamphlets. His instrument, a mural quadrant by Bird, was scarcely sufficient for the purpose, and his opinion, that, “satellites” of the brighter fixed stars were found at a distance of as much as three degrees, was certainly wrong in the instances he adduced, though Mädler has shown that stars as far asunder may possibly be physically connected.2 We must, therefore, consider William Herschel to be the first who proved the existence of double stars. This he did by aid of micrometric measures,3 which he originally had made with the view of finding the parallax of fixed stars, similar observations having previously been attempted by the Rev. Roger Long, of Pembroke College, Cambridge, who, however, had not been very successful.