The Children's Book of Stars

by G. E. Mitton

STAR CLUSTERS AND NEBULÆ

Could you point out any star cluster in the sky? You could if you would only think for a minute, for one has been mentioned already. This is the cluster known as the Pleiades, and it is so peculiar and so different from anything else, that many people recognize the group and know where to look for it even before they know the Great Bear, the favourite constellation in the northern sky, itself. The Pleiades is a real star cluster, and the chief stars in it are at such enormous distances from one another that they can be seen separately by the eye unaided, whereas in most clusters the stars appear to be so close together that without a telescope they make a mere blur of brightness. For a long time it was supposed that the stars composing the Pleiades could not really be connected because of the great distances between them; for, as you know, even a hair's-breadth apparently between stars signifies in reality many millions of miles.

Light travelling from the Pleiades to us, at that incomprehensible pace of which you already know, takes a hundred and ninety years to reach us! At this incredibly remote distance lies the main part of the cluster from us; but it is more marvellous still that we have every reason to believe that the outlying stars of this cluster are as far from the central ones as the nearest star we know of, Alpha Centauri, is from us! Little wonder was it, then, that men hesitated to ascribe to the Pleiades any real connection with each other, and supposed them to be merely an assemblage of stars which seemed to us to lie together.

With the unaided eye we see comparatively few stars in the Pleiades. Six is the usual number to be counted, though people with very good sight have made out fourteen. Viewed through the telescope, however, the scene changes: into this part of space stars are crowded in astonishing profusion; it is impossible to count them, and with every increase in the power of the telescope still more are revealed. Well over a thousand in this small space seems no exaggerated estimate. Now, it is impossible to say how many of these really belong to the group, and how many are seen there accidentally, but observations of the most prominent ones have shown that they are all moving in exactly the same direction at the same pace. It would be against probability to conceive that such a thing could be the result of mere chance, considering the infinite variety of star movements in general, and so we are bound to believe that this wonderful collection of stars is a real group, and not only an apparent one.

So splendid are the great suns that illuminate this mighty system, that at least fifty or sixty of them far surpass our own sun in brilliancy. Therefore when we look at that tiny sparkling group we must in imagination picture it as a vast cluster of mighty stars, all controlled and swayed by some dominant impulse, though separated by spaces enough to make the brain reel in thinking of them. If these suns possess also attendant planets, what a galaxy of worlds, what a universe within a universe is here!

Other star clusters there are, not so conspicuous as the Pleiades, and most of these can only be seen through a telescope, so we may be thankful that we have one example so splendid within our own vision. There are some clusters so far and faintly shining that they were at first thought to be nebulæ, and not stars at all; but the telescope gradually revealed the fact that many of these are made up of stars, and so people began to think that all faint shining patches of nebulous light were really star clusters, which would be resolved into stars if only we had better telescopes. Since the invention of the spectroscope, however, fresh light has been thrown on the matter, for the spectrum which is shown by some of the nebulous patches is not the same as that shown by stars, and we know that many of these strange appearances are not made up of infinitely distant stars.

We are talking here quite freely about nebulæ because we have met one long ago when we discussed the gradual evolution of our own system, and we know quite well that a nebula is composed of luminous faintly-glowing gas of extreme fineness and thinness. We see in the sky at the present time what we may take to be object-lessons in our own history, for we see nebulæ of all sorts and sizes, and in some stars are mixed up, and in others stars are but dimly seen, so that it does not require a great stretch of the imagination to picture these stars as being born, emerging from the swaddling bands of filmy webs that have enwrapped them; and other nebulæ seem to be gas only, thin and glowing, with no stars at all to be found in it. We still know very little about these mysterious appearances, but the work of classifying and resolving them is going on apace. Nebulæ are divided into several classes, but the easiest distinction to remember is that between white nebulæ and green nebulæ. This is not to say that we can see some coloured green, but that green appears in the spectrum of some of the nebulæ, while the spectrum of a white nebula is more like that of a star.

It is fortunate for us that in the sky we can see without a telescope one instance of each of the several objects of interest that we have referred to.

We have been able to see one very vivid example of a variable star; we have seen one very beautiful example of a star cluster; and it remains to look for one very good example of a white nebula.

Just as in finding Algol you were doing a little bit of practical work, proving something of which you had read, so by seeing this nebula you will remember more about nebulæ in general than by reading many chapters on the subject. This particular nebula is in Andromeda, and is not far from Algol; and it is not difficult to find. It is the only one that can be well seen without a telescope, and was known to the ancients; it is believed to have been mentioned in a book of the tenth century!