GIANT SUNS TURNER. 177 



2,500° for M. Does this settle the matter? We know that there 

 is a general tendency for all bodies to cool which points to the 

 direction O B — M N as the order of events; but it was 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 deter- 

 mined with the spectroscope) 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 O B — M 

 was indeed pretty firmly established at the time when the revolution 

 came. 



The revolution began with the advent ol a young American 

 research student, Mr. H. N. Eussell, to Cambridge in 1904^6. 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 the 

 distances of a number of stars by photography with the instruments 

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

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

 return to America he got from the Harvard Observatory — 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 were 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 was 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 stepladder, 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 

 year had passed the great war had shattered them all. It was on 

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