ORIGIN OP THE SOLAR SYSTEM JEANS 151 



in diameter, the most gigantic of known giant stars may be repre- 

 sented by a pin head one-thirtieth of an inch in diameter. The pres- 

 ent spacing of the stars is such that on this scale there is less than 

 one star to a volume equal to the interior of St. Paul's Cathedral. 

 Space then can not be said to be overcrowded, and although it is 

 possible that the stars may disturb one another as they move in their 

 courses, it is clear that any serious disturbance of one star by another 

 must be a rather exceptional event. Obviously we have been right 

 in regarding the evolution of a star entirely undisturbed by its 

 neighbors as the normal course of evolution, and we can now see 

 why the vast majority of stars follow this normal course. 



To all appearances, the stars which have been sidetracked off this 

 normal course are extraordinarily few in number. The total number 

 of stars in the sky is about equal to the total population of the earth ; 

 the number of known exceptional systems would at most populate 

 one small town, although, of course, we can scarcely even conjecture 

 how many exceptional systems there may be which are still un- 

 known to us. There is no reason for supposing that the sidetracking 

 influence has in every case been a neighboring star, but the systems 

 known to be exceptional are sufficiently few to suggest that this may 

 have been the cause in a large proportion of cases. 



The immediate question before us, however, is not that of the 

 exceptional systems in general, but of our own solar system. Was 

 it a neighboring star that threw it off the main line of evolutionary 

 development? Here, for the first time, observational astronomy 

 denies us any help. Not a single system is known outside our solar 

 system which resembles it in the least degree. The reason is not 

 that no such system exists, but that we could not see it if it did. 

 An astronomer on a distant star observing our system would see 

 Jupiter as the brightest object after our sun, but the ratio of their 

 luminosities would be as 300,000,000 to 1. Seen from our nearest 

 known neighbor in space, Proxima Centauri, the sun would appear 

 as a first magnitude star, and Jupiter as a star of magnitude 22.2, 

 the distance between them being at most four seconds of arc. A 

 star of magnitude 22.2 is still well beyond the range of our largest 

 telescopes, and would be doubly invisible if it had a first magnitude 

 star only four seconds away. We must wait for a very great in- 

 crease in the power of our telescopes before there will be any hope of 

 seeing systems similar to our own in the sky, even if they exist 

 no further away from us than Proxima Centauri. Thus it is clear 

 that our discussion has now left the regions in which observation 

 can be called upon to make suggestions or to check our conclusions : 

 henceforth we have theory alone to guide us. 



Let us start on our quest by noticing that our solar system has 

 quite clearly marked characteristics. It is no mere jumble of bodies 



