Sir William Herschel: His Life and Works

by Edward S. Holden

It was in the search for such pairs of stars that the Catalogue of Double Stars (1782) was formed. Herschel's first idea of a double star made such pairs as he found, to consist of two stars accidentally near to each other. A was near to us, and appeared projected in a certain place on the celestial sphere. B was many times more distant, but, by chance, was seen along the same line, and made with A an optical double. If the two stars were at the same distance from the earth, if they made part of the same physical system, if one revolved around the other, then this method of gaining a knowledge of their distance failed. Even in his first memoir on the subject, a surmise that this latter state might occur in some cases, was expressed by Herschel. The notes on some of the pairs declare that a motion of one of them was suspected. But this motion might be truly orbital—of one star about the other as a centre—or it might simply be that one star was moving by its own proper motion, and leaving the other behind. It was best to wait and see. The first Catalogue of Double Stars contained two hundred and three instances of such associations. These were observed from time to time, and new pairs discovered. The paper of Michell, "An Inquiry into the probable Parallax and Magnitude of the Fixed Stars, from the Quantity of Light which they Afford, and the Particular Circumstances of their Situation" (1767), was read and pondered. By 1802 Herschel had become certain that there existed in the heavens real pairs of stars, both at the same distance from the earth, which were physically connected with each other. The arguments of Michell have been applied by Bessel to the case of one of Herschel's double stars, in much the same order in which the argument ran in Herschel's own mind, as follows:

The star Castor (α Geminorum) is a double star, where A is of the second, and B of the fourth, magnitude. To the naked eye these two appear as one star. With a telescope this is seen to be two stars, some 5″ apart. In the whole sky there are not above fifty such stars as the brighter of the two, and about four hundred of the brilliancy of B. These fifty and four hundred stars are scattered over the vault of heaven, almost at random. No law has yet been traced by which we can say that here or here there shall be a bright star like A, or a fainter one like B. In general the distribution appears to be fortuitous. How then can we account for one of the four hundred stars like B placed so close to one of the fifty like A?

The chances are over four hundred thousand to one that the association in position is not accidental. This argument becomes overwhelming when the same association is found in many other cases. There were two hundred and three doubles in the Catalogue of 1782 alone, and many thousands are now known.

By a process like this, Herschel reached his grand discovery of true binary systems, where one sun revolves about another. For he saw that if the two stars are near together in space, they could not stand still in face of each other, but that they must revolve in true orbits. Here was the discovery which came to take the place of the detection of the parallaxes of the fixed stars.

He had failed in one research, but he was led to grand conclusions. Was the force that these distant pairs of suns obeyed, the force of gravitation? This he could not settle, but his successors have done so. It was not till about 1827 that Savary, of the Paris Observatory, showed that one of Herschel's doubles was subjected to the law of gravitation, and thus extended the power of this law from our system to the universe at large. Herschel himself lived to see some of his double stars perform half a revolution.

Of Herschel's discoveries, Arago thinks this has "le plus d'avenir." It may well be so. The laws which govern our solar system have been extended, through his researches, to regions of unknown distance. The binary stars will afford the largest field for research into the laws which govern them, and together with the clusters and groups, they will give a firm basis by which to study the distribution of stars in general, since here we have the great advantage of knowing, if not the real distance of the two stars from the earth, at least that this distance is alike for both.

Researches on Planets and Satellites.

After Herschel's first publication on the mountains of the Moon (1780), our satellite appears to have occupied him but little. The observation of volcanoes (1787) and of a lunar eclipse are his only published ones. The planets Mercury, Venus, Mars, and Jupiter, although they were often studied, were not the subjects of his more important memoirs. The planet Saturn, on the contrary, seems never to have been lost sight of from the time of his first view of it in 1772.

The field of discovery always appears to be completely occupied until the advent of a great man, who, even by his way of putting old and familiar facts, shows the paths along which discoveries must come, if at all. This faculty comes from profound reflection on the nature of the subject itself, from a sort of transmuting power which changes the words of the books into the things of reality. Herschel's paper on Saturn, in 1790, is an admirable example of this.

Herschel's observations on Saturn began in 1772. From 1790 to 1808 he published six memoirs on the figure, the ring, and the satellites of this planet. The spheroidal shape of the ball was first discovered by him, and we owe much of our certain knowledge of the constitution of the rings to his work. The sixth and seventh satellites, Mimas and Enceladus, were discovered by him in 1789. The periods of rotation of the ball and of the ring were also fixed. In his conclusions as to the real figure of the rings, there is a degree of scientific caution which is truly remarkable, and which to-day seems almost excessive.