Sir William Herschel: His Life and Works

by Edward S. Holden

This question of variability in general is an important one, then. It can only be tested by making accurate catalogues of the relative brilliance of stars at various times, and by comparing these. No such general catalogue existed before Herschel's time, and led by the discrepancies in isolated cases, which he found between his own estimates and those of his predecessors, he made from observation a series of four catalogues, in which were set down the order of sequence of the stars of each constellation.

The method adopted by Herschel was perfectly simple in principle, though most laborious in practice. Suppose any number of stars, A, B, C, D, E, . . . etc., near enough to each other to be well compared. The process consists simply in writing down the names of the stars, A, B, C, etc., in the order of their relative brightness. Thus if for a group of eight stars we have found at one epoch A, B, C, D, E, F, G, H, and if at another time the order was A, B, C, D, F, E, G, H, symptoms of variability are pointed out. Repeated observations, where the same star is found in different sequences, will decide the question. Thus, for the stars visible to the naked eye, we know exactly the state of the sky in Herschel's day, now nearly a century ago. Any material change cannot escape us. These catalogues have been singularly overlooked by the observers of our generation who have followed this branch of observation, and it was not till 1876 that they received proper attention and a suitable reduction (at the hands of Mr. C. S. Pierce).

We owe to Herschel the first trustworthy account of the stars visible to the naked eye, and since the date of his labors (about 1800) we have similar views published by Argelander (1839), Heis (1848), Argelander and Schönfeld (1857), Gould (1860 and 1872), and Houzeau (1875). Thus his labors have been well followed up.

In the prosecution of this work Herschel found stars whose light was progressively diminishing, others which regularly increased, one star whose light periodically varies (α Herculis), and at least one star (55 Herculis) which has utterly disappeared. On October 10, 1781, and April 11, 1782, he observed this latter star, but in May, 1791, it had totally vanished. There was no trace remaining.

The discovery of the variability of α Herculis was a more important one than would at first sight appear. Up to that time the only variable stars known were seven in number. Their-periods were four hundred and ninety-four, four hundred and four, three hundred and thirty-four, seven, six, five, and three days. These periods seemed to fall into two groups, one of from three hundred to five hundred days, the other comparatively much shorter, of three to seven days. α Herculis came to occupy the middle place between these groups, its period being about sixty days.

The cause of these strange and regular variations of brightness was supposed by Herschel to be the rotation of the star bodily on an axis, by which revolution different parts of its surface, of different brilliancy, were successively and periodically presented to us. This explanation it might have been difficult to receive, when the periods of the known variables were so markedly various in length. His own discovery came to bridge over the interval, and quite confirmed him in his belief. He returned to the subject of the revolution of stars about their axes again and again, and connected it with the revolution of satellites.

He found that the satellites of Jupiter and one of Saturn's periodically changed in brightness, and by quite simple means showed that their periods of rotation were at least approximately the same as their periods of revolution about their primaries. In this case, as in every other, he considered a discovery in each and every one of its possible bearings. There are no instances where he has singularly overlooked the consequences of his observations.

Researches on Double Stars.

The double stars were the subject of Herschel's earliest and of his latest papers. In 1782 he published his "Catalogue of Double Stars," and his last published memoir (1822) was on the same subject.

The question of determining the parallax of stars first brought Herschel to the discovery of double stars. If two stars, A and B, appear very close together, and if, in reality, the star B is very many times more distant from the earth than A, although seen along the same line of sight, then the revolution of the earth in its orbit will produce changes in the relative situation of A and B, and, in fact, B will describe a small orbit about A, due to this revolution. This idea had been proposed by Galileo, and measures on this plan had been made by Long, with negative results. But Herschel, in reviewing their work, declares that the stars chosen by Long were not suitable to the purpose. It is necessary, among other things, to the success of this method, that it should be certain that the star B is really very much more distant than the star A. The only general test of the distance of stars is their brilliancy, and Herschel decided to use only stars for this research which had two components very greatly different in brightness. A must be very bright (and presumably near to us), and B must be very close to A, and very faint (and thus, presumably, very distant).