the fact that the Sun and stars have, for untold millions of years, been radiating heat into space. If we refrain from considering the basis on which this conclusion rests, it is not so much because we consider it unquestionable, as because the discussion would be too long and complex for the present work.
One of the great problems of modern science has been to account for the source of this heat. Before the theory of energy was developed this problem offered no difficulty. In the time of Newton, Kant and even of La Place and Herschel, no reason was known why the stars should not shine forever without change. Now we know that when a body radiates heat, that heat is really an entity termed energy, of which the supply is necessarily limited. Kelvin compared the case of a star radiating heat with that of a ship of war belching forth shells from her batteries. We know that if the firing is kept up, the supply of ammunition must at some time be exhausted. Have we any means of determining how long the store of energy in Sun or star will suffice for its radiation?
We know that the substances which mainly compose the Sun and stars are similar to those which compose our earth. We know the capacity for heat of these substances, and we also have determined how much the Sun radiates annually. From these data, it is found by a simple calculation that the temperature of the Sun would be lowered annually by more than two degrees Fahrenheit, if its capacity for heat were the same as that of water. If this capacity were only that of the substances which compose the great body of the earth, the lowering of temperature would be from 5° to 10° annually. Evidently, therefore, the actual heat of the Sun would only suffice for a few thousand years' radiation, if not in some way replenished.
When the difficulty was first attacked, it was supposed that the supply might be kept up by meteors falling into the Sun. We know that in the region round the Sun, and, in fact, in the whole Solar System, are countless minute meteors some of which may from time to time strike the Sun. The amount of heat that would be produced by the loss of energy suffered by a meteor moving many hundred miles a second would be enormously greater than that which would be produced by combustion. But critical examination shows that this theory cannot have any possible basis. Apart from the fact that it could at best be only a temporary device there seems to be no possibility that meteors sufficient in mass can move round the Sun or fall into it. Shooting stars show that our earth encounters millions of little meteors every day; but the heat produced is absolutely insignificant.
It was then shown by Kelvin and Helmholtz that the Sun might radiate the present amount of heat for several millions of years, simply from the fund of energy collected by the contraction of its volume
