In the early days of science, the unquestioning acceptance of the law of causation as a guiding principle in the natural world led to the discovery and formulation of laws of the general type “an assigned cause A leads to a known effect B.” For instance the addition of heat to ice causes it to melt, or stated in more detail, heat decreases the amount of ice in the universe and increases the amount of water.

Primitive man would become acquainted with this law very easily—he had only to watch the action of the sun on hoar-frost, or the effect of the long summer days on the mountain glaciers. In winter he would notice that cold changed water back into ice. At a farther stage it might be discovered that the re-frozen ice was equal in amount to the original ice before melting. It would then be a natural inference that something belonging to a more general category than either water or ice had remained unaffected in amount throughout the transformation

Modern physics is familiar with laws of this type, which it describes as “conservation laws.” The discovery we have just attributed to primitive man is a special case of the law of conservation of matter. The law of “conservation of X,” whatever X may be, means that the total amount of X in the universe remains perpetually the same: nothing can change X into something which is not X.

It has already been noticed how the “great nebulae” form what Herschel described as a system of “island-universes,” distinct and detached both from one another and from the galactic system of stars. Hubble has found that these nebulae are all of comparable size, being, as fig. 2 (p. 15) has shewn, of size comparable with, although smaller than, the galactic system.

This of itself would encourage the conjecture that the great nebulae may be star-clouds, of the same general nature as the cloud of stars surrounding the sun. This view of the nature of the great nebulae has been very prevalent since the time of the Herschels, and various items of recently gained knowledge appear to give it support rather than the reverse.

Viewed from a fairly remote nebula, our galactic system of stars would appear as a cloud of faint light, which telescopes of terrestrial power would be unable to resolve into separate stars. Since the average light from these stars gives a spectrum of F or G type, the composite spectrum of this cloud of stars would closely resemble a stellar spectrum of F or G type, and this is precisely the type of spectrum shewn by the great nebulae, their spectra even being crossed by dark lines of the same general character as the Fraunhofer lines in the solar spectrum.

My book attempts to describe the present position of Cosmogony and of various closely associated problems of Astronomy, as, for instance, the physical state of astronomical matter, the structure of the stars, the origin of their radiation, their ages and the course of their evolution.

In a subject which is developing so rapidly, few problems can be discussed with any approach to finality, but this did not seem to be a reason against writing the book. Many years have elapsed since the last book on general Cosmogony appeared, and the interval has seen the whole subject transformed by new knowledge imported from observational astronomy and atomic physics. It has also witnessed the growth of an interest in the results of Cosmogony, which now extends far beyond the ranks of professional astronomers, and indeed beyond scientific circles altogether.

With this in my mind, I have tried to depict the present situation in the simplest language consistent with scientific accuracy, avoiding technicalities where possible, and otherwise explaining them. As the book is intended to be, first and foremost, a rigorously argued scientific treatise, the inclusion of a substantial amount of mathematical analysis was inevitable, but every effort has been made to render the results intelligible to readers with no mathematical knowledge, of whom I hope the book may have many.

THE GENERAL CONDITION OF LIQUID STARS.

In Chapter III we investigated the internal equilibrium of the stars on the supposition that they were masses of gravitating gas, in which the gas-laws were obeyed throughout. The investigation was abandoned when it was found to lead to impossibly high values for the atomic weights of the stellar atoms. This created a suspicion that the hypothesis on which it was based was unfounded, and that the gas-laws are not obeyed in stellar interiors.

The last chapter provided further evidence to the same effect. We there investigated the mode of generation of stellar energy, using the guiding principle that all modes of generation of energy which make the stars dynamically unstable can be ruled out of the list of practical possibilities. We found that when the gas-laws are supposed to be obeyed, no possibilities remain for stars of enormously great mass. Further the only mode of generation of energy which was both physically acceptable and consistent with the stability of actual stars proved to be one in which the rate of generation of energy is uninfluenced by changes of density and temperature, as in radioactive substances, and this, as we shall see at once (§ 134), requires substantial deviations from the gas-laws in stars of all masses.

We now rediscuss the problem of the physical constitution of the stars, and examine the form it assumes when the gas-laws are no longer obeyed.

As we have seen (§ 13), our sun is a member of a huge system of stars whose number must be counted in thousands of millions. In general shape this system may be compared to an oblate spheroid with very unequal axes, or, less mathematically, to a coin or round biscuit. The stars are not uniformly distributed throughout this system, being much more thickly scattered in its central parts than in its outer regions. Probably there is no clearly defined boundary, the star-density diminishing indefinitely as we recede from the centre, but never becoming quite zero. The sun lies almost exactly in the central plane of the system, although not precisely at the centre. Those stars which lie near the edge of the coin or biscuit are so remote as to appear very faint to us and constitute the Milky Way. The system of stars bounded by the Milky Way is commonly called the Galactic System.

