HEAT OF THE SUN

Evening Star, Issue 18137, 29 November 1922, Page 5

HEAT OF THE SUN

IS IT DIMINISHING! The astronomical correspondent of the ‘Morning Post’ writes that it is generally accepted that the surface temperature of tho sun is not very far from o ) 6oodeg centigrade; probably not more tham 7,ooodeg, nor less than 6,2oodeg v Tins is practically constant, with certain limitations; that is to say ( that there is no evidence of a progressive diminution of tho sun’s heat, though opinion is being continually confirmed that there is a marked variation from year to year in tho sunspot period, and that fluctuations from day to day can be detected by means of tho spectrobolomcter, from which attempts have been made to forecast tropical rainfall. These rapid fluctuations seem on the face of it extremely probable, since tho sun’s surface shows such changes in regard to spots, jaculae, and prominences, so that the heat, radiated must apparently be subject to variation. In the main, however, it would be certainly plausible to suggest that tho sun is cooling, and it is very probable that this may be the case, but at so slow a rato as to be imperceptible to our most delicate instruments. If, then, it is not cooling, how is its heat maintained? On the analogy of an ordinary furnace it has been suggested that tho sun is constantly receiving fresh supplies of fuel from meteors falling into it. It has been estimated that eight millions of meteors strike the earth every day, so that a very much larger number may be expected to reach tho sun. _ Moreover, meteors will strike the sun with a much higher velocity than they will tho earth, on account of tho greatly increased force of gravity. Taking the average velocity of meteors striking the earth, at twentyfive miles per second, they will roach the sun at 384 miles per second, and the heat generated will be in tho proportion of tho squares of these numbers, so that •236 times as much heat will be produced by striking tho sun as by striking the earth. It has been computed that a kilogram falling into the sun would piovide nearly forty-six million calories. If we assume that the sun’s heat is maintained in this way, it fellows that the heat radiated from tho sun’s surface is just equal to that produced _by the meteors falling on it. If tho distribution is uniform, the earth will intercept tho share that would fall on that portion of the sun enclosed in a cone with it-s vertex at the sun’s centre, and just touching the earth on all sides—the only portion of the sun, in fact, which radiates heat to the earth. Thus the heat received from the sun is equal to that which' would be produced by the interrupted meteors failing into the sun. These, however, falling on the earth instead, will provide l-236Ui of this heat. Therefore the earth should receive from meteors l-236th of the heat received from the sun. This is very far from being the case, however, so the sun s heat cannot be maintained by meteors alone, even if we assume that all tho meteors approaching the sun actually reach it, ignoring tho great probability that a large proportion of them would merely trace out orbits round it, or would certainly fail to reach it with the assumed velocity of 384 miles per second. _ Doubtless the meteors have some share in maintaining the sun’s heat, but wo must look elsewhere for its main support, and, failing these external supplies it is evident that the cause must be internal. Work is the production of motion against resistance, and it is clear that if the sun is shrinking a great deal of work is being performed in the motion of every particle towards the centre. Helmholtz assumed that the sun contracts in such a way that it always remains homogeneous, and from this hypothesis, though it cannot be true, an approximate idea may be obtained of the process and results of contraction. Reduced to figures, the conclusion reached is that, if it always remained homogeneous, tho sun, in contracting from infinity to its present size, would generate enough heat to raise the temperature of an equal mass of water more than 27,000,000 deg centigrade. This may be regarded as almost meaningless, since contraction from infinity ignores all the rest of the’ universe; but if an assumed original solar nebula reached as far out as Neptune’s orbit these 27,000,000 deg must bo divided by about 6,600, making the increase of temperature more than 4,ooodeg centigrade. . It would seem, then, that if this were the sole source of heat maintenance the original solar nebula must have extended further than Neptune’s orbit. In view of the attempts that have been made to compute positions for extra Neptunian planets in the same way in which the investigations of Adam and Le Verrier led to tho discovery of Neptune, this seems quite reasonable, if Laplan’s nebular hypothesis can be accepted. But the contraction theory affords a due to the age of the earth, or at least points to an upper limit for it, and this upper limit is far too small for tho geological evidence, especially that founded on the ago of some uranium ores, deduced from the percentage of lead contained. Evidently the contraction theory will not account for everything, but it can be safely assumed that it will cover a great deal of the ground. If the sun radiates equally in all directions, it has been deduced by Abbott from boloiretric experiments that the amount of heat emitted annually would raise the temperature of a mass of water equal to that of tho sun through Ideig 44niin centigrade, and to maintain this rate of emission for 10,000 years would only require a shrinkage of about one second of arc in the sun’s diameter, which averages rather more than 1,900 seconds of arc. Tins means a shrinkage of about 43yds per annum, and it would take a great many years, even with the best of modern instruments, to provide at this rate a measurable diminution of the sun’s diameter. Tho reference to uranium ores reminds us that another possible source nf supply for heat energy may be found in the disintegration of radium or similar substances. We cannot assume very much in tho way of dissociation of chemical compounds on account of the high temperature of tho eun; but it cannot be denied that enormous possibilities lie in the disintegration of the atom itself, which forms tho basis of the uranium-lead argument for the earth’s age. Investigation of this process, under the name of ionisation, is one of tho most fascinating developments of modern science, and has its influence not only on ( solar physics, but on meteorology, on the theory of the aura, and on terrestrial magnetism, to name only a few of the related problems. In any event, it seems perfectly clear that there is no immediate prospect of a great diminution in tho sun’s heat, though it has been maintained with some probability that the sun has reached its maximum temperature, and must inevitably decline after some thousands, or perhaps millions, of years, without any apparent change.