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NATURE 



[September 2, 1920 



The Internal Constitution of the Stars.» 



By Prof. A. S. Eddington, M.A., M.Sc, F.R.S. 



LAST year at Bournemouth we listened to 

 a proposal from the President of the 

 Association to bore a hole in the crust of the 

 earth and discover the conditions deep down 

 below the surface. This proposal may remind 

 us that the most secret places of Nature are, perhaps, 

 not 10 to the nth miles above our heads, but lo miles 

 below our feet. In the last five years the outward 

 march of astronomical discovery has been rapid, and 

 the most remote worlds are now scarcely safe from 

 its inquisition. By the work of H. Shapley the globu- 

 lar clusters, which are found.to be at distances scarcely 

 dreamt of hitherto, have been explored, and our know- 

 ledge of them is in some respects more complete than 

 that of the local aggregation of stars which includes 

 the sun. Distance lends not enchantment, but preci- 

 sion, to the view. Moreover, theoretical researches of 

 Einstein and Weyl make it probable that the space 

 which remains beyond is not illimitable ; not merely 

 the material universe, but also space itself, is perhaps 

 finite ; and the explorer must one day stay his con- 

 quering march for lack of fresh realms to invade. 

 But to-day let us turn our thoughts inwards to that 

 other region of mystery — a region cut off by more 

 substantial barriers, for, contrary to many anticipa- 

 tions, even the discovery of the fourth dimension has 

 not enabled us to get at the inside of a body. Science 

 has material and non-material appliances to bore into 

 the interior, and I have chosen to devote this address 

 to what may be described as analytical boring devices 

 - — ahsit omen! 



The analytical appliance is delicate at present, and, 

 I fear, would make little headwav against the solid 

 crust of the earth. Instead of letting it blunt itself 

 against the rocks, let us look round for something 

 easier to penetrate. The sun? Well, perhans. Many 

 have struggled to penetrate the mystery of the interior 

 of the sun ; but the difficulties are great, for its sub- 

 stance is denser than water. It mav not be quite so 

 bad as Biron makes out in "Love's Labour's Lost" : 



The heaven s glorious sun 

 That will not be deep-search'd with saucy looks : 

 Small have continual plodders ever won 

 Save base authority from others' books. 



But it is far better if we can deal with 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 sav 

 where, except perhaps 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, although at first sight they are scarcely 

 to be discriminated from dense stars like our sun. Not 

 only so, but the gaseous stars are the most powerful 

 light-givers, so that they force themselves 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 

 condition has been made known through the researches 

 of H. N. Russell (Nature, vol. xciii., pp. 227, 252, 281) 

 and E. Hertzsprung; the way in which their conclu- 

 sions, which ran counter to the prevailing thought of 

 the time, have been substantiated on all sides bv over- 

 whelming evidence is the outstanding feature of recent 

 progress in stellar astronomy. 



The diffuse gaseous stars are called giants, and 



*Opening address of the president o^Pection A (Mat^^emstical and Physical 

 Science) delivered at ih© Cardiff meeting of the British Association on 

 August 24. 



NO. 2653, VOL. 106] 



the dense stars dwarfs. During the life of a star 

 there is presumably a gradual increase of density 

 through contraction, so that these terms distinguish 

 the earlier and later stages of stellar history. It 

 appears that 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 contracts its tem- 

 perature 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 maximum temperature is attained, def)ending 

 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 descending 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 ori- 

 ginally 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 determinations of stellar densi- 

 ties, which are possible in the case of eclipsing vari- 

 ables like Algol. Finally, Adams and Kohlschiitter 

 have set the seal on this discussion by showing that 

 there are actual spectral differences between the ascend- 

 ing 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 disen- 

 tangled what is the true order of continuous increase of 

 density, which was hidden by superficial resemblances. 



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. I^t us try to picture the conditions 

 inside a giant star. We need not dwell on the vast 

 dimensions — a mass like that of the sun, but swollen 

 to much greater volume on account of the low density, 

 often below that of our own atmosphere. It is the 

 star as a storehouse of heat which especially engages 

 our attention. In the hot bodies familiar to us the 

 heat consists in the energy of motion of the ultimate 

 particles, flying at great speeds hither and thither. So, 

 too, in the stars a great store of heat exists in this 

 form ; but a new feature arises. A large proportion, 

 sometimes more than half the total heat, consists of 

 imprisoned radiant energy — aether-waves travelling in 

 all directions trying to break through the material 

 which encages them. The star is like a sieve, which 

 can retain them onlv temporarily ; they are turned 

 aside, scattered, absorbed for a moment, and flung out 

 again in a new direction. .An element of energy may 

 thread the maze for hundreds of vears before it attains 

 the freedom of outer space. Nevertheless, the sieve 

 lealvs, and a steady stream permeates outwards, supply- 

 ing the light and heat which the star radiates all round. 



That some aethereal heat as well as material heat 

 exists in anv hot bodv would naturallv be admitted ; 

 hut the point on which we have here to lay stress is 



