THE GIANT TELESCOPE

Evening Star, Issue 20058, 26 December 1928, Page 7

THE GIANT TELESCOPE

CALIFORNIA'S NEW SCHEME WONDERS OF STELLAR PHOTOGRAPHY THE LIMITS OF THE UNIVERSE? [By Professor H, H. Tcrnkr, F.R.S., in the ‘Observer.’] It has been recently announced that funds have been provided for the construction and maintenance of a telescope with a reflector of 200 in in diameter, double the width of the largest existing reflector, which is that of tho Hooker telescope on Mount Wilson. Although this comes as a definitely approved project with some suddenness, i* was known to astronomers that the construction of a large telescope, even so large as 3001 n in diameter, had been under the most careful consideration for some years; indeed, specifications and estimates on this scale appeared in August, 1926, in the ‘ Publications of tlie Astronomical Society ot the Pacific,’ with the confident statement that such a telescope could be built and used if only the money could be provided. But that money ran into millions of pounds, and no person or body of persons seemed ready to face. so largo an expenditure. Recently’ it was whispered, almost with awe," that the proposal might not remain entirely vicionary. Tho man who has more than once made possible a great scientific advance, George Ellory Hale, though his health had given way under the strain of his devoted work lor science, has fortunately so far recovered as to “ take hold ” ’, an “ Observatory Council ” has been constituted under Ins chairmanship, and though the proposed diameter of the mirror has been reduced Irom JOOin to 2UUuv, lor reasons easily understood, there is now every prospect that within ten years this giant will come into being and open our eyes to new wonders. LORD ROSSE’S TELESCOPE. What advance in particular do we expect to make? Judging from the case of its’predecessor, the lOOin, its most striking usefulness may turn out to be in some direction at present uuloreseeu or barely foreseen. But it will almost certainly extend the achievements ol the LOOin, one of which has been sensational enough to deserve special mention; indeed, it is largely the prospect of bettering this particular achievement that has crystallised a vision of the future into a definite and practical scheme of the present. Let us cast our memories back nearly a century to 1845, wdien Lord Rosse completed his great 6ft which excited the wonder of the world. De Quincey wrote an essay about it, which is still so much alive that a passage from it was set for translation into Latin prose as one of this year’s prism compositions at Oxford (voi. HI. in Black’s edition, pp. 169-176). The importance of the results expected may bo gauged from these few sentences of De Quincey;— “ 1 ask peremptorily, docs it stand with good sense, is it reasonable, that earth is waning, .science drooping, man looking downward, precisely in that epoch when, first of all, man’s eye is arming itself lor looking effectively into the mighty depths of space? A new era lor the human intellect upon a path that lies amongst its most aspiring is promised, is inaugurated, by Lord Rossc’s almost awful telescope.” STARS AND NEBUL/E. Such expectations, high as they were, were not altogether disappointed. Lord Rosse secured an advance in two directions: he could see fainter stars, qnd be saw them more clearly defined. Hence some of the nebula) which smaller telescopes had seen as continuous hazy masses, presumably of gas, were with the new weapon “ resolved ” into myriads of tiny faint stars. Two inferences naturally presented themselves: the first, that tlie nebuhu which this great telescope failed so to resolve might be resolved into stars if an even bigger telescope could be made; and, secondly, that all the nebuhu were consequently distant communities of stars —how distant it was scarcely possible even to guess, and quite impossible to measure. At that time astronomers could only measure the distance of one or two stars comparatively close to us, the light from which could reach us hi a few years. In subsequent years three important events modified the situation thus sketched. Firstly, in 1864 the spectroscope showed that some of the nebuhu, at any rate, were masses of luminous gas, not collections of stars. Secondly, although no telescope larger than Lord Rosse’s was built until well on into the present century, photography (about 18SU) provided an alternative to increasing the size of telescopes in the prolongation of exposure. Expose a plate in a telescope for a few minutes and images will be obtained of tlie Jiiirily bright stars; expose for an \wn and the hunter stars appear in addition; expose it for a hundred hours and stars much fainter still will be photographed. It might seem that the shortness of a night would prevent such long exposures, but with proper care it is tonnd passible to close up the telescope us daylight approaches, and to resume the exposure on the next fine night. Cumulative exposures have been given in this way for more than 100 hundred hours, and it might seem that by mere patience and industry the possessor of a moderate telescope could secure the advantages of a larger one, by reaching fainter and fainter stars to an almost indefinite extent. He can certainly secure some of these advantages, but not all, as will appear in a moment. LIGHT A MILLION YEARS ON THE WAY. The third event which has completely altered the situation is that within tlio last decade a method of measuring the distances ol the stars and of tlie nebuho which contain stars, even when they are almost incredibly remote, has been discovered. Among the millions of stars which shine steadily there are some hundreds, called Ccpheids, which vary their light in a peculiar way, some oi*Jheni in a few hours, others in a low days; and the gist of tlie discovery is that those which take longest to vary are the brightest. By merely watching such a star so as to note how soon tlie changes repeat we can tell how bright it really is ; however hunt it may seem to us owing to its great distance. And, in fact, by comparing what seems withkivhat really is we can infer that distance. Thus has an immense step forward been taken in the exploration of the universe. Dr Hubble trained the great lOOin telescope on the Andromeda nebula.and photographed it repeatedly. Among the marly tiny stars shown half a dozen were found which varied their light in this way. Their periods of variation were found, their real brightnesses were inferred', and by comparison with their seeming brightnesses the distance of the nebula; was found to be a million light years. ; All the half-dozen gave consistent testimony that tho light which reaches us now was sent’out by them a million years ago. And this was not the only success of the kind. Smaller and fainter nebula) were similarly photographed,

