Age and Duration of the The great est problem connected with the sun is suggested by modern science. Up to 100 years ago stu dents and philosophers paw no reason the sun should not continue to shed heat and light on the earth for an indefinite period without undergoing any change whatever. But toward the middle of the 19th century the laws of energy were developed and understood. These laws set forth that the radiation of heat always in volves the expenditure of something called energy; and that the latter is necessarily limited in supply. It was also seen that the sun must be a hot body, and must lose all the heat it radiated. To make this subject clear, we must remark that what the sun really radiates is not properly called heat in scientific nomenclature; the more exact term is radiant energy. But this differs in no respect from what is radiated into a room from a hot fire. Radiant energy goes out from the fire and strikes the walls of the room, where it creates heat, and thus warms the walls. All the heat thus transmitted to the walls comes from the coal, although in its pas sage from the fire to the walls it passes through an intermediate stage called radiant energy. That the latter does not necessarily warm a me dium through which it passes is shown by the striking experiment of making a large lens out of ice instead of glass. When the sun's rays are concentrated on a point by passing through this mass of ice they will burn the substance on which they fall, as if they had passed through glass. We see, then, that however cold the space between us and the sun may be, all the radiant energy reaching us from the sun must come from a source in the sun limited in supply.
If the sun were merely losing energy like an ordinary hot body cooling off, a very simple calculation will show that it would be so cooled off in the course of 3,000 or 4,000 years as no longer to radiate much heat. It is clear that such has not been the case. Yet the most care ful study shows no possibility that it can be receiving any considerable part of its energy from any outside source. Moreover, the geol ogists assure us that the stratification of the rocks, as well as many other other phenomena associated with them, proves that the sun has been radiating heat to the earth at not much less than its present rate for hundreds of millions of years.
The only solution of the puzzle that was sug gested until 1903 was based on the mutual con vertibility of heat and motion. From the time that the theory of energy was developed it was known that when the motion of a material sub stance is arrested without any other effect being produced, heat is generated. For example, the waters of Niagara are warmed by about one quarter of a degree Fahrenheit in striking the bottom of the falls. The blacksmith by ham mering a piece of cold iron can make it hot, be cause the energy which he puts into the motion of the hammer is converted into heat when the latter strifes the iron. It follows that if bodies of any sort are falling into the sun, heat will be generated by the fall. Moreover, owing to the power of the sun's attraction, such bodies may fall with great velocity; and the heat thus gen erated increases as the square of the velocity. Thus arose the first theory as to how the sun's heat could be kept up. It was supposed that meteors were continually falling into that lumi nary. But further study showed it to be im possible that meteors could fall in such quantity as to have this effect.
Then it was suggested by Helmholtz and Thomson that if the sun were a gaseous body, as it is now supposed to be, radiating energy, the loss of the latter would continually be made up by the fall of its outer portions involved in the continual contraction of the sun through loss of heat. All bodies, and gaseous ones in .a
higher degree than any others, diminish in vol ume when they cool off. Accordingly, when the photosphere of the sun cools off, it diminishes in volume, grows smaller and falls down upon the mass of the sun below it. Careful calcula tion shows that if the sun contracted about 250 feet per annum, the energy thus generated would keep up all the heat which the sun radi ates. An important addition to this theory was made by J. Homer Lane, who showed that if the sun contracted like a mass of pure gas it would continually grow hotter as it con tracted. This is now known as Lane's law. But there is a necessary limit to the quantity of heat which can thus be generated. If the sun has been thus growing smaller through long ages, there must have been a time when it filled the whole space now occupied by the solar sys tem. What is more, the contraction must have been far more rapid the larger the sun was; because the force of attraction at the sun's sur face diminishes as the inverse square of the diameter of the sun. For example, when the sun was twice as large as it is now, this force was only one-quarter as great; consequently it would have to contract four times as much to generate a given amount of energy as it does now. Finally, exact computation showed that even on this theory there was still a limit to the existence of the sun too narrow to satisfy the demands of geology. It could not have been radiating heat for more than 50,000,000 or 100,000,000 years. Before that time it must, ac cording to the theory, have been a gaseous mass filling the whole space now occupied by the solar system, which contracted and formed sun and planets, in accordance with the theory known as the ((nebular hypothesis.D It also seemed very improbable that the sun's heat could have been at all constant for even 20,000,000 years; on the other hand, geologists went hundreds of millions of years. Thus ap parently an irreconcilable contradiction was pre sented to scientific investigators when in 1900 the discovery of radium began to put a new face upon the fundamental theories of physical science. We now lcnow that there is an im mense amount of energy stored in the atom, which is a very. complex thing. With the so called ((radioactive) substances, the atoms may be broken up, the result of the process being an element of lower atomic weight than the original substance. And in this brealcing up of the atom a great amount of energy is liberated. Though it has been known for many years that Helm holtz' theory was inadequate, whether a large part of the sun's energy is of this sub-atomic origin, we do not know, but it is reascmable to suppose that it is. And it is only necessary to suppose that a part of the energy of the atom is in this way changed into heat energy to al most indefinitely prolong.the life of the sun.
The most recent semi-popular, but authen tic work is 'The Sun' by C. G. Abbot (New York 1911) •, this contains many references to more extended worlcs or detailed publications. A larger and very important work is (Physik der Sonne,' by E. Pringsheim (Leipzig 1910). Numerous papers will be found in the Proceed ings of the Royal Society, London, and in the Astrophysical Journal, Chicago.