IN STARRY SKIES

Evening Post, Volume CXXII, Issue 33, 7 August 1936, Page 18

IN STARRY SKIES

MESSAGES FROM AFAR

(By "Omega Centauri.")

Every luminous body or system is incessantly sending out radio messages in every direction through space. An inconceivably minute fraction of this celestial-broadcast is intercepted, by the earth. . Most of this falls on unresponsive •■-soil; An infinitesimal, fraction- enters the pupil of some human eye,..or, after passing through .a telescopic tube, falls on a photographic plate. Even then it may be lost unless the-message is decoded and interpreted by a human brain. . Out of this microscopic fraction of a fraction, all our knowledge.of the external universe has been built up. A message, .when it corheSj will hot wait. Unless it is intercepted it passes on and is gone for ever. Notwithstanding the almost insuperable difficulties of . reception and interpretation a mass of knowledge has been accumulated that is far beyond the capacity of any single brain to assimilate. A specialist is able to acquire a fairly clear conception of a strictly limited field but must be content with a comparatively hazy and ill-defined picture of all the rest. In recent years observation has gathered such a vast store of facts that for the moment interpretation is lagging hopelessly behind. Astronomers can give vi wonderfully precise information as to what innumerable heavenly bodies are like, how far off they are, how large and how not, whether they are moving towards us or from us, and with what speed. But when they speculate about the life history of a star or. a galaxy, when they inquire how the sun gained its planetary system, or what made the galaxy or the spiral nebulae rotate, their words fail to carry conviction. But this need

not.interfere with anyone's delight in astronomy. Though we cannot understand, we can be impressed by the beauty and the grandeur of the portion of the universe that is open to our ken, and we can appreciate the thrill that is experienced by those who are privileged to join in the exploration when any new fact is discovered. Amidst the glorious display of colour that marked the.opening'of-the new Art Gallery and Museum, a whisper reached us of the reception in Wellington of a message which came in the form of a minute colour change, from a distance of thousands of millions of. miles. To the ordinary eye few stars show characteristic colours, but some observers can detect extremely small differences in tint. A slight change in the colour of a faint star led recently to a careful examination, which revealed the existence of an eclipsing binary, hitherto unknown, with a period.of about eleven days. But the r,_adio messages that we have been considering lately come from much vaster distances than this and show that the huge galactic system is but one unit in a system of far greater immensity. It is impossible to form ( a clear conception of the dis-i tances that "separate the units from one another, and of tha times that are taken by light, the swiftest messenger; we know, to travel. across these intervening spaces. Attempts are continually being made to give. some idea of the stupendous size of the portion- of. the universe so far revealed io us: One of the most striking and most successful of these attempts. was described fairlyv fully in this column nearly nine years ago. It was given by Dr. I. S. Plaskett, Director of the Dominion Astronomical Observatory, Victoria, 8.C., in an address entitled "Sixty Years' Progress in Astronomy." This address was published in the journal of the R.A.S. of Canada in October, 1927. Dr. Plaskett returned to the subject and reviewed all the recent evidence in the Halley Lecture delivered at r Oxford last year. A summary of the principal features of this address is published in the last number of the Canadian Journal, under the title "The Dimensions and Structure of the Galactic System." A comparison of these two addresses gives, a, good idea of the progress made during the last nine years in clarifying our knowledge of the place of the Galaxy in the universe. In the earlier lecture Dr. Plaskett adopted the method of imagining that we can travel away into space in a series of steps each ahundred times as long as the last, and after each step

looking back towards our starting point. The view from each standpoint is pictured within a. 4in square. Using this geometrical progression it is possible to traverse immense distances in astonishingly few steps. Dr. Plaskett uses twenty to pass from the electron to the limits of the visible universe as disclosed by the 100-inch telescope. We adopted a slower progression, in which the ratio of the linear dimensions is ten to one. The area shown in each picture is then a hundred times, and the volume of space represented a thousand times, that in the preceding one. Our, imaginary journey then consists of forty steps, though there are now indications that we may soon need a forty-first. We reprint a diagram giving a rough indication of what might be seen in lour pictures selected out of the forty. We assume, of course, .that we have sight of infinite. .space-penetrating power and that we are able" to represent what we see. '.-v ■ ■ ■ .■-.•■-.

Nine years ago the diameter of the galactic system was estimated to be 300,000 light years, and its thickness about 12,000 light years, the sun being in a star cloud some 5000 light years in diameter situated about 60,000 light years from the centre of the galaxy. The spiral nebulae •in Triangulum and Andromeda were said to be 870,000 and 900,000 light years away, and to have, diameters of 16,000 and 45,000 light years respectively. During the last few years, however, the importance of diffuse matter throughout the galaxy has been more generally recognised. A dozen years ago Dr. Plaskett found that the Hand X lines of Calcium and the D lines of Sodium, which appear in the spectra of high-temperature stars, are due to gas'which does not share in the radial motions of the individual stars. It was soon found that the lines become more' and more prominent as the distance of the stars that are studied increases. The abundance of dark obstructing matter shown in photographs of the Milky Way has

[also to be taken into account. Now the distances of all but the nearest j stars have to be found by indirect methods, the best of which depend on the difference between the absolute and the apparent luminosity. Now both the calcium cloud and the dust I clouds make the stars which shine j through them appear fainter than they would otherwise' have done.: Distances ■within the galaxy were thus lively to be i over-estimated. The spirals.and the other extra galactic nebulae are seen only in the regions that are comparatively free from all such obstructing matter. It has thus come about that the galaxy is no longer considered to be very much vaster than the other systems.. The Astronomer Royal last year gave its diameter as 150,000 light years, the thickness of the system bounded by the globular clusters as between 25,000 and 40,000 light years, the diameter of the local star-cloud as 2000 light years, and its distance from the centre of the galaxy as 32,000 light years. The distance of the most remote nebula whose spectrum^ has been photographed he estimates as 230,000,000 light years. Dr. Plaskett is inclined to reduce the ■ size of our galaxy still further. He puts the effective daimeter of the system as nearly 100,000 light years, but adds that ..scattered variable and highvelocity stars may extend 15,000 light years /beyond these limits. The thickness of the system is given as only 3200 to 6500 light years, with a spheroidal enlargement in the centre, which is perhaps 16,000 - light years through. The mass of the system is given as about that of 165,000,000,000 suns, as indicated by the apparent speed' of rotation. There must, however, be considerable uncertainty in this estimate, for Dr. Plaskett gives the distance of the sun from the centre of the system as about 18,500 light years, whilst the Astronomer Royal gives 32,000 light years.

On the other hand, estimates of the size of the Andromeda nebula tend to increase, and most of the authorities now agree that its diameter* is at lease 60,000 or 65,000 light years,; or between half and two-thirdfi of that of the galaxy.

The spiral and other forms of extragalactic nebulae thus appear similar in character to,' and comparable in size with,. the galaxy itself. There is no indication of any thinning out within the region explored at present, and their number must be reckoned in scores of millions.