Double Stars.—To the unaided eye, these appear as single points of light; but, when observed with a telescope of sufficient magnifying power, their dual nature can be detected.
The first double star discovered was Mizar, the middle star of the three in Ursa Major which form the tail of the bear. The components are of the fourth and fifth magnitudes, of a brilliant white colour, and distant fourteen seconds of arc.
In 1678, Cassini perceived stars which appeared as single points of light when viewed with the naked eye, but when observed with the telescope presented the appearance of being double.
The astronomer Bode, in 1781, published a list of eighty double stars, and, in a few years after, Sir William Herschel discovered several hundreds more of those objects. They are now known to exist in thousands, Mr. Burnham, of the Lick Observatory, having, by his keen perception of vision, contributed more than any other observer to swell their number.
All double stars are not binaries; many of them are known as ‘optical doubles’—an impression created by two stars when almost in the same line of vision, and, though apparently near, are situated at a great distance apart and devoid of any physical relationship.
Binary stars consist of two suns which revolve round their common centre of gravity, and form real dual systems.
The close proximity of the components of double stars impressed the minds of some astronomers with the belief that a physical bond of union existed between them. In the interval between 1718 and 1759, Bradley detected a change of 30° in the position angle of the two stars forming Castor, and was very nearly discovering their physical connection.
In 1767, the Rev. John Michell wrote: ‘It is highly probable in particular, and next to a certainty in general, that such double stars as appear to consist of two or more stars placed very near together do really consist of stars placed near together and under the influence of some general law.’ Afterwards he says: ‘It is not improbable that a few years may inform us that some of the great number of double and triple stars which have been observed by Mr. Herschel are systems of bodies revolving about each other.’ Christian Mayer, a German astronomer, formed a list of stellar pairs, and announced, in 1776, the supposed discovery of ‘satellites’ to many of the principal stars. His observations were, however, not exact enough to lead to any useful results, and the existence of his ‘planet stars’ was at that time derided, and believed to find a place only in his imagination.
The conclusions arrived at by some astronomers with regard to double stars were afterwards confirmed by Herschel, when, by his observation of a change in the relative positions of many of their components, he was able to announce that they form independent systems in mutual revolution, and are controlled by the law of gravitation.
The number of binary stars in active revolution is known to exceed 500; but, besides these, there are doubtless numerous other compound stars which, on account of their extreme remoteness and the close proximity of their components, are irresolvable into pairs by any optical appliances which we possess.