Kitabı oku: «The Children's Book of Stars», sayfa 8

CHAPTER XIII
RESTLESS STARS

You remember we have already remarked upon the difficulty of telling how far one star lies behind another, as we do not know their sizes. It is, to take another similar case, easy enough to tell if a star moves to one side or the other, but very difficult by ordinary observation to tell if it is advancing toward us or running away from us, for the only means we have of judging is if it gets larger or smaller, and at that enormous distance the fact whether it advances or recedes makes no difference in its size. Now, the spectroscope has changed all this, and we can tell quite as certainly if a star is coming toward us as we can if it moves to one side. I will try to explain this. You know, perhaps, that sound is caused by vibration in the air. The noise, whatever it is, jars the air and the vibrations strike on our ears. It is rather the same thing as the result of throwing a stone into a pond: from the centre of the splash little wavelets run out in ever-widening circles; so through the air run ever-widening vibrations from every sound. The more vibrations there are in a second the shriller is the note they make. In a high note the air-vibrations follow one another fast, pouring into one's ear at a terrific speed, so that the apparatus in the ear which receives them itself vibrates fiercely and records a high note, while a lower note brings fewer and slower vibrations in a second, and the ear is not so much disturbed. Have you ever noticed that if a railway engine is sweeping-toward you and screaming all the time, its note seems to get shriller and shriller? That is because the engine, in advancing, sends the vibrations out nearer to you, so more of them come in a second, and thus they are crowded up closer together, and are higher and higher.

Now, light is also caused by waves, but they are not the same as sound waves. Light travels without air, whereas sound we know cannot travel without air, and is ever so much slower, and altogether a grosser, clumsier thing than light. But yet the waves or rays which make light correspond in some ways to the vibrations of sound. What corresponds to the treble on the piano is the blue end of the spectrum in light, and the bass is the red end. Now, when we are looking at the spectrum of any body which is advancing swiftly toward us, something of the same effect is observed as in the case of the shrieking engine. Take any star and imagine that that star is hastening toward us at a pace of three hundred miles a second, which is not at all an unusual rate for a star; then, if we examine the band of light, the spectrum, of such a star, we shall observe an extraordinary fact – all these little lines we have spoken of are shoved up toward the treble or blue end of the spectrum. They still remain just the same distances from each other, and are in twos and threes or single, so that the whole set of lines is unaltered as a set, but everyone of them is shifted a tiny fraction up toward the blue end of the spectrum, just a little displaced. Now if, instead of advancing toward us, this same star had been rushing away from us at a similar pace, all these lines would have been moved a tiny bit toward the red or bass end of the spectrum. This is known to be certainly true, so that by means of the spectroscope we can tell that some of these great sun-stars are advancing toward us and some receding from us, according to whether the multitudes of little lines in the spectrum are shifted slightly to the blue or the red end.

You remember that it has been surmised that the pace the sun moves with his system is about twelve miles a second. This seems fast enough to us, who think that one mile a minute is good time for an express train, but it is slow compared with the pace of many of the stars. As I have said, some are travelling at a rate of between two hundred and three hundred miles a second; and it is due to the spectroscope that we know not only whether a star is advancing toward us or receding from us, but also whether the pace is great or not; it even tells us what the pace is, up to about half a mile a second, which is very marvellous. It is a curious fact that many of the small stars show greater movement than the large ones, which mayor may not mean that they are nearer to us.

It may be taken as established that there is no such thing as absolute rest in the universe: everything, stars and nebulæ alike, are moving somewhere; in an infinite variety of directions, with an infinite variety of speed they hasten this way and that. It would be impossible for any to remain still, for even supposing it had been so 'in the beginning,' the vast forces at work in the universe would not let it remain so. Out of space would come the persistent call of gravitation: atoms would cry silently to atoms. There could be no perfect equality of pull on all sides; from one side or another the pull would be the stronger. Slowly the inert mass would obey and begin falling toward it; it might be an inch at a time, but with rapid increase, until at last it also was hastening some whither in this universe which appears to us to be infinite.

It must be remembered that these stars, even when moving at an enormous pace, do not change their places in the sky when regarded by ordinary observers. It would take thousands of years for any of the constellations to appear at all different from what they are now, even though the stars that compose them are moving in different directions with a great velocity, for a space of many millions of miles, at the distance of most of the stars, would be but as the breadth of a fine hair as seen by us on earth. So thousands of years ago men looked up at the Great Bear, and saw it apparently the same as we see it now; yet for all that length of time the stars composing it have been rushing in this direction and that at an enormous speed, but do not appear to us on the earth to alter their positions in regard to each other. I know of nothing that gives one a more overwhelming sense of the mightiness of the universe and the smallness of ourselves than this fact. From age to age men look on changeless heavens, yet this apparently stable universe is fuller of flux and reflux than is the restless ocean itself, and the very wavelets on the sea are not more numerous nor more restless than the stars that bestrew the sky.

