Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/325

There is quite a different method of approaching the subject, which leads in a very striking . manner to conclu sions practically identical with those we have just sketched. We may commence by dealing with the sun as we find it at the present moment, and then reasoning back to what must have been the case in the earlier epochs of the history of our system. The stupendous daily outpour of heat from the sun at the present time is really, when properly studied, a profound argument in support of the nebular theory. The amount of the sun s heat has been estimated. We receive on the earth less than one two-thousand- millionth part of the whole radiation. It would seem that the greater part of the rest of that torrent flows away to be be lost in space. Now what supplies this heat ? We might at first suppose that the sun was really a mightily heated body radiating out its heat as white hot iron does, but this explanation cannot be admitted in face of the notorious fact that there is no historical evidence that the sun is growing colder. We have not the slightest reason to think that the radiation from the sun is measurably weaker now than it was a couple of thousand years ago, yet it can be shown that, if the sun were merely radiating heat as simply a hot body, then it would cool some degrees every year, and must have cooled many thousands of degrees within the time covered by historical records. We there fore conclude that the sun has some other source of heat than that due simply to incandescence. We can also con ceive that the heat of the sun might be supplied by some thing analogous to combustion. It would take 20 tons of coal a day burned on each square foot of the sun s surface to supply the daily radiation. Even if the sun were made of one mass of fuel as efficient as coal, that mass must be entirely expended in a few thousand years. We cannot therefore admit that the source of the heat in the sun is to be found in any chemical combination taking place in its mass. Where then can we find an adequate supply of heat 1 Only one external source can be named : the falling of meteors into the sun must yield some heat just as the flash of a shooting star yields some heat to our atmosphere, but the question is whether the quantity of heat obtainable from the shooting stars is at all adequate for the purpose. It can be shown that unless a quantity of meteors in collective mass equal to our moon were to plunge into the sun every year the supply of heat could not be sustained from this source. Now there is no reason to believe that meteors in anything like this quantity can be supplied to the sun, and therefore we must reject this source as also inadequate. The truth about the sun s heat appears to be that the sun is really an incandescent body losing heat, but that the operation of cooling is immensely retarded owing to a curious circumstance due jointly to the stupendous mass of the sun and to a remarkable law of heat. It is of course well known that if energy disappears in one form it reappears in another, and this principle applied to the sun will explain the famous difficulty. As the sun loses heat it contracts, and every pair of particles in the sun are nearer to each other after the contraction than they were before. The energy due to their separation is thus less in the contracted state than in the original state, and as that energy cannot be lost it must reappear in heat. The sun is thus slowly contract ing ; but as it contracts it gains heat by the operation of the law just referred to, and thus the further cooling and further contraction of the sun is protracted until the additional heat obtained is radiated away. In this way we can reconcile the fact that the sun is certainly losing heat with the fact that the change in temperature has not been large enough to be perceived within historic times. 311 It can be shown that the sun is at present contracting, so that its diameter diminishes four miles every century. This is of course an inappreciable distance when compared with the diameter of the sun, which is nearly a million of miles, but the significance for our present purpose depends upon the fact that this contraction is always taking place. A thousand years ago the sun must have had a diameter 40 miles greater than at present, ten thousand years ago that diameter must have been 400 miles more than it is now, and so on. We cannot perhaps assert that the same rate is to be continued for very many centuries, but it is plain that the further we look back into past time the greater must the sun have been. Dealing then simply with the laws of nature as we know them, we can see no boundary to the growth of the sun as we look back. "We must conceive a time when the sun was swollen to such an extent that it filled up the entire space girdled by the orbit of Mercury. Earlier still the sun must have reached to the Earth. Earlier still the sun must have reached to where Neptune now revolves on the confines of our system, but the mass of the sun could not undergo an expansion so prodigious without being made vastly more rarefied than at present, and hence we are led by this mode of reasoning to the conception of the primaeval nebula from which our system has originated. Considering that our sun is but a star, or but one of the millions of stars, it becomes a question of great interest to see whether any other systems present indication of a nebulous origin analogous to that which Laplace proposed for the solar system. In one of his most memorable papers, Sir W. Herschel marshals the evidence which can be collected on this point. He arranges in this paper a selection from his observations on the nebulae in such a way as to give great plausibility to his view of the gradual transmutation of nebulae into stars. Herschel begins by showing us that there are regions in the heavens where a faint diffused nebulosity is all that can be detected by the telescope. There are other nebulae in which a nucleus can be just discerned, others again in which the nucleus is easily seen, and still others where the nucleus is a brilliant star-like point. The transition from an object of this kind to a nebulous star is very natural, while the nebulous stars pass into the ordinary stars by a few graduated stages. It is thus possible to enumerate a series of objects beginning at one end with the most diffused nebulosity and ending at the other with an ordinary fixed star or group of stars. Each object in the series differs but slightly from the object just before it and the object just after it. It seemed to Herschel that he was thus able to view the actual changes by which masses of phosphorescent or glowing vapour became actually condensed down into stars. The conden sation of a nebula could be followed in the same manner as we can study the growth of the trees in the forest, by comparing the trees of various ages which the forest con tains at the same time. In attempting to pronounce on the evidence with regard to Herschel s theory, we must at once admit that the transmutation of a nebula into a star has never been seen. It is indeed very doubtful whether any changes of a nebula have ever been seen which are of the same character as the changes Herschel s theory would require. It seems, however, most likely that the periods of time required for such changes are so stupendous that the changes accomplished in a century or two are absolutely inappreciable. The nebular theory is a noble speculation supported by plausible argument, and the verdict of science on the whole subject cannot be better expressed than in the words of Newcomb : "At the present time we can only "say that the nebular hypothesis is indicated by the general tend encies of the laws of nature, that it has not been proved