History of Double Star Observation

In 1610 the invention of the telescope by Galileo gradually led to the discovery of telescopic double stars but these were noted merely by the way. In 1617 Castelli found that Mizar was itself double4 and he later added a few more pairs. In 1664 Robert Hooke was observing the comet discovered by Hevelius when he came across Y Arietis, a pair of pure-white stars of 4th magnitude separated by some 8''.

Over the next hundred years or so a few more double stars were noted but not catalogued in any determined manner, but this was to change when the Reverend John Michell first suggested that double stars were not merely a line-of-sight effect but that the two components really revolved around each other under a mutual gravitational influence, implying that Newton's laws applied to objects outside the Solar System. In Philosophical Transactions for 1767, Michell says: "it is highly probable in particular, and next to a certainty in general, that such double stars, &c, as appear to consist of two or more stars placed close together, do really consist of stars placed near together, and under the

Table 1.3. The first ten telescopic double star discoveries





1617 Jan


ß Mon

1617 Jan 30


9 Orionis ABC

1617 Feb


ß Sco



Y Ari





G.D. Cassini

Z Cnc AB-C

1680 Mar 22


a Crucis



a Centauri



Y Virginis

1 71 8


influence of some general law, whenever the probability is very great, that there would not have been any such stars near together, if all those that are not less bright than themselves had been scattered at random throughout the whole heavens".

A small catalogue of double stars was compiled in 1780 by Christian Mayer of Mannheim but the next great step was taken by William Herschel who turned his unprecedently powerful telescopes on many bright stars to find that even at high power, some stars appeared as very close pairs. In an attempt to measure stellar parallax, Herschel argued that in unequally bright, close pairs by measuring the position of the faint (hence distant and fixed) star with respect to the bright (or nearby) star he should be able to measure the parallactic shift and hence the distance of the latter. This idea he attributes to Galileo. To prove this he used filar micrometers of his own construction to measure the position of the fainter star with respect to the brighter. However, instead of seeing a six-monthly "wobble" in the position of the bright star with respect to the faint, Herschel found that the relative motion between the two stars was curved and could only be explained if the stars were revolving around a common centre of gravity. He had proved that binary stars existed but the mathematical confirmation came six years after his death, in 1828, when the French scientist Savary used the pair £ UMa (which Herschel had discovered) to show that the apparent orbit of the fainter star around the brighter (assuming the latter was fixed) was an ellipse.

The significance of this work was that it gave an estimate for the ratio of the stellar masses in a binary star system. This resulted in a great impetus in the visual observation of double stars and over the next 50 years or so many rich amateur astronomers in Europe dedicated time and money to making micrometric measurements, or paid someone to do it for them. Dawes, in England, and particularly Baron Ercole Dembowski, in Italy, and others, flourished but without the excitement of discovery the work lost momentum and became largely unfashionable by the turn of the century.

In 1857 when Bond first imaged Mizar with the Harvard 15-inch refractor the advantages of photography for double-star astronomy were not immediately realised, partly because the resolution obtained initially did not allow much work to be done in the orbital pairs of relatively short period. For those bright pairs where the separation was such that both components could be imaged at all parts of the orbital cycle, such as 70 Oph, it was possible to determine individual masses from the size of the apparent ellipses that each star traced out against the stellar background. It was not until the middle of the last century that observers such as Willem Luyten, Peter van de Kamp and Wulff Heintz used photography much more purposefully. Luyten, in a long career, found many pairs of stars with common proper motion, indicative of orbital pairs but with a long period. van de Kamp concentrated on those systems where the only evidence of duplicity was a periodic wobble of a bright star with respect to the background, indicating a faint and close but nonetheless significantly massive companion star.

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