Cepheids found in them, and distances hundreds of times as great were mcas r ured. And then even the great lUUin telescope could do no more; it had reached its natural limits. Why was this r* Why could not still fainter nebulto be reached by prolonging tile exposure as above indicated? The answer is that the exposures must necessarily be short if changes in a few hours are to be noted. For a picture of a horse race it is useless to take a long exposure; we must take snapshots, because the horses are changing their positions rapidly. The stars are changing, not their positions, but their brightnesses, and by long exposures we should lose just what we want to find. The only resource is to have a larger telescope. Hence the project which had been under serious, but not very hopeful, consideration for some years has now taken definite shape. THE LIMITS OF THE UNIVERSE? Dr Hubble’s successes, and even more perhaps the cheek which put an end to them, have given a new impulse to the project. There is, further, a feature of the situation which can only be handled somewhat delicately lest too great an impression may inadvertently be made. If tiie distances already gauged could lie extended as the substitution of a OOOiii telescope for a lUOiu might extend them they would approach, not perhaps Einstein’s suggested limit to space, but at any rate a respectable fraction of it. Einstein’s work leads him to think that space is not indefinitely extended like a fiat plane, but limited like the surface ;of a npfierc which returns into itself, and that just as there, is a radius for this surface in two dimensions so there is a radius for space in three, for which he has assessed a value. If we could measure distances comparable with this value we might learn something, even if at present we scarcely know what. We may be tempted to sneer at such strange ideas; many people sneered at Einstein’s predictions that rays of light could be bent by gravity, until the eclipses of 1919 and 1922 proved that he was triumphantly right. In any case it is clear that a larger telescope can make an advance which pinnot be made in any other way, and it is delightful to find that the decision to make one (even though ■ produce liiio restricted the size to 200 in rather than 300) has been taken and the money actually provided. It is delightful to find that Dr Hale is again able to take the lead in the enterprise, and. especially is it delightful to us in England to see prominent among the names of those whose help has been gratefully acknowledged that of Sir Charles Parsons—son of the Lord Rosse who made the great Gft telescope eighty years age—all the more because-he has himself come nobly to the aid of the British optical industry in the great troubles which belcll it after the war,