GIANT TELESCOPE

Dominion, Volume 22, Issue 81, 29 December 1928, Page 21

GIANT TELESCOPE

CALIFORNIA’S NEW SCHEME

(By Professor H. H. Turner, F.R.S., in the London “Observer.”)

It has been recently announced that funds have been provided for the construction and maintenance of a telescope with a reflector of 200 inches in diameter, double the width of the largest existing reflector, which is that of the Hooker telescope on Mount Wilson. Although this comes as a deflnitely approved project with some suddenness, it was known to astronomers that the construction of a large telescope, even so large'as 300 inches 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 the Astronomical Society of 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 large an expenditure. Recently it was whispered, almost with awe. that the proposal might riot remain entirely visionary. The mau who has more than once made pos-

sible a great scientific advance, George .EUery Hale, though his health had given away under the strain of his devoted work for science, has fortunately so far recovered as to “take hold”: an “Observatory Council” has been constituted under his chairmanship, and though the proposed diameter of the mirror has been reduced from 300 inches to 200 inches, for. 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. • What advance in particular do we expect to make? Judging from the case of its predecessor, the 100-lnch, its most striking usefulness may turn out to be in some direction at present unforeseen, or barely foreseen. But it will almost certainly extend the achievements of the 100-inch, one of which has been sensational enough to deserve special mention; indeed, it is largely the prospect of bettering this particular achievement that has crystalised 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, when Lord Rosse completed his great six-foot reflector, which excited the wonder of the world. De Quincey wrote an essay about it, which is still so much alive i that a passage from it was set for translation into Latin prose as one of this year’s prize compositions at Oxford. The importance of the results expected may be gauged from these few sentences of De Quincey :— I ask premptorily, Does 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 for look--1 ing effectively into the mighty depths of space? A new era for the human Intellect upon a path that lies amongst its most aspiring, is promised, is inaugurated, by Lord Rosse’s almost awful telescope. Such expectations, high as they Were, were not altogether disappointed. / Lord Rosse secured an advance in two directions: he could see fainter stars, and he saw them more clearly defined. Hence some of the nebulae 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 the nebulae 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 nebulae were consequently distant

communities of stars—how distant it was scarcely possible even to guess, and quite impossible to measure. At that- time astonomers could only measure the distance of one or two stars comparatively close to us, the light from which could reach us in a few years.

In subsequent years three important events modified the situation thus sketched. Firstly, in 1864, the spectroscope showed that some of the riebulae, 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 1880) provided al al- ' ternative 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 the fairly bright stars; expose for an hour and the ; fainter 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 found possible to close up the telescope as daylight approaches, and to . resume the exposure on the next fine night. Cumulative exposures have , been given In this way for more than one hiindred 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.

1 The third event which has completely altered the situation is that within the last decade a method of measuring the distances of the stars and of the nebulae 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 Cepheids, which vary their light in a peculiar way, some of them in a few hours, others in a few days; and the gist of the discovery is that those which take longest to vary are the . brightest. By merely watching such a star so as to note how soon the changes repeat we can tell how bright it really is. however faint it may seem to us owing to its great distance. And, in fact, by comparing what seems with what 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 100-. inch telescope on the Andromeda nebulae and photographed it repeatdly. Among the many 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 nebulae was found to be a million light-years. All the half-dozen gave consistent testimony tllat the 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 nebulae were similarly photographed. Cepheids found in them, and distances 'hundreds of times as great were measured. And then even the great 100inch telescope could do no more: it had reached its natural limits.

Why was this? Why could not still fainter nebulae be reached by prolonging the exposeure is that the exposures must necessarily be short if changes fin a few hours are to be noted. For a picture of a horse-race ii is useless to take a long exposure; we must take snap-shots because the horses are changing their positions rapidly. The stars are changing, not 1 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. Dr. Hubble’s successes, and even more perhaps the check 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 macle. If the distances already gauged could be extended as the substitution of a 300-inch telescope for a jOO-inch might extend them, they would approach, not perhaps Einstein’s suggested limit of 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 flat plane, but limited like the surface of a sphere 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 prediction that rays of light could be bent by gravity, until the eclipses of 1919 and 1922 p.roved that he was triumphantly right. In any case it is clear that a larger telescope can make an advance, which cannot be made in any other way; and it is delightful to find that the decision to make one (even though prudence has restricted the size to 200 inches 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 Rose who made the great Six-foot telescope eighty years ago—all the more because he has hiriiself come nobly to the aid of the British optical industry in the great troubles which befell it after the war.