Much useful work has been done also in the analysis of the various spectra. Julius Scheiner, now “chief observer” at Potsdam Astrophysical Observatory, has, since 1890, done much valuable work in this direction. Special attention was devoted to the spectrum of Capella, 490 lines in the spectrum of which were measured by Scheiner. In his own words, “he believes a complete proof of the absolute agreement between its spectrum and that of the Sun to be thereby furnished.” Other stars of the Sirian and solar classes were exhaustively studied by Scheiner.
The study of the exact brilliance of the stars was a branch of research long neglected, yet it is of much importance in astronomy, for it is only through exact measurement of stellar brilliance that stellar variation can be detected. Herschel commenced the study, which was continued by his son at the Cape, but it is only within the last twenty years that stellar photometry has become a recognised branch of astronomy; and the credit of this is due to the energy and zeal of the great American observer, Edward Charles Pickering.
Born in Boston in 1846, Edward Charles Pickering was appointed in 1865 Instructor of mathematics in the Lawrence Scientific School at Harvard, after a distinguished university career. In 1876 he succeeded Winlock as director of the Harvard Observatory, and in the following year he commenced his photometric studies. He invented an instrument named the meridian photometer, with the aid of which he succeeded in determining, in the years 1879 to 1882, the exact brilliance of 4260 stars to the sixth magnitude between the north celestial pole and thirty degrees of south declination. At a later date he devised a larger photometer, with which he made over one million observations. Pickering next extended his survey to the southern hemisphere, erecting the photometer on the slope of the Andes, where the Harvard auxiliary station at Arequipa is now located, and where 8000 determinations of stellar brilliance were made. Meanwhile Pritchard, at Oxford, published in 1885 his ‘Uranometria Nova Oxoniensis,’ with photometric determinations of the brilliance of 2784 stars from the pole to ten degrees of south declination. Both of these catalogues were epoch-making works, and testify to the enthusiasm and perseverance of the astronomers who designed them.
The study of stellar photometry glides into that of stellar variation. At the beginning of the nineteenth century the number of known variable stars was very small, as a glance at the list given in Brewster’s edition of Ferguson’s Astronomy (1811) will show. Some remarkable investigations were due to the English astronomer, John Goodricke (1764-1786), who rediscovered the variability of the star Algol, and accurately determined its period in 1782. Goodricke suggested that the regular variations in the light of Algol were due to the partial eclipse of its light by a dark satellite, a hypothesis now fully confirmed. Two years later, in 1784, Goodricke discovered other two variables, δ Cephei and β Lyræ. He died in 1786 at the age of twenty-one, and thus variable-star astronomy was deprived of its founder.
The foundation of variable-star astronomy as an exact branch of the science is due to Argelander. In the years 1837-1845, while residing at Bonn during the erection of the observatory, of which he had been made director, he erected a temporary observatory, and there he carefully determined the magnitudes of all stars visible in Central Europe. From this he was led to the discussion of stellar variation, to which subject he continued to give much attention. He was the first to describe a method of observing variable stars scientifically and accurately,—a method consisting in estimating in “steps” or “grades” the difference in brilliance between the variable, or suspected variable, and other stars which are selected for comparison, and which are of various degrees of brilliance, so that they may be available for comparison with the variable throughout its fluctuations. Argelander’s “steps” are tenths of a magnitude, and Gore describes the method of observation as follows: “If we call a and b the comparison stars, and v the variable, a being brighter than b, and if v is judged to be midway in brightness between a and b, we write a5v5b. If v is slightly nearer to b, we write a6v4b. We may also write a3v7b, or a7v3b, the sum of the steps being always ten.”
This method, described in 1844, led to many discoveries at Bonn in the following twenty years by Argelander and his assistants Schmidt and Schönfeld. At this time Eduard Heis (1806-1877), at Münster, who also ranks as one of the founders of meteoric astronomy, while engaged on the construction of his great atlas, attentively determined the change of magnitude of stars visible to the naked eye; and by means of the naked eye, the opera-glass, and a small telescope, he amassed a large number of observations. At the same time two English observers, Hind and Pogson, were making remarkable discoveries which greatly increased the number of known variables. Among Hind’s discoveries were S Cancri of the Algol type; while Schmidt discovered another of the same class, δ Libræ, and also the famous ζ Geminorum. While director of the Observatory of Mannheim, an institution equipped with very antiquated instruments, Schönfeld devoted himself to the study of variable stars, and increased the number of known variables considerably. In the southern hemisphere Gould, in South America, did for the observation of variable stars what Argelander did in the northern.
In 1874 a very important, although not so obvious, service to variable-star astronomy was rendered by the Danish observer, Hans Carl Fredrik Christian Schjellerup (1827-1887). He translated from Arabic into French the works of the Persian astronomer of a thousand years ago, Al-Sufi, and thus rendered his observations available to modern astronomers. Al-Sufi was a most accurate observer, and, by comparing his catalogue with those of modern observers, it can be found whether stars have changed in brilliance during the past thousand years.
The study of variable stars has been pursued in recent years by many astronomers, both by means of photography and by the visual method. The most important names in the visual discovery of variables are Gustav Müller and Paul Friedrich Ferdinand Kempf (born 1856) of Potsdam; Alexander William Roberts of Lovedale, South Africa; Seth Carlo Chandler of Boston; Nils Christopher Dunér at Upsala; and John Ellard Gore (born 1845) in Dublin.
The researches of J. E. Gore are a brilliant example of how much may be done for astronomy by means of very moderate instruments. Born in 1845 at Athlone, in Connaught, he went to India in 1868 as engineer on the Sirhind Canal in the Punjab. While in India he erected his small telescopes on brick pillars, and took observations, many of them of stellar brilliance. In 1879 he returned to Ireland, and since then has devoted himself to astronomy with zeal and enthusiasm. His discoveries and investigations of variables have been made by means of the binocular. On December 13, 1885, he discovered a remarkable star in Orion, which was at first considered to be temporary, and called “Nova Orionis,” but was afterwards found to be a long-period variable star.
Recently photography has come much to the front in the discovery of variable stars. Pickering at Harvard, and Wolf at Heidelberg, have particularly distinguished themselves in this branch, and the number of known variables is now very large, as every year brings fresh discoveries, mostly by aid of photography. Many of these newly-discovered variables are in star-clusters and nebulæ.