The Children's Book of Stars

CHAPTER X

THE GLITTERING HEAVENS

On a clear moonless night the stars appear uncountable. You see them twinkling through the leafless trees, and covering all the sky from the zenith, the highest point above your head, down to the horizon. It seems as if someone had taken a gigantic pepper-pot and scattered them far and wide so that some had fallen in all directions. If you were asked to make a guess as to how many you can see at one time, no doubt you would answer 'Millions!' But you would be quite wrong, for the number of stars that can be seen at once without a telescope does not exceed two thousand, and this, after the large figures we have been dealing with, appears a mere trifle. With a telescope, even of small power, many more are revealed, and every increase in the size of the telescope shows more still; so that it might be supposed the universe is indeed illimitable, and that we are only prevented from seeing beyond a certain point by our limited resources. But in reality we know that this cannot be so. If the whole sky were one mass of stars, as it must be if the number of them were infinite, then, even though we could not distinguish the separate items, we should see it bright with a pervading and diffused light. As this is not so, we judge that the universe is not unending, though, with all our inventions, we may never be able to probe to the end of it. We need not, indeed, cry for infinity, for the distances of the fixed stars from us are so immeasurable that to atoms like ourselves they may well seem unlimited. Our solar system is set by itself, like a little island in space, and far, far away on all sides are other great light-giving suns resembling our own more or less, but dwindled to the size of tiny stars, by reason of the great void of space lying between us and them. Our sun is, indeed, just a star, and by no means large compared with the average of the stars either. But, then, he is our own; he is comparatively near to us, and so to us he appears magnificent and unique. Judging from the solar system, we might expect to find that these other great suns which we call stars have also planets circling round them, looking to them for light and heat as we do to our sun. There is no reason to doubt that in some instances the conjecture is right, and that there may be other suns with attendant planets. It is however a great mistake to suppose that because our particular family in the solar system is built on certain lines, all the other families must be made on the same pattern. Why, even in our own system we can see how very much the planets differ from each other: there are no two the same size; some have moons and some have not; Saturn's rings are quite peculiar to himself, and Uranus and Neptune indulge in strange vagaries. So why should we expect other systems to be less varied?

As science has advanced, the idea that these faraway suns must have planetary attendants as our sun has been discarded. The more we know the more is disclosed to us the infinite variety of the universe. For instance, so much accustomed are we to a yellow sun that we never think of the possibility of there being one of another colour. What would you say then to a ruby sun, or a blue one; or to two suns of different colours, perhaps red and green, circling round each other; or to two such suns each going round a dark companion? For there are dark bodies as well as shining bodies in the sky. These are some of the marvels of the starry sky, marvels quite as absorbing as anything we have found in the solar system.

It requires great care and patience and infinite labour before the very delicate observations which alone can reveal to us anything of the nature of the fixed stars can be accomplished. It is only since the improvement in large telescopes that this kind of work has become possible, and so it is but recently men have begun to study the stars intimately, and even now they are baffled by indescribable difficulties. One of these is our inability to tell the distance of a thing by merely looking at it unless we also know its size. On earth we are used to seeing things appear smaller the further they are from us, and by long habit can generally tell the real size; but when we turn to the stars, which appear so much alike, how are we to judge how far off they are? Two stars apparently the same size and close together in the sky may really be as far one from another as the earth is from the nearest; for if the further one were very much larger than the nearer, they would then appear the same size.

At first it was natural enough to suppose that the big bright stars of what we call the first magnitude were the nearest to us, and the less bright the next nearest, and so on down to the tiny ones, only revealed by the telescope, which would be the furthest away of all; but research has shown that this is not correct. Some of the brightest stars may be comparatively near, and some of the smallest may be near also. The size is no test of distance. So far as we have been able to discover, the star which seems nearest is a first magnitude one, but some of the others which outshine it must be among the infinitely distant ones. Thus we lie in the centre of a jewelled universe, and cannot tell even the size of the jewels which cover its radiant robe.

