The Theory of Island Universes
The first notions of the scale of stellar distances in the new universe were derived by Newton and by Huygens, using the principle of the uniformity of nature. If the sun were a star, they argued, it would be reasonable to assume that all the stars were like the sun - in particular that they all had the same intrinsic luminosity, the same candle-power, as the sun. The assumption is now known to be only a very rough approximation to the truth. Nevertheless, it justified the estimation of the general orders of distances from the apparent faintness of the stars. If a star were really as bright as the sun, but appeared a million million times fainter than the sun, then it would be a million times as far away - about 16 light-years. Using this method, Huygens estimated that the distance of Sirius, the brightest star in the sky, was about half a light-year, and Newton estimated that the distance of stars of the first magnitude was about 15 light-years.
These estimates are only a few per cent. of the true distances, but they gave the first intimation of the immense scale on which the stars are scattered. Speculation could now orient its flight, and soon a still larger vision emerged. In 1750 Thomas Wright published the first approximately correct explanation of the Milky Way. The stars, he said, are not scattered indefinitely through the universe; they form a limited system, isolated in space. The stellar system is rather flat, like a disk or a coin, and the sun is near the centre. When we look towards the top or bottom of the disk, the distance to the boundary is short and the eye encounters few stars. When we look towards the rim, the distance to the boundary is great and the eye encounters many stars, which, in projection, appear crowded together to form the Milky Way.
Wright carried his speculations still farther. The notion of a single stellar system, alone in the universe, was unwelcome to his sense of proportion. He dreamed of a universe populated by countless similar stellar systems, separated by vast intervals. As visible evidence he pointed to certain faint cloudy patches in the sky, known as nebulae. These mysterious bodies, he suggested, were the nearest of the neighbouring systems.
Five years later Kant developed. these speculations in a form which was immediately accepted, and which persisted unchanged until recent years. Eventually, the conception was called the theory of island universes - stellar systems scattered through the ocean of space.
Observations followed as rapidly as instruments and technique developed, and, step by. step, they tended to confirm this particular line of speculation. Sir William Herschel sketched the rough outlines of our stellar system by counting the numbers of stars his telescope revealed in different directions. The relative numbers of stars, he considered, indicated the relative distances to the boundary of the system along the various lines of sight. About a century ago distances of stars were measured by direct triangulation. Then powerful indirect methods of estimating distances were developed, and the realm of the stars could be explored with a measuring rod. Finally, the stellar system was mapped, using the very luminous globular clusters as landmarks. The stars, it was fully demonstrated, do form a definite system, isolated in space. Beyond the boundaries, the universe stretched on and on, inaccessible to actual investigation but populated in fancy by other stellar systems, comparable with our own.
Thus the second great chapter in the exploration of space was developed. The first had been confined to the realm of the planets; the second ranged through the realm of the stars. And now, in our day, the third chapter has opened. For the explorations have at least won their way beyond the stars into the realm of the nebulae.