The Children's Book of Stars

CHAPTER XVI

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!

If you take an imaginary line down from the two left-hand stars of Cassiopeia, and follow it carefully, you will come before long to a rather faint star, and close to it is the nebula.

When you catch sight of it you will, perhaps, at first be disappointed, for all you will see is a soft blur of white, as if someone had laid a dab of luminous paint on the sky with a finger; but as you gaze at it night after night and realize its unchangeableness, realize also that it is a mass of glowing gas, an island in space, infinitely distant, unsupported and inexplicable, something of the wonder of it will creep over you.

Thousands of telescopic nebulæ are now known, and have been examined, and they are of all shapes. Roughly, they have been divided up into several classes – those that seem to us to be round and those that are long ovals, like this one in Andromeda; but these may, of course, be only round ones seen edgewise by us; others are very irregular, and spread over an enormous part of the sky. The most remarkable of these is that in Orion, and if you look very hard at the middle star in the sword-hilt of Orion, you may be able to make out a faint mistiness. This, when seen through a telescope, becomes a wonderful and far-spreading nebula, with brighter and darker parts like gulfs in it, and dark channels. It has been sometimes called the Fish-mouth Nebula, from a fanciful idea as to its shape. Indeed, so extraordinarily varied are these curious structures, that they have been compared with numbers of different objects. We have some like brushes, others resembling fans, rings, spindles, keyholes; others like animals – a fish, a crab, an owl, and so on; but these suggestions are imaginative, and have nothing to do with the real problem. In The System of the Stars Miss Clerke says: 'In regarding these singular structures we seem to see surges and spray-flakes of a nebulous ocean, bewitched into sudden immobility; or a rack of tempest-driven clouds hanging in the sky, momentarily awaiting the transforming violence of a fresh onset. Sometimes continents of pale light are separated by narrow straits of comparative darkness; elsewhere obscure spaces are hemmed in by luminous inlets and channels.'

One curious point about the Orion Nebula is that the star which seems to be in the midst of it resolves itself under the telescope into not one but six, of various sizes.

Nebulæ are in most cases too enormously remote from the earth for us to have any possible means of computing the distance; but we may take it that light must journey at least a thousand years to reach us from them, and in many cases much more. Therefore, if at the time of the Norman Conquest a nebula had begun to grow dim and fade away, it would, for all intents and purposes, still be there for us, and for those that come after us for several generations, though all that existed of it in reality would be its pale image fleeting onward through space in all directions in ever-widening circles.

That nebulæ do sometimes change we have evidence: there are cases in which some have grown indisputably brighter during the years they have been under observation, and some nebulæ that have been recorded by careful observers seem to have vanished. When we consider that these strange bodies fill many, many times the area of our whole solar system to the outermost bounds of Neptune's orbit, it is difficult to imagine what force it is that acts on them to revive or quench their light. That that light is not the direct result of heat has long been known; it is probably some form of electric excitement causing luminosity, very much as it is caused in the comets. Indeed, many people have been tempted to think of the nebulæ as the comets of the universe, and in some points there are, no doubt, strong resemblances between the two. Both shine in the same way, both are so faint and thin that stars can be seen through them; but the spectroscope shows us that to carry the idea too far would be wrong, as there are many differences in constitution.

We have seen that there are dark stars as well as light stars; if so, may there not be dark nebulæ as well as light ones? It may very well be so. We have seen that there are reasons for supposing our own system to have been at first a cool dark nebula rotating slowly. The heavens may be full of such bodies, but we could not discern them. Their thinness would prevent their hiding any stars that happened to be behind them. No evidence of their existence could possibly be brought to us by any channel that we know.

It is true that, besides the dark rifts in the bright nebulæ, which may themselves be caused by a darker and non-luminous gas, there are also strange rifts in the Milky Way, which at one time were conjectured to be due to a dark body intervening between us and the starry background. This idea is now quite discarded; whatever may cause them, it is not that. One of the most startling of these rifts is that called the Coal-Sack, in the Southern Hemisphere, and it occurs in a part of the sky otherwise so bright that it is the more noticeable. No possible explanation has yet been suggested to account for it.

Thus it may be seen that, though much has been discovered, much remains to be discovered. By the patient work of generations of astronomers we have gained a clear idea of our own position in the universe. Here are we on a small globe, swinging round a far mightier and a self-luminous globe, in company with seven other planets, many of which, including ourselves, are attended by satellites or moons. Between the orbits of these planets is a ring or zone of tiny bodies, also going round the sun. Into this system flash every now and then strange luminous bodies – some coming but once, never to return; others returning again and again.

Far out in space lies this island of a system, and beyond the gulfs of space are other suns, with other systems: some may be akin to ours and some quite different. Strewn about at infinite distances are star clusters, nebulæ, and other mysterious objects.

The whole implies design, creation, and the working of a mighty intelligence; and yet there are small, weak creatures here on this little globe who refuse to believe in a God, or who, while acknowledging Him, would believe themselves to know better than He.

THE END