CHAPTERS ON THE STARS. 139 



through the mutual gravitation of its parts. As the Sun cools it con- 

 tracts; the fall of its substance toward the center, produced by this 

 contraction, generates energy, which energy is constantly turned into 

 heat. The amount of contraction necessary to keep up the present 

 supply may be roughly computed; it amounts in round numbers to 220 

 feet a year, or four miles in a century. 



Accepting this view, it will almost necessarily follow that the great 

 body of the Sun must be of gaseous constitution. Were it solid, its sur- 

 face would rapidly cool off, since the heat radiated would have to be 

 conducted from the interior. Then, the loss of heat no longer going on 

 at the same rate, the contraction also would stop and the generation 

 of heat to supply the radiation would cease. Even were the Sun a 

 liquid, currents of liquid matter could scarcely convey to the surface a 

 sufficient amount of heated matter to supply the enormous radiation. 

 Thus the reason of the case combines with observation of the density 

 of the Sun to show that its interior must be regarded as gaseous rather 

 than solid or liquid. 



A difficult matter, however, presents itself. The density of the Sun 

 is greater than we ordinarily see in gases, being, as we have remarked, 

 even greater than the density of water. The explanation of this diffi- 

 culty is very simple: the gaseous interior is subject to compression by 

 its superficial portions. The gravitation on the surface being 27 times 

 what it is on the earth, the pressure increases 27 times as fast when we 

 go toward the center as it does on the earth. We should not have to go 

 very far within its body to find a pressure of millions of tons on the 

 square inch. Under such pressure and at such an enormous tempera- 

 ture as must there prevail, the distinction between a gas and a liquid 

 is lost; the substance retains the mobility of a gas, while assuming the 

 density of a liquid. 



It does not follow, however, that the visible surface of the Sun is a 

 gas, pure and simple. The sudden cooling which a mass of gaseous 

 matter undergoes on reaching the surface may liquefy it or even change 

 it into a solid. But, in either case, the sudden contraction which it thus 

 undergoes makes it heavier and it sinks down again to be remelted in 

 the great furnace below. It may well be, therefore, that the description 

 of the Sun as a vast bubble is nearly true. It may be added that all we 

 have said about the Sun may very well be presumed to apply to the 

 stars. We have now to consider the law of change as a sun or star con- 

 tracts through the loss of heat suffered by its radiation into space. 



This subject was very exhaustively developed by Bitter some years 

 since.* It is not practicable to give even an abstract of Bitter's results 

 at the present time, especially as every mathematical investigation of 

 the subject must either rest on hypotheses more or less uncertain, or 



* Wiedemann's 'Annalen der Physik u. Chemie,' 1878 to 1883, etc. 



