Te Aroha & Ohinemuri News

Te Aroha News, 15 February 1890, Page 3

Te Aroha & Ohinemuri News

EDITED BY MRS FRANCES HODGSON BURNETT.

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[All Rights Reserved.]

The Outskirts of our Universe

By Sir Robert Ball.

In discussing the extent of the visible universe, it must always be borne in mind that the farther a source of light is from us the fainter is the illumination which we receive from it. Snppose that a star which just lies on the limit of naked eye visibility was somehow to be transported to a distance which is twice as great, then the lustre of that star would be reduced to one-fourth of its original amount. It would, therefore, be of course invisible to the unaided eye, bat it could still be easily perceived by a telescope. Indeed, the very word telescope means an instrument for looking at objects a long way ofi, and the effect of the telescope is to reduce the apparent distance of the object. Thus the binocular glass that the mariner uses at sea has the seeming effect of reducing distances to about one-third of their amount, so that if a star were carried oil to three times its actual distance from us, it would still be shown as clearly in a binocular as it would be seen by the anaidea eye when placed at its original distance. A star just on the verge of naked-eye visibility Would still remain within the ■ grasp of such an instrument even were it, three times as far away. The instrument the astronomer uses must, however, be much more powerful than that which suffices ioi the mariner. A telescope small enough to be held in the hands would be competent to retain a star within its ken even though the star was removed to a distance, ten times as great as when it was just visible with the unaided eye. The larger instruments would be sufficiently powerful to follow a star even though it was removed to a distance one hundred times as great as that at which it could be just glimpsed by the unaided eye, while the greatest instruments of which our observatories can boast would still hold every lucid star in the heavens within view even were those stars wafted off one thousand times as far as they are at present. These facts give us some notion ot tne extraordinary extent to which great teleg copes increase the range of our vision. yy T e are permitted by their aid to sound to in space one thousand times as great' as that to which the unaided eye would enable us bo penetrate. Above and on each" side and around us in every direction the range of our vision is increased a thousand' fold. At first iff might appear that the e xtent of space accessible to oui telescopes would also be increased to one thousand times the extent ot naked eye vision. This would, however, give a veiy inadequate conception of the truth. W ith our unaided eyes we are able to see all the objects around ns which lie within a certain mighty globe of which our earth is the centre When we employ great telescopes we can explore a globe of which the radius has been enlarged one thousand-fold, therefore the quantity of space that we can examine in the two cases must be represented by two globes, one of which has a diameter a thousand times as great as the other. The volumes of two such spheres have a ratio so great that perhaps we do not readily comprehend it. . The point \vhich I now wish to illustrate is one which v. cry frequently arises in,the consideration of problems in astronomy : and, indeed, m other subjects as well.. _ us take a very simple one. Suppose a cow is tethered by a chain in a held ; she will, of course, be only able to graze over the jgrass which grows within the circle to ‘ which her movements are confined. It we desired to give the cow double as much grass as she had before would that end be accomplished bv doubling tne length of the chain? - It would be more than accomplished, in fact; she would then have four times as much pasture as she had before. - The circle over which she can graze has double the diameter no doubt, but then its area is four times as great. Thus we see that the area increases in a far more rapid ratio than that in which the radius increases. Think next of two bird cages. 1 We may suppose them both shaped lixe globes, the diameter of the larger being double that of the smaller. What will be t the freedom enjoyed by the bird in the • larger cage as compared with that of the .’ less fortunate bird n» the other. . The -volume of space over which the bird in the : big globe can fly is not twice nor even lour ’times as great as that allotted to the other '■ !bird; it is no less than eight times as The bulk of a grain of sand as compared with the bulk of a football may illustrate ithe space accessible to our eyes when com■tpared with the space accessible to one ot the -.great telescopes. The larger of these ©paces has a thousand times the diameter of the other-therefore the relative. quantities of these spaces are to be obtained by multiplying 1,000 by 1,000 again. Thus we finally learn that the amplitude of our vision is augmented to one thousand million times its original extent by the use of oui greatest telescopes. It need, therefore, be no matter for surprise that the number of stars visible through our great telescopes or recorded on the sensitive films of photocraphfe plates should number scores oi millions. In fact, it would sometimes seem surprising that the number of telescopic stars is not even greater than it actually ap- . mears to be. If we are able to explore one thousand million times as much space, we might expect that the number of objects disclosed would be also increased about a thousand million fold ; but this is certainly •c. i-Up page The truth seems to be that 5S i, but one star of a mighty cluster of Stars; we' happen to he near the middle oilh * cluster, and the rest of the stars belonging to it form what we know as the ‘ Milk y Way.’ There are of course other clusters scattered through the J ea Y®“ 8 ’ some of them perhaps as great as that body of stars which form the ‘Milky Way. Owing to our residence in this cluster we see the neighbouring suns in multitudes, • and thus we receive the impression that the •solar svstem lies in an exceptionally rich A >part of .the universe in as far as the distri--button-of stars is concerned. On the outskirts of the universe he those jfaintest and dimmest of objects WPldb

wo can just perceive through our greatest . telescopes. We know that many of the stars around us would still remain visible in great instruments even though they were removed a thousand times as far off. Among the myriads of faint stars which w" from our observatories there may rr -ny, indeed there must be many, which are fuiiy a thousand times as distant as the bright stars which twinkle in our comparative neighbourhood- We thus obtain some conception of the stupendous distances at which the outskirts are situated. There are differentwaysof illustrating this point, but I think the simplest as well as the most striking is that which is founded on the velocity of light. It is a remarkable fact that the beautiful star known as Vega has a distance from us so tremendous that its light must have taken somewhere about eighteen years to travel hither from thence. Notwithstanding that the light dashes along with such inconceivable speed that it will cover 180,000 miles in every second, notwithstanding that a journey at this pace will complete the entire circuit of this globe seven or eight times between two successive ticks of a clock, the light will nevertheless take eighteen years to reach the naked eye from the time it leaves Vega. We do not therefore see the star as it is at present; we see it as it was eighteen years ago. For the light which this evening enters our eyes has been all that time on its journey ; indeed, it Vega were actually bo be blotted out from existence it would still continue to shine out as vividly as ever for 18 years before the light on its way had reached us.

We have been led to the belief that among the more distant stars in the universe there must be many which are fully a thousand times as far from us as is Vega, hence we arrive at the startling conception that the light they enjoy has been on its journey for 18,000 years before it reached us. When we look at those lights to night we are actually viewing them as they were 18,000 years ago. In fact, those stars might have totally vanished 17,000 years ago, though we and our descendants may still see them glittering for yet another thousand years. At the moment of extinction there was a column of light on its way and until the whole of that column has reached our system we receive no intention of the fact that the star from which it came has ceased to twinkle. We shall realise a little more fully what this reasoning involves if we suppose that astronmers dwelt on such a star, and that they had eyes and telescopes sufficiently keen nob only to discern our little earth, but even to scrutinise its surface with attention. Let us suppose that the stellar astronomers looked at England, do you think they would see a network of railways joining mighty and populous cities, furnished with immense manufactories and with countless institutions which would be the England of to-day ? But from the distance at which these astronomers are situated light takes 18,000 years for its journey, and, therefore, what they would see would be England as it was 18,000 years ago. To them England would even now appear as a country mainly covered with forests inhabited by bears and wolves, and totally free from any trace of civilisation. This illustration will at all events serve to convey some conception of the distance at which the outskirts of our visible universe are plunged in the depths of space. The End.