PAPERS PRESENTED AT GENERAL SESSIONS 71 



dwell upon this point — not only are the giants exceptional 

 wdth regard to their masses, they may be exceptional 

 also with regard to their velocities. 



It will be observed that if the radiations of the stare are 

 derived at the expense of their masses and if the masses 

 are renewed by material gathered in from the snn-onnd- 

 ing space, then there is no necessary upper limit to the 

 life of a star, nor to that group of stars which we call the 

 galaxy. The long intervals of time which are demanded 

 by the dynamics of the galaxy have a real significance. 

 In particular, the close approach of two stars, which, on 

 account of the limited time admitted into the discussion, 

 has hitherto been regarded largely as an imaginary event 

 now becomes an event of fundamental importance, for 

 they are certain to occur in sufficiently extended inter- 

 vals of time. If the distance from the earth to the sun 

 be taken as the measure of a close approach then for any 

 one star there Avill be such a close approach once in every 

 four million billion (10'^) years on the average: and if 

 there are a billion stars in the galaxy there should be such 

 a close approach somewhere in the galaxy once in eveiw 

 eight millions of years. 



The consequences of such a close approach have been 

 very carefully studied by Chamberlin and Moulton, and 

 forms the basis for the Planetesimal Hypothesis as to the 

 origin of our planetary system. The splendid harmonies 

 exhibited by this h>i)othesis in the domains of d^niamical 

 astronomy and of geology impress upon us the conviction 

 that our own sun experienced such a close encounter 

 some ten or twenty billions of years ago, and that our 

 planetary system is the result of the encounter. If this 

 conviction is justified, then planetary systems must be 

 fairly numerous throughout the galaxy and they should 

 exist in varying stages of development. As ten billions 

 of years is a very short interval of time from a galactic 

 point of view our planetary system must be relatively 

 young. If the sun should plunge into a relatively dense 

 nebulous region and add to its mass, so also would the 

 planets add to their mass. In the course of time the sun 

 would lose its extra mass by radiation Avhile the planets 

 would not. After many such experiences Jupiter and 



