It was noticed by Gould, from observations made at Cordova, that “a belt or stream of bright stars appears to girdle the heavens very nearly in a great circle which intersects the Milky Way.” According to Gould, the belt includes Orion, Canis Major, Argo, Crux, Centaurus, Lupus, and Scorpio in the southern hemisphere, and Taurus, Perseus, Cassiopeia, Cepheus, Cygnus, and Lyra in the northern. This was interpreted by Celoria as indicating the existence of two galactic rings, but Gould considered the zone of bright stars to form with the Sun a subordinate cluster of about five hundred stars within the Galaxy.
Perhaps the most elaborate investigations on the structure of the Universe have been those of Kapteyn, commenced in 1891. In that year he demonstrated that stars are bluer and more easily photographed in the Galaxy than elsewhere, a discovery independently made by Gill at the Cape, and Pickering at Harvard. In 1893 Kapteyn announced his conclusions, derived from a novel method of studying the distance of the stars from their proper motions. In order to reach a definite idea of the distances of the stars, he made use of the component of the proper motion, measured at right angles to a great circle of the sphere which passes through a given star and the apex of the solar motion. He found that stars of the first spectral type have smaller proper motions than those of the second, indicating that stars of the second type are on the average nearer to the Solar System than those of the first, the near vicinity containing almost exclusively second-type stars. Kapteyn concluded that the group of second-type stars formed one system, named the solar cluster, which he considered to be roughly spherical in shape. In 1902 he abandoned this idea, retaining, however, his opinions as to the relative distances of the different types. That the second-type stars are nearer to the Sun than the first is, he remarked in a letter to the writer, incontrovertible.
In the investigation of the motions in, and extent of, the Universe, the name of Simon Newcomb stands out pre-eminently. Born in 1835 at Wallace, in Nova Scotia, he went to the States in 1853. In 1862 he received an appointment at Washington Observatory, and he retained an official position until 1897. Throughout his scientific career he has been specially attracted by the question of the construction of the heavens, which he fully discussed in his book on ‘The Stars’ in 1901. Newcomb’s investigations have shown that some of the stars are not permanent members of the Sidereal System, among them the swiftly-moving 1830 Groombridge. He has shown that the Stellar Universe does not possess that form of stability which is seen in the Solar System. Newcomb considers the Universe to be limited in extent, as opposed to the opinions of Struve and others, who believed it to be infinite. He has brought clearly before his readers a calculation, based on the known law that there are three times as many stars of any given magnitude as of that immediately brighter, the increase of number compensating for the decrease of brilliance. Were the Universe infinitely extended, the whole heavens would shine with the brilliance of the Sun. Newcomb, therefore, concludes that “that collection of stars which we call the Universe is limited in extent.”
Positive evidence that this is the case was obtained by Giovanni Celoria, now director of the Milan Observatory, in the course of a series of star-gauges at the north galactic pole. Using a small refractor, showing stars barely to the eleventh magnitude, he found he could see exactly the same number of stars as Herschel’s large reflector, indicating that increase of optical power will not increase the number of stars visible in that direction. Celoria’s observation can only be explained on the assumption that the Universe is limited in extent, as otherwise Herschel’s telescope should have shown more stars than Celoria’s, even granting an extinction of light,—a theory which Newcomb, Schiaparelli, and others have shown to be quite untenable. That the Universe is limited in extent is about all that is known for certain, although even this has been called in question, notably by E. W. Maunder and H. H. Turner. The problem of the construction of the heavens is by no means solved, although several more or less probable theories have been advanced.
A series of investigations on stellar distribution, from 1884 to 1898, led Hugo Seeliger, director of the Munich Observatory, to some remarkable deductions. He believes the Universe to be flattened at the galactic poles. The Galaxy is the zone of stellar condensation, and he concludes the distance of the Solar System from the inner border of the zone to be 500 times the distance of Sirius, while the external border is 1100 times that distance. The Universe is finite in extent, its limits being about 9000 light years from the Solar System. In Seeliger’s opinion the extinction of light may come into play beyond our Universe, and prevent us seeing other collections of stars.
The question of external universes is purely a hypothetical one, although there is undoubtedly much to be said in its favour. These universes have never been seen, and we can only speculate as to their existence. The last word on the subject is by Gore, in 1893, in his elaborate work, ‘The Visible Universe.’ He regards the Solar System as a system of the first order, and the Galaxy and its fellow-universes of the second. He makes a calculation of the possible distance of an external universe of his second order. He assumes the distance of the nearest universe from our Galaxy as proportional to that separating the Sun from α Centauri, and reaches the amazing conclusion that the distance of the nearest Galaxy is no less than 520,149,600,000,000,000,000 miles,—a distance which light, with its inconceivable velocity of 186,000 miles a second, would take almost ninety millions of years to traverse.
These calculations absolutely overwhelm the mind, which is unable to comprehend such vast distances. Our universe is indeed, as Flammarion expresses it, a point in the infinite. The calculations of J. E. Gore represent our highest scientific conception of the universe. He sums up his investigations with the following words: “Although we must consider the number of visible stars as strictly finite, the numbers of stars and systems really existing, but invisible to us, may be practically infinite. Could we speed our flight through space on angel wings beyond the confines of our limited universe to a distance so great that the interval which separates us from the remotest fixed star might be considered as merely a step on our celestial journey, what further creations might not then be revealed to our wondering vision? Systems of a higher order might there be unfolded to our view, compared with which the whole of our visible heavens might appear like a grain of sand on the ocean shore,—systems perhaps stretching out to infinity before us, and reaching at last the glorious ‘mansions’ of the Almighty, the Throne of the Eternal.”
CHAPTER XIII.
CELESTIAL EVOLUTION.
In the second chapter we outlined the nebular hypothesis as propounded by Herschel. Some time earlier the French mathematician, Laplace, had put forward his theory of the evolution of the Solar System. Pierre Simon Laplace was born at Beaumont-en-Auge, near Honfleur, in 1749, and was educated in the Military School of his native town. In 1767 he became Assistant Professor of Mathematics at Beaumont, and some years later at the Military School in Paris, which position he retained for many years. Member of the Institute and Minister of the Interior under Napoleon, he was created a Marquis by Louis XVIII., and died at Arcuile on March 5, 1827.
In the last chapter of his popular work, the ‘Système du Monde,’ Laplace put forward his nebular theory “with that distrust which everything ought to inspire that is not the result of observation or calculation.” Laplace noticed that in the Solar System all the planets revolved round the Sun in the same direction, from west to east, and that the satellites of the planets obeyed the same law. He also observed that the Sun, Moon, and planets rotated on their axes in the same direction as they revolved round the Sun; also that the planets moved round the Sun, and the satellites round their primaries, in almost the same plane as the Earth’s orbit, the plane of the ecliptic. It was evident that these remarkable congruities were not the result of chance, and accordingly Laplace expressed his belief that the Solar System originated from a great nebula, which in condensing detached various rings in the process of rotation. These rings condensed into the various planets and their satellites.