I say 'lie,' but that is really not the correct word. So far as we have been able to find out, there is no such thing as absolute rest in the universe – in fact, it is impossible; for even supposing any body could be motionless at first, it would be drawn by the attraction of its nearest neighbours in space, and gradually gain a greater and greater velocity as it fell toward them. Even the stars we call 'fixed' are all hurrying along at a great pace, and though their distance prevents us from seeing any change in their positions, it can be measured by suitable instruments. Our sun is no exception to this universal rule. Like all his compeers, he is hurrying busily along somewhere in obedience to some impulse of which we do not know the nature; and as he goes he carries with him his whole cortège of planets and their satellites, and even the comets. Yes, we are racing through space with another motion, too, besides those of rotation and revolution, for our earth keeps up with its master attractor, the sun. It is difficult, no doubt, to follow this, but if you think for a moment you will remember that when you are in a railway-carriage everything in that carriage is really travelling along with it, though it does not appear to move. And the whole solar system may be looked at as if it were one block in movement. As in a carriage, the different bodies in it continue their own movements all the time, while sharing in the common movement. You can get up and change your seat in the train, and when you sit down again you have not only moved that little way of which you are conscious, but a great way of which you are not conscious unless you look out of the window. Now in the case of the earth's own motion we found it necessary to look for something which does not share in that motion for purposes of comparison, and we found that something in the sun, who shows us very clearly we are turning on our axis. But in the case of the motion of the solar system the sun is moving himself, so we have to look beyond him again and turn to the stars for confirmation. Then we find that the stars have motions of their own, so that it is very difficult to judge by them at all. It is as if you were bicycling swiftly towards a number of people all walking about in different directions on a wide lawn. They have their movements, but they all also have an apparent movement, really caused by you as you advance toward them; and what astronomers had to do was to separate the true movements of the stars from the false apparent movement made by the advance of the sun. This great problem was attacked and overcome, and it is now known with tolerable certainty that the sun is sweeping onward at a pace of about twelve miles a second toward a fixed point. It really matters very little to us where he is going, for the distances are so vast that hundreds of years must elapse before his movement makes the slightest difference in regard to the stars. But there is one thing which we can judge, and that is that though his course appears to be in a straight line, it is most probably only a part of a great curve so huge that the little bit we know seems straight.

When we speak of the stars, we ought to keep quite clearly in our minds the fact that they lie at such an incredible distance from us that it is probable we shall never learn a great deal about them. Why, men have not even yet been able to communicate with the planet Mars, at its nearest only some thirty-five million miles from us, and this is a mere nothing in measuring the space between us and the stars. To express the distances of the stars in figures is really a waste of time, so astronomers have invented another way. You know that light can go round the world eight times in a second; that is a speed quite beyond our comprehension, but we just accept it. Then think what a distance it could travel in an hour, in a day; and what about a year? The distance that light can travel in a year is taken as a convenient measure by astronomers for sounding the depths of space. Measured in this way light takes four years and four months to reach us from the nearest star we know of, and there are others so much more distant that hundreds – nay, thousands – of years would have to be used to convey it. Light which has been travelling along with a velocity quite beyond thought, silently, unresting, from the time when the Britons lived and ran half naked on this island of ours, has only reached us now, and there is no limit to the time we may go back in our imaginings. We see the stars, not as they are, but as they were. If some gigantic conflagration had happened a hundred years ago in one of them situated a hundred light-years away from us, only now would that messenger, swifter than any messenger we know, have brought the news of it to us. To put the matter in figures, we are sure that no star can lie nearer to us than twenty-five billions of miles. A billion is a million millions, and is represented by a figure with twelve noughts behind it, so – 1,000,000,000,000; and twenty-five such billions is the least distance within which any star can lie. How much farther away stars may be we know not, but it is something to have found out even that. On the same scale as that we took in our first example, we might express it thus: If the earth were a greengage plum at a distance of about three hundred of your steps from the sun, and Neptune were, on the same scale, about three miles away, the nearest fixed star could not be nearer than the distance measured round the whole earth at the Equator!

