The Children's Book of Stars

by G. E. Mitton

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.