1919] on Giant Suns 415- 



painful experience. And yet the outside of a star is hot enouojh. 

 The temperature is again estimated from the spectrum, though this 

 time not from the hues but from the relative intensities of the ends, 

 and the B A end is undoubtedly hotter than the other. We may 

 give as iUustrations 15,000° for B, 5,000^ for G, and 2,500° for M. 

 Does this settle the matter r We know that there is a general 

 tendency for all bodies to cool which points to the direction C B — 

 M N as the order of events ; but it w^as also known that under the 

 stress of gravitation a star might rise in temperature, in which case 

 the growth might be the other way. Still the former alternative 

 commended itself more generally ; and when Prof. W. W. Campbell 

 found that the velocities of stars (also determined with the spectro- 

 scope) were smaller for type B than for type M, the facts were 

 interpreted to mean that a star moved more quickly with advancing 

 age (^because M stars were older than B). The idea that the life of 

 a star was spent in passage down the series B — M was indeed 

 pretty firmly established at the time when the revolution came. 



The revolution began with the advent of a young American 

 research student, Mr. H. N. Russell, to Cambridge in lOOd-6. It is 

 to the credit of Mr. A. R. Hinks that he made so much of this 

 brilliant young student, setting him on the way to determine tue 

 distances of a number of stars by photography with the instruments 

 which he (Mr. Hinks) has spent much time and labour in perfecting. 

 This was the first element in his success. The next was that on his 

 return to America he got from the Harvard Observator} — that store- 

 house of astronomical facts— the spectral types of his stars ; and 

 combining these with the measures of distance (which told him the 

 intrinsic brightness or luminosities of the stars) he found that stars 

 of the same spectral type M fell into two distinct groups separated 

 by an interval. There w'ere very bright stars, now called Giants, and 

 there were very faint ones, now called Dwarfs, but none of inter- 

 mediate stature. 



The same was true in minor degree for stars of other types, but 

 as the B end of the series w^as approached the gap gradually dis- 

 appeared much in the way that the gap between the legs of a step- 

 ladder gradually lessens as we approach the top. Indeed Russell's 

 diagram of his results is very like a step-ladder, the top representing 

 the B stars followed by A, F, G, K, M, in descending order, and the 

 gap between the two legs of the ladder representing the difference 

 in luminosity, as the intrinsic brightness of a star has come to be 

 called. Russell brought this diagram with him when he came to 

 attend the meeting of the Solar Union at Bonn in 1913. It is sad 

 to remember the occasion, for the most friendly relations seemed to 

 have been permanently established between the various nations 

 assembled. We remember with especial regret the trip on a great 

 steamer on the Rhine which ended the meeting, and, alas ! was the 

 end also of our hopes of a permanent friendliness, for before the 



