CHAPTERS ON THE STARS. 147 



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 con- 

 tinual 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 nebulae offer a similar difficulty. Their extreme tenuity and 

 their seemingly almost unmaterial structure appear inadequate to ac- 

 count for any such mutual gravitation of their parts as would result in 

 the generating of the flood of energy which they are constantly radiat- 

 ing. 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 

 nebulae 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.' " 