All this must provoke the question, How can anyone find out these things? Well, for a long time the problem of the distances of the stars was thought to be too difficult for anyone to attempt to solve it, but at last an ingenious method was devised, a method which shows once more the triumph of man's mind over difficulties. In practice this method is extremely difficult to carry out, for it is complicated by so many other things which must be made allowance for; but in theory, roughly explained, it is not too hard for anyone to grasp. The way of it is this: If you hold up your finger so as to cover exactly some object a few feet distant from you, and shut first one eye and then the other, you will find that the finger has apparently shifted very considerably against the background. The finger has not really moved, but as seen from one eye or the other, it is thrown on a different part of the background, and so appears to jump; then if you draw two imaginary lines, one from each eye to the finger, and another between the two eyes, you will have made a triangle. Now, all of you who have done a little Euclid know that if you can ascertain the length of one side of a triangle, and the angles at each end of it, you can form the rest of the triangle; that is to say, you can tell the length of the other two sides. In this instance the base line, as it is called – that is to say the line lying between the two eyes – can easily be measured, and the angles at each end can be found by an instrument called a sextant, so that by simple calculation anyone could find out what distance the finger was from the eye. Now, some ingenious man decided to apply this method to the stars. He knew that it is only objects quite near to us that will appear to shift with so small a base line as that between the eyes, and that the further away anything is the longer must the base line be before it makes any difference. But this clever man thought that if he could only get a base line long enough he could easily compute the distance of the stars from the amount that they appeared to shift against their background. He knew that the longest base line he could get on earth would be about eight thousand miles, as that is the diameter of the earth from one side to the other; so he carefully observed a star from one end of this immense base line and then from the other, quite confident that this plan would answer. But what happened? After careful observations he discovered that no star moved at all with this base line, and that it must be ever so much longer in order to make any impression. Then indeed the case seemed hopeless, for here we are tied to the earth and we cannot get away into space. But the astronomer was nothing daunted. He knew that in its journey round the sun the earth travels in an orbit which measures about one hundred and eighty-five millions of miles across, so he resolved to take observations of the stars when the earth was at one side of this great circle, and again, six months later, when she had travelled to the other side. Then indeed he would have a magnificent base line, one of one hundred and eighty-five millions of miles in length. What was the result? Even with this mighty line the stars are found to be so distant that many do not move at all, not even when measured with the finest instruments, and others move, it may be, the breadth of a hair at a distance of several feet! But even this delicate measure, a hair's-breadth, tells its own tale; it lays down a limit of twenty-five billion miles within which no star can lie!

This system which I have explained to you is called finding the star's parallax, and perhaps it is easier to understand when we put it the other way round and say that the hair's-breadth is what the whole orbit of the earth would appear to have shrunk to if it were seen from the distance of these stars!

Many, many stars have now been examined, and of them all our nearest neighbour seems to be a bright star seen in the Southern Hemisphere. It is in the constellation or star group called Centaurus, and is the brightest star in it. In order to designate the stars when it is necessary to refer to them, astronomers have invented a system. To only the very brightest are proper names attached; others are noted according to the degree of their brightness, and called after the letters of the Greek alphabet: alpha, beta, gamma, delta, etc. Our own word 'alphabet' comes, you know, from the first two letters of this Greek series. As this particular star is the brightest in the constellation Centaurus, it is called Alpha Centauri; and if ever you travel into the Southern Hemisphere and see it, you may greet it as our nearest neighbour in the starry universe, so far as we know at present.

CHAPTER XI

THE CONSTELLATIONS

From the very earliest times men have watched the stars, felt their mysterious influence, tried to discover what they were, and noted their rising and setting. They classified them into groups, called constellations, and gave such groups the names of figures and animals, according to the positions of the stars composing them. Some of these imaginary figures seem to us so wildly ridiculous that we cannot conceive how anyone could have gone so far out of their way as to invent them. But they have been long sanctioned by custom, so now, though we find it difficult to recognize in scattered groups of stars any likeness to a fish or a ram or a bear; we still call the constellations by their old names for convenience in referring to them.

Supposing the axis of the earth were quite upright, straight up and down in regard to the plane at which the earth goes round the sun, then we should always see the same set of stars from the Northern and the same set of stars from the Southern Hemispheres all the year round. But as the axis is tilted slightly, we can, during our nights in the winter in the Northern Hemisphere, see more of the sky to the south than we can in the summer; and in the Southern Hemisphere just the reverse is the case, far more stars to the north can be seen in the winter than in the summer. But always, whether it is winter or summer, there is one fixed point in each hemisphere round which all the other stars seem to swing, and this is the point immediately over the North or the South Poles. There is, luckily, a bright star almost at the point at which the North Pole would seem to strike the sky were it infinitely lengthened. This is not one of the brightest stars in the sky, but quite bright enough to serve the purpose, and if we stand with our faces towards it, we can be sure we are looking due north. How can we discover this star for ourselves in the sky? Go out on any starlight night when the sky is clear, and see if you can find a very conspicuous set of seven stars called the Great Bear. I shall not describe the Great Bear, because every child ought to know it already, and if they don't, they can ask the first grown-up person they meet, and they will certainly be told. (See map.)