288 The Origin and Evolution of the Solar System [on. xn 



bursts into incandescence as a giant Jl/-star. During this period energy is 

 radiated in the form of heat, but observation tells us nothing as to its 

 amount. Theory, as developed in Chap. VIII, shews that the early heat- 

 radiation will be the same in amount as the later light-radiation provided 

 the opacity c remains constant. The opacity, however, is not likely to remain 

 constant; the radiation in the interior of a luminous star is of very short 

 wave-length*, comparable with that of soft X-radiation, and matter might be 

 expected to be much more transparent for such radiation than for the radiation 

 which conveys the energy in the interior of a non-luminous star. Against 

 this must be set the fact that a comparison of the total emission of a star 

 with the temperature gradient in its interior shews that the matter of 

 luminous stars must be very much more opaque than ordinary gaseous matter ; 

 Eddingtonf calculates an opacity such that light is reduced to 1/e times its 

 original intensity after passing through -fa gm. per sq. cm. of stellar matter. 

 Imagine, however, that we could in some way suppose the mean opacity c for 

 the earlier dark sun to have been forty times as great as for the present sun. 

 Then the rate of emission of radiation by the dark sun would have been only 

 a fortieth of the present rate, and the time-scale may be extended about forty 

 times. More precisely we may perhaps suppose that three-quarters of the 

 total energy of contraction has been radiated from the sun in its luminous 

 period, this lasting about 15 million years, while the remaining quarter was 

 radiated in a non-luminous period lasting for about 200 million years. 



Some such course seems to be the only one open if we accept the geological 

 estimates of the earth's age, while refraining from introducing unknown sources 

 of solar energy such as that suggested in 308. Our conjecture will not, how- 

 ever, satisfy those geologists who maintain that the earth has remained at a 

 temperature near to its present temperature for a period of hundreds of 

 millions of years. 



CONCLUSION 



It has not been part of our task to arrive at a conclusion ; the time for 

 arriving at conclusions in cosmogony has not yet come. Our object has rather 

 been to consider different hypotheses in turn, pointing out and perhaps to 

 some extent balancing the advantages and disadvantages of each, leaving it to 

 future investigators, armed with more mathematical and observational know- 

 ledge than we at present possess, to pronounce a final decision. 



In so far as one conclusion has seemed to us more probable than another, it 

 has been something of the following kind. Some hundreds of millions of years 

 ago all the stars within our galactic universe formed a single mass of excessively 



* Eddington, Monthly Notices R.A.S. 77 (1917), p. 34. 

 t I.e. p. 28. 



