234 



SCIENCE 



[N. S. Vol. LII. No. 1341 



of letting it blunt itself against the rocks, let 

 us look round for sometliing easier to pene- 

 trate. The sun? Well, perfiaps. Many have 

 struggled to penetrate the mystery of the in- 

 terior of the sun; but the difficulties are great, 

 for its substance is denser than water. It may 

 not be quite so bad as Biron makes out in 

 " Love's La;bour's Lost " : 



The teaven 's glorious sun ; 

 That will not be deep-searched with saucy looks; 

 Small have continual plodders ever won 

 Save base authority from others' books. 



But it is far better if we can deal vcith matter 

 in that state known as a perfect gas, which 

 charms away difficulties as by magic. Where 

 shall it be found? 



A few years ago we should have been puzzled 

 to say where, except peAaps in certain nebulae ; 

 but now it is known that abundant material of 

 this kind awaits investigation. Stars in a 

 truly gaseous state exist in great numbers, al- 

 though at first sight they are scarcely to be 

 discriminated from dense stars like oux sun. 

 Not only so, but the gaseous stars are the most 

 powerful light-givers, so that they force them- 

 selves on our attention. Many of the familiar 

 stars are of this kind — Aldebaran, Canopus, 

 Arcturus, Antares ; and it would be safe to say 

 that three quarters of the naked-eye stars are 

 in this diffuse state. This remarkable condi- 

 tion has been made known through the re- 

 searches of H. ]Sr. Eussell and E. Hertzsprung ; 

 the way in which their conclusions, which 

 ran counter to the prevailing thought of the 

 time, have been substantiated on all sides by 

 overwhelming evidence, is the outstanding 

 feature of recent progress in stellar astronomy. 

 , The diffuse gaseous stars are called giants, 

 and the dense stars are called dwarfs. Dur- , 

 ing the life of a star there is presumably a 

 gradual increase of density through contrac- 

 tion, so that these terms distinguish the earlier 

 and later stages of stellar history. It appears 

 lihat a star begins its effective life as a giant of 

 comparatively low temperature — a red or M- 

 type star. As this diffuse mass of gas con- 

 tracts its temperature must rise, a conclusion 

 long ago pointed out by Homer Lane. The 



rise continues until the star becomes too dense, 

 and ceases to behave as a perfect gas. A max- 

 imum temperature is attained, depending on 

 the mass, after which the star, which has now 

 become a dwarf, cools and further contracts. 

 Thus each temperature-level is passed through 

 twice, once in an ascending and once in a de- 

 scending stage — once as a giant, once as a 

 dwarf. Temperature plays so predominant a 

 part in the usual spectral classification that 

 the ascending and descending stars were not 

 originally discriminated, and the customary 

 classification led to some perplexities. The 

 separation of the two series was discovered 

 through their great difference in luminosity, 

 particularly striking in the case of the red and 

 yellow stars, where the two stages fall widely 

 apart in the star's history. The bloated giant 

 has a far larger surface than the compact 

 dwarf, and gives correspondingly greater light. 

 The distinction was also revealed by direct de- 

 terminations of stellar densities, which are 

 possible in the case of eclipsing variables like 

 Algol. Finally, Adams and Kohlschiitter have 

 set the seal on this discussion by showing that 

 there are actual spectral differences between 

 the ascending and descending stars at the same 

 temperature-level, which are conspicuous 

 enough — ^when they are looked for. 

 ' Perhaps we should not 'too hastily assume 

 that the direction of evolution is necessarily in 

 the order of increasing density, in view of our 

 ignorance of the origin of a star's heat, to 

 which I must allude later. But, at any rate, it 

 is a great advance to have disentangled what 

 is the true order of continuous increase of 

 density, which was hidden by superficial re- 

 semblances. 



The giant stars, representing the first half of 

 a star's life, are taken as material for our first 

 boring experiment. Probably, measured in 

 time, this stage corresponds to much less than 

 half the life, for here it is the ascent which is 

 easy and the way down is long and slow. Let 

 us try to picture the conditions inside a giant 

 star. We need not dwell on the vast dimen- 

 sions — a mass like that of the sun, but swoUen 

 to much greater volume on account of the low 

 density, often below that of our own atmos- 