CHAPTER XIV
THE COLOURS OF THE STARS

Has it ever occurred to you that the stars are not all of the same colour? It is true that, just glancing at them casually, you might say they are all white; but if you examine them more carefully you cannot help seeing that some shine with a steely blue light, while others are reddish or yellowish. These colours are not easy to distinguish with the naked eye, and might not attract any attention at all unless they were pointed out; yet when attention is drawn to the fact, it is impossible to deny the redness of some, such as Aldebaran. But though we may admit this, we might add that the colours are so very faint and inconspicuous, that they might be, after all, only the result of imagination.

To prove that the colours are constant and real we must use a telescope, and then we need have no further doubt of their reality, for instead of disappearing, the colours of some stars stand out quite vividly beyond the possibility of mistake. Red stars are a bright red, and they are the most easily seen of all, though the other colours, blue and yellow and green, are seen very decidedly by some people. The red stars have been described by various observers as resembling 'a drop of blood on a black field,' 'most magnificent copper-red,' 'most intense blood-red,' and 'glowing like a live coal out of the darkness of space.' Some people see them as a shining red, like that of a glowing cloud at sunset. Therefore there can be no doubt that the colours are genuine enough, and are telling us some message. This message we are able to read, for we have begun to understand the language the stars speak to us by their light since the invention of the spectroscope. The spectroscope tells us that these colours indicate different stages in the development of the stars, or differences of constitution – that is to say, in the elements of which they are made. Our own sun is a yellow star, and other yellow stars are akin to him; while red and blue and green stars contain different elements, or elements in different proportions.

Stars do not always remain the same colours for an indefinite time; one star may change slowly from yellow to white, and another from red to yellow; and there are instances of notable changes, such as that of the brilliant white Sirius, who was stated in old times by many different observers to be a red star. All this makes us think, and year by year thought leads us on to knowledge, and knowledge about these distant suns increases. But though we know a good deal now, there are still many questions we should like to ask which we cannot expect to have answered for a long time yet, if ever.

The star colours have some meanings which we cannot even guess; we can only notice the facts regarding them. For instance, blue stars are never known to be solitary – they always have a companion, but why this should be so passes our comprehension. What is it in the constitution of a blue star which holds or attracts another? Whatever it may be, it is established by repeated instances that blue stars do not stand alone. In the constellation of Cygnus there are two stars, a blue and a yellow one, which are near enough to each other to be seen in the same telescope at the same time, and yet in reality are separated by an almost incredible number of billions of miles. But as we know that a blue star is never seen alone, and that it has often as its companion a yellowish or reddish star, it is probable that these two, situated at an enormous distance from one another, are yet in some mysterious way dependent on each other, and are not merely seen together because they happen to fall in the same field of view.

Many double stars show most beautifully contrasted colours: among them are pairs of yellow and rose-red, golden and azure, orange and purple, orange and lilac, copper-colour and blue, apple-green and cherry-red, and so on. In the Southern Hemisphere there is a cluster containing so many stars of brilliant colours that Sir John Herschel named it 'the Jewelled Cluster.'

I expect most of you have seen an advertisement of Pear's Soap, in which you are asked to stare at some red letters, and then look away to some white surface, such as a ceiling, when you will see the same letters in green. This is because green is the complementary or contrasting colour to red, and the same thing is the case with blue and yellow. When any one colour of either of these pairs is seen, it tends to make the other appear by reaction, and if the eye gazed hard at blue instead of red, it would next see yellow, and not green. Now, many people to whom this curious fact is known argue that perhaps the colours of the double stars are not real, but the effect of contrast only; for instance, they say a red star near a companion white one would tend to make the companion appear green, and so, of course, it would. But this does not account for the star colours, which are really inherent in the stars themselves, as may be proved by cutting off the light of one star, and looking only at the other, when its colour still appears unchanged. Another argument equally strong against the contrast theory is that the colours of stars in pairs are by no means always those which would appear if the effect was only due to complementary colours. It is not always blue and yellow or red and green pairs that we see, though these are frequent, but many others of various kinds, such as copper and blue, and ruddy and blue.

We have therefore come to the conclusion that there are in this astonishing universe numbers of gloriously coloured suns, some of which apparently lie close together. What follows? Why, we want to know, of course, if these stars are really pairs connected with each other, or if they only appear so by being in the same line of sight, though one is infinitely more distant than the other. And that question also has been answered. There are now known thousands of cases in which stars, hitherto regarded as single, have been separated into two, or even more, by the use of a telescope. Of these thousands, some hundreds have been carefully investigated, and the result is that, though there are undoubtedly some in which the connexion is merely accidental, yet in by far the greater number of cases the two stars thus seen together have really some connexion which binds them to one another; they are dependent on one another. This has been made known to us by the working of the wonderful law of gravitation, which is obeyed throughout the whole universe. We know that by the operation of this law two mighty suns will be drawn toward each other with a certain pull, just as surely as we know that a stone let loose from the hand will fall upon the earth; so by noting the effect of two mighty suns upon each other many facts about them may be found out. By the most minute and careful measurements, by the use of the spectroscope, and by every resource known to science, astronomers have, indeed, actually found out with a near approach to exactness how far some of these great suns lie from each other, and how large they are in comparison with one another.

