The present book contains an expansion of the Rede Lecture delivered before the University of Cambridge in November 1930.

There is a widespread conviction that the new teachings of astronomy and physical science are destined to produce an immense change on our outlook on the universe as a whole, and on our views as to the significance of human life. The question at issue is ultimately one for philosophic discussion, but before the philosophers have a right to speak, science ought first to be asked to tell all she can as to ascertained facts and provisional hypotheses. Then, and then only, may discussion legitimately pass into the realms of philosophy.

With some such thoughts as these in my mind, I wrote the present book, obsessed by frequent doubts as to whether I could justify an addition to the great amount which has already been written on the subject. I can claim no special qualifications beyond the proverbially advantageous position of the mere onlooker; I am not a philosopher either by training or inclination, and for many years my scientific work has lain outside the arena of contending physical theories.

The first four chapters, which form the main part of the book, contain brief discussions, on very broad lines, of such scientific questions as seem to me to be of interest, and to provide useful material, for the discussion of the ultimate philosophical problem.

A few stars are known which are hardly bigger than the earth, but the majority are so large that hundreds of thousands of earths could be packed inside each and leave room to spare; here and there we come upon a giant star large enough to contain millions of millions of earths. And the total number of stars in the universe is probably something like the total number of grains of sand on all the seashores of the world. Such is the littleness of our home in space when measured up against the total substance of the universe.

This vast multitude of stars are wandering about in space. A few form groups which journey in company, but the majority are solitary travellers. And they travel through a universe so spacious that it is an event of almost unimaginable rarity for a star to come anywhere near to another star. For the most part each voyages in splendid isolation, like a ship on an empty ocean. In a scale model in which the stars are ships, the average ship will be well over a million miles from its nearest neighbour, whence it is easy to understand why a ship seldom finds another within hailing distance.

We believe, nevertheless, that some two thousand million years ago this rare event took place, and that a second star, wandering blindly through space, happened to come within hailing distance of the sun.

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.