The general fact that every star has a life history—that this history will ultimately come to an end—that it must have had a beginning in time—is indicated by so great a number of concurring facts that no one who has most profoundly studied the subject can have serious doubts upon it. Yet there are some unsolved mysteries connected with the case, which might justify a waiting for further evidence, coupled with a certain degree of skepticism. Of the questions connected with the case the most serious one is: How is the supply of energy radiated by the Sun and stars kept up? Only one answer is possible in the light of recent science. It is that already given in the last chapter—the continual contraction of volume. The radiant energy sent out is balanced by the continual loss of potential energy due to the contraction.
On this theory the age of the Sun can be at least approximately estimated. About twenty millions of years is the limit of time during which it could possibly have radiated anything like its present amount of energy. But this conclusion is directly at variance with that of geology. The age of the earth has been approximately estimated from a great variety of geological phenomena, the concurring result being that stratification and other geological processes must have been going on for hundreds—nay, thousands of millions of years. This result is in direct conflict with the only physical theory which can account for the solar heat.
The nebulæ offer a similar difficulty. Their extreme tenuity and their seemingly almost unmaterial structure appear inadequate to account for any such mutual gravitation of their parts as would result in the generating of the flood of energy which they are constantly radiating. What we see must, therefore, suggest at least the possibility that all shining heavenly bodies have connected with them some form of energy of which science can, as yet, render no account. This suspicion cannot, however, grow into a certainty until we have either seen the nebulæ contracting in volume or have made such estimates of their probable masses that we can compute the amount of contraction they must undergo to maintain the supply of energy.
In the impressive words of Sir William Huggins:
"We conclude filled with a sense of wonder at the greatness of the human intellect, which from the impact of waves of ether upon one sense-organ, can learn so much of the Universe outside our earth; but the wonder passes into awe before the unimaginable magnitude of Time, of Space and of Matter of this Universe, as if a Voice were heard saying to man: 'Thou art no Atlas for so great a weight.'"
