182 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



Mr. Russell, again specially in response to a request mentioning this 

 lecture : 



What appeals to me as the big thing is Eddington's work on radiative equi- 

 librium (MN 77, p. 16 and p. 596). The importance of this can hardly be 

 exaggerated ; it is not too much to say that it is the first rational theory of 

 stellar constitution. 



Eddington has in fact given us a rough attempt at tracing the 

 history of a star of given mass. By way of illustration let us con- 

 sider our own sun. He is now a " dwarf star," on the descending 

 leg of the ladder, of spectral type G, and with a surface temperature 

 of about 5,000° C, and an absolute magnitude 5.1. Looking back 

 into the past he was at one time much hotter and of type F, and 

 probably never rose much higher than this on the ladder. Before 

 that his history lay on the ascending leg, and there was a time when 

 his spectral type was just as at present, but his absolute magnitude 

 was near zero, five magnitudes greater than at present. This means 

 that the total light was 100 times greater than now, and since the 

 surface was in a similar radiative state, it must have been 100 times 

 more extensive. The diameter of the sun was therefore 10 times the 

 present diameter — 10,000,000 miles instead of 1,000,000. Where our 

 little earth may have been at that time we can scarcely con- 

 jecture, but supposing for a moment that we had been able to regard 

 the sun in our present conditions, he would have taken nearly an 

 hour to rise instead of a few minutes ; and when risen, his disk would 

 be 10 times as great in all directions — a " giant " sun indeed ! And 

 yet this magnification of 10 to 1 is only modest compared with the 

 extreme possibilities. 



We set out by the recollection of Jack the Giant Killer, but our 

 road has led us rather to think perhaps of Jack and the Beanstalk. 

 We have climbed up to giant land, the land of the giant suns, not 

 by a beanstalk, but by means of the trembling rays of light, a ladder 

 which does not grow upward from our earth, but is let down to it 

 by the giants themselves. " Fee fo f um ! " said the giant, " I smell 

 the blood of an Englishman." In our analogy the giants have been 

 invaded, not by an Englishman, but chiefly by an American ; but at 

 any rate we have the satisfaction of reflecting that his work began 

 in Cambridge when he was a student, and that at the end of it 

 there has emanated from Cambridge this brilliant confirmation by 

 Prof. Eddington of which the discoverer has himself expressed 

 such generous appreciation. 



