Double-star Systems.— Whether the earth and moon originally formed one body which afterward separated into two through excess of rotation, followed by the action of bodily tides, is subject to grave doubt, but there is but little doubt that the evolution of double-star systems has taken place in substantial accordance with this theory. For here are numerous examples of two suns of approximately equal mass re volving about one another with their surfaces almost in contact, and it is most significant that in every such known case the spectrum of the system is that of the very early types of stars. The densities of these pairs are also very low, and on the whole if cannot be doubted that they represent a very early stage of the system's development. When we pass to a consideration of the wider (visual), double-star systems it is found that the circularity of the orbits has disappeared and that the stars have aged to later spectral types. It can be shown mathe matically that the continued effect of tidal action is not only to push the bodies farther apart but also to increase the eccentricities (that is, to diminish the roundness), of the orbits.
We have thus apparently found a true and satisfactory account of the genesis and develop ment of many of the double-star systems. But it should be added that there are many ex amples of pairs of stars which are drifting through space together, and which therefore belong to one system, but which are separated by distances so vast that it is quite inconceivable that any form of tidal action could have been effective in pushing them apart from a single body. The periods of such pairs are doubtless to be reckoned in tens, or even in hundreds, of thousands of years. It is reasonable to suppose that the common motion observed in these cases is merely the original motion of the nebula from which they, and perhaps other stars of the same stream, were a part. But when this explanation is to be replaced by that one which, as we have seen, accounts so fully for the development of closer pairs, we are not able to say.
Development of the Stars.— It is uni versally believed by astronomers that the stars have developed from nebulae, but the exact mode of this development, and especially a knowledge of precisely what is taking place in certain phases of the nebulae, is very uncertain. Roughly speaking, the known nebula: can be divided into four great classes: (1) The vastly extended, faintly shining, very irregular, nebu lous clouds, of which the nebula in Orion is a well-known example; (2) the small, round, planetary nehula, which frequently have a con densed nucleus in the centre; () the hazy, nebulous stars; and (4) the great host of Spiral nebula, which far exceed in number all others combined. It is a remarkable fact, thus far
quite unexplained, that while the first three classes are most numerous in the neighborhood of the Milky Way, the spiral nebula are almost wholly wanting in this region but are found by tens of thousands in the most vacant areas of the sky.
It is probable that the first class of nebula represent the simplest form of inorganic sub stance known to us. Whether these inconceiv ably extended and tenuous clouds can under their own gravity become denser clouds, from which ultimately even stars may develop, we do not know. It is believed by some astronomers that certain of them, as, for example, the faint nebulosity in the Pleiades and the nebulous streams, are made up of inconceivably small particles, or ions, which have been repelled from hot stars, and thus are in a sense a dis integrated product, rejected in the course of the general stellar evolution. The spiral nebula are of such great size that it is doubtful whether the smallest of them visible to us could develop into a solar system more or less closely re sembling our own. It seems more probable that from these a star stream, or even a star cluster, containing multitudes of stars, will eventually be produced.
All observation leads us to the conclusion that the majority of the nebula are of a hetero geneous structure,• that is, that they are very far from being of a uniform density through out their extent. It is not unreasonable to sup pose that much of their material may condense into solid bodies, or meteoric particles, before the general development has proceedeirlar toward a condensation into many separate star centres. But this possibility has no important bearing upon the general history of the sys tem's development. And whether the nebula is of so great extent that it develops into a mul titude of suns or whether it is so very small that there results but one, the fact that the growth of each sun takes place In a resisting medium leads us to the conclusion that some of the nebulous matter will escape being drawn into the centre but will be left revolving around the central sun. Accordingly, it appears to us very probable that a vast number of the stars have worlds revolving around them and that our solar system is not unique in this respect.