PLURALITY OF WORLDS 169 



laid down 1200 million years ago. The sun must have 

 been burning still longer, living (we now think) on its 

 own matter which dissolves bit by bit into radiation. 

 According to the theoretical time-scale, which seems 

 best supported by astronomical evidence, the beginning 

 of the sun as a luminous star must be dated five billion 

 (5-I0 12 ) years ago. The theory which assigns this date 

 cannot be trusted confidently, but it seems a reasonably 

 safe conclusion that the sun's age does not exceed this 

 limit. The future is not so restricted and the sun may 

 continue as a star of increasing feebleness for 50 or 

 500 billion years. The theory of sub-atomic energy 

 has prolonged the life of a star from millions to bil- 

 lions of years, and we may speculate on processes 

 of rejuvenescence which might prolong the exist- 

 ence of the sidereal universe from billions to trillions 

 of years. But unless we can circumvent the second 

 law of thermodynamics — which is as much as to 

 say unless we can find cause for time to run back- 

 wards — the ultimate decay draws surely nearer and the 

 world will at the last come to a state of uniform 

 changelessness. 



Does this prodigality of matter, of space, of time, 

 find its culmination in Man? 



Plurality of Worlds. I will here put together the present 

 astronomical evidence as tQ the habitability of other 

 worlds. The popular idea that an answer to this ques- 

 tion is one of the main aims of the study of celestial 

 objects is rather disconcerting to the astronomer. Any- 

 thing that he has to contribute is of the nature of frag- 

 mentary hints picked up in the course of investigations 

 with more practicable and commonplace purposes. 

 Nevertheless, the mind is irresistibly drawn to play with 