The stars shew so little motion that for a long time astronomers failed to detect any motion at all, and they became known as “fixed stars” to distinguish them from the planets or “wandering stars” whose motion was obvious to everyone. But modern astronomy finds it possible to measure the motions of a great number of stars.

The discussion of the last chapter shewed that the orbits of binary stars, both spectroscopic and visual, are still far from conforming to the statistical laws which must finally prevail after the stars have interacted with one another for an unlimited length of time. The same is true of the components of the velocities of the stars in space. After a sufficiently long time of interaction between stars, these ought to conform to the well-known Maxwell law of distribution of velocities. The investigation of Seares already given has shewn that the resultant velocities conform well enough, at least to the extent of obeying the law of equipartition of energy, but the distribution of their directions is far from conforming to this law. After a sufficiently long time of interaction, stellar velocities must be distributed in all directions equally, their motion not favouring any one special direction. As Kapteyn shewed in 1904, the actual velocities of the stars shew a very marked favouritism for a definite direction in space, so that the law of distribution appropriate to the final state is far from being obeyed.

If the statistical laws which specify the final steady state had proved to be exactly obeyed, we could have concluded that the stars had been interacting with one another for a very long time, but we could not have estimated the length of this time except possibly in terms of a lower limit.

Now that the detailed discussion of particular problems is ended, we may perhaps attempt to summarise our results and tentative conclusions, sacrificing logical and chronological order in favour of the arrangement which offers the broadest and simplest view of the whole subject.

The easiest part of the problem of cosmogony is the interpretation of the observed shapes of astronomical bodies and formations. Here the effects of rotation have proved to be of primary importance. The earth and many of the planets have the shape of flattened oranges. The degree of flattening is such as would be produced by quite slow rotation about an axis, and there is no room for doubt that this is the actual cause of the observed flattening. It is possible to trace out theoretically the shapes assumed by astronomical bodies having all possible amounts of rotation. Mathematical investigation shews that the flattened-orange shape is assumed by all bodies in slow rotation, no matter what their internal constitution and arrangement may be, but that with more rapid rotation the shape depends on the internal arrangement of the body, being especially affected by the extent to which its mass is concentrated at or near its centre.

Two special and quite extreme types of arrangement have been considered in detail. In the first the body is supposed to consist of matter which cannot be compressed and is of uniform density throughout; to fix our ideas, we may think of a mass of water.

The moon, our nearest neighbour in the sky, is 240, 000 mites away from us; a distance which light, travelling at 186, 000 miles a second, traverses in a little over one second. The farthest astronomical objects whose distances are known are so remote that their light takes over one hundred million years to reach us. The ratio of these two periods of time–a hundred million years to a second–is the ratio of the greatest to the least distance with which the astronomer has to deal, and within this range of distances lie all the objects of his study.

As he wanders through this vast range with the aid of his telescope, he finds that the great majority of the objects he encounters fall into well-defined classes; they may almost be said to be “manufactured articles” in the sense in which Clerk Maxwell applied the phrase to atoms. Just as atoms of hydrogen or of oxygen are believed to be of similar structure and properties wherever they occur in nature, so the various astronomical objects–common stars, binary stars, variable stars, star-clusters, spiral nebulae, etc.–are believed to be, to a large extent at least, similar structures no matter whore they occur.

The similarity, it is true, is not so definite or precise as that between the atoms of chemistry, and perhaps a better comparison is provided by the different species of vegetation which inhabit a country.

GENERAL PRINCIPLES

The early spectroscopists believed that the spectrum of a star provided a sure indication of the star's age. Huggins and Lockyer had found, for instance, that the spectrum of Sirius exhibited hydrogen lines very strongly and calcium lines rather weakly; in the solar spectrum the relative strength of these two sets of lines was reversed, calcium being strong and hydrogen weak. They concluded that hydrogen was specially prominent in the constitution of Sirius and calcium in that of the sun. Believing that Sirius must one day develop into a star similar to our sun, they conjectured that its substance must gradually change from hydrogen into calcium and other more complex elements, thus finding support for the long-established hypothesis that the more complex elements were formed by gradual evolution out of the simplest. In this way they were led to regard a star's spectrum as an index to its age.

As we have seen, the true interpretation of these observations is merely that the surface of Sirius is at a temperature at which hydrogen is specially active in emitting and absorbing radiation, while the sun's surface is at a lower temperature at which hydrogen is comparatively inert, while calcium, iron, etc., have become active in its place.