through the mutual gravitation of its parts. As the Sun cools it contracts; 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 surface 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 difficulty 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 temperature 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 contracts through the loss of heat suffered by its radiation into space.
This subject was very exhaustively developed by Ritter some years since.[1] It is not practicable to give even an abstract of Ritter'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.