The very first double star ever discovered was one which you have already seen, the middle one in the tail of the Great Bear. If you look at it you will be delighted to find that you can see a wee star close to it, and you will think you are looking at an example of a double star with your very own eyes; but you will be wrong, for that wee star is separated by untold distances from the large one to which it seems so near. In fact, any stars which can be seen to be separate by the naked eye must lie immeasurably far apart, however tiny seems the space between them. Such stars may possibly have some connexion with each other, but, at any rate in this case, such a connexion has not been proved. No, the larger star itself is made up of two others, which can only be seen apart in a telescope. Since this discovery double stars have been plentifully found in every part of the sky. The average space between such double stars as seen from our earth is – what do you think? It is the width of a single hair held up thirty-six feet from our eyes! This could not, of course, be seen without the use of a telescope or opera-glasses. It serves to give some impression of star distances when we think that the millions and millions of miles lying between those stars have shrunk to that hair's-breadth seen from our point of view.

Twin stars circle together round a common centre of gravity, and are bound by the laws of gravitation just as the planets are. Our sun is a solitary star, with no companion, and therefore such a state of things seems to us to be incredible. Fancy two gigantic suns, one topaz-yellow and the other azure-blue, circling around in endless movement! Where in such a system would there be room for the planets? How could planets exist under the pull of two suns in opposite directions? Still more wonders are unfolded as the inquiry proceeds. Certain irregularities in the motions of some of these twin systems led astronomers to infer that they were acted upon by another body, though this other body was not discernible. In fact, though they could not see it, they knew it must be there, just as Adams and Leverrier knew of the existence of Neptune, before ever they had seen him, by the irregularities in the movements of Uranus. As the results showed, it was there, and was comparable in size to the twin suns it influenced, and yet they could not see it. So they concluded this third body must be dark, not light-giving like its companions. We are thus led to the strange conclusion that some of these systems are very complicated, and are formed not only of shining suns, but of huge dark bodies which cannot be called suns. What are they, then? Can they be immense planets? Is it possible that life may there exist? No fairy tale could stir the imagination so powerfully as the thought of such systems including a planetary body as large or larger than its sun or suns. If indeed life exists there, what a varied scene must be presented day by day! At one time both suns mingling their flashing rays may be together in the sky; at another time only one appears, a yellow or blue sun, as the case may be. The surface of such planets must undergo weird transformations, the foliage showing one day green, the next yellow, and the next blue; shadows of azure and orange will alternate! But fascinating as such thoughts are, we can get no further along that path.

To turn from fancy to facts, we find that telescope and spectroscope have supplied us with quite enough matter for wonder without calling upon imagination. We have discovered that many of the stars which seem to shine with a pure single light are double, and many more consist not only of two stars, but of several, some of which may be dark bodies. The Pole Star was long known to be double, and is now discovered to have a third member in its system. These multiple systems vary from one another in almost every case. Some are made up of a mighty star and a comparatively small one; others are composed of stars equal in light-giving power – twin suns. Some progress swiftly round their orbits, some go slowly; indeed, so slowly that during the century they have been under observation only the very faintest sign of movement has been detected; and in other systems, which we are bound to suppose double, the stars are so slow in their movements that no progress seems to have been made at all.

The star we know as the nearest to us in the heavens, Alpha Centauri, is composed of two very bright partners, which take about eighty-seven years to traverse their orbit. They sometimes come as near to each other as Saturn is to the sun. In the case of Sirius astronomers found out that he had a companion by reason of his irregularities of movement before they discovered that companion, which is apparently a very small star, only to be discerned with good telescopes. But here, again, it would be unwise to judge only by what we see. Though the star appears small, we know by the influence it exercises on Sirius that it is very nearly the same size as he is. Thus we judge that it is poor in light-giving property; in fact, its shining power is much less than that of its companion, though its size is so nearly equal. This is not wonderful, for Sirius's marvellous light-giving power is one of the wonders of the universe; he shines as brilliantly as twenty-nine or thirty of our suns!

In some cases the dark body which we cannot see may even be larger than the shining one, through which alone we can know anything of it. Here we have a new idea, a hint that in some of these systems there may be a mighty earth with a smaller sun going round it, as men imagined our sun went around the earth before the real truth was found out.

So we see that, when we speak of the stars as suns comparable with our sun, we cannot think of them all as being exactly on the same model. There are endless varieties in the systems; there are solitary suns like ours which may have a number of small planets going round them, as in the solar system; but there are also double suns going round each other, suns with mighty dark bodies revolving round them which may be planets, and huge dark bodies with small suns too. Every increase of knowledge opens up new wonders, and the world in which we live is but one kind of world amid an infinite number.

In this chapter we have learnt an altogether new fact – the fact that the hosts of heaven comprise not only those shining stars we are accustomed to see, but also dark bodies equally massive, and probably equally numerous, which we cannot see. In fact, the regions of space may be strewn with such dark bodies, and we could have no possible means of discovering them unless they were near enough to some shining body to exert an influence upon it. It is not with his eyes alone, or with his senses, man knows of the existence of these great worlds, but often solely by the use of the powers of his mind.