(From Mr. Roberts’ Photograph.)
It is almost impossible to apply the methods which we use in finding the distance of a star to the discovery of the distance of the nebulæ. These flimsy bodies are usually too ill-defined to admit of being measured with the precision and the delicacy required for the determination of distance. The measurements necessary for this purpose can only be made from one star-like point to another similar point. If we could choose a star in the nebula and determine its distance, then, of course, we should have the distance of the nebula itself; but the difficulty is that we have, in general, no means of knowing whether the star does actually lie in the object. It may, for anything we can tell, lie billions of miles nearer to us, or billions of miles further off, and, by merely happening to lie in the line of sight, appear to glimmer in the nebula itself.
Fig. 91.—To show how Small the Solar System is compared with a Great Nebula.
If we have any assurance that the star is surrounded by a mass of this glowing vapor, then it may be possible to measure that nebula’s distance. It will occasionally happen that grounds can be found for believing that a star which appears to be in the glowing gas does veritably lie therein, and is not merely seen in the same direction. There are hundreds of stars visible on a good drawing or a good photograph of the famous object in Andromeda, and doubtless large numbers of these are merely stars which happen to lie in the same line of sight. The peculiar circumstances attending the history of one star seem, however, to warrant us in making the assumption that it was certainly in the nebula. The history of this star is a remarkable one. It suddenly kindled from invisibility into brilliancy. How is a change so rapid in the lustre of a star to be accounted for? In a few days its brightness had undergone an extraordinary increase. Of course, this does not tell us for certain that the star lay in the glowing gas; but the most rational explanation that I have heard offered of this occurrence is that due, I believe, to my friend Mr. Monck. He has suggested that the sudden outbreak in brilliancy might be accounted for on the same principles as those by which we explain the ignition of meteors in our atmosphere. If a dark star, moving along with terrific speed through space, were suddenly to plunge into a dense region of the nebula, heat and light must be evolved in sufficient abundance to transform the star into a brilliant object. If, therefore, we knew the distance of this star at the time it was in Andromeda, we should, of course, learn the distance of that interesting object. This has been attempted, and it has thus been proved that the Great Nebula must be very much further from us than is that star of whose distance I attempted some time ago to give you a notion.
We thus realize the enormous size of the Great Nebula. It appears that if, on a map of this object, we were to lay down, accurately to scale, a map of the solar system, putting the sun in the centre and all the planets around in their true proportions out to the boundary traced by Neptune, this area, vast though it is, would be a mere speck on the drawing of the object. Our system would have to be enormously bigger before it sufficed to cover anything like the area of the sky included in one of these great objects. Here is a sketch of a nebula ([Fig. 91]), and near it I have marked a dot which is to indicate our solar system. We may feel confident that the Great Nebula is at the very least as mighty as this proportion would indicate.
CONCLUSION.
And now, my young friends, I am drawing near the close of that course of lectures which has occupied us, I hope you will think not unprofitably, for a portion of our Christmas holidays. We have spoken of the sun and of the moon, of comets and of stars, and I have frequently had occasion to allude to the relative position of our earth in the universe. No doubt it is a noble globe which we inhabit, but I have failed in my purpose if I have not shown you how insignificant is this earth when compared with the vast extent of some of the other bodies that abound in space. We have, however, been endowed with a feeling of curiosity which makes us long to know of things beyond the confines of our own earth. Astronomers can tell us a little, but too often only a little. They will say—That is a star, and That is a planet, and That is so big, and That so far; such is the meagre style of information with which we often have to be content. The astronomers who live on other worlds, if their faculties be in any degree comparable with ours, must be similarly ignorant with regard to this earth. Inhabitants of our fellow-planets can know hardly anything more than that the earth on which we dwell is a globe 8000 miles across, with many clouds around us. Some of the planets would not even pay us the compliment of recognizing our existence; while from the other systems—the countless other systems—of space we are absolutely imperceptible and unknown.
Out of all the millions of bodies which we can see, you could very nearly count on your fingers those from which our earth would be visible. This reflection is calculated to show us how vast must be the real extent of that universe around us. Here is our globe, with its inhabitants, with its great continents, with its oceans, with its empires, its kingdoms, with its arts, its commerce, its literature, its sciences, and yet it would seem that all these things are absolutely unknown to any inhabitants that may exist elsewhere. I do not think that any reasonable person will doubt that there must be inhabitants elsewhere. There are millions of globes, many of them more splendid than ours. Surely it would be presumptuous to say that this is the only one of all the bodies in the universe on the surface of which life, with all that life involves, is manifested. You will rather think that our globe is but one in the mighty fabric, and that other globes may teem with interest just as ours does. We can, of course, make no conjecture as to what the nature of the life may be elsewhere. Could a traveller visit some other globes and bring back specimens of the natural objects that he found there, no collections that the world has ever seen could rival them in interest. When I go into the British Natural History Museum and look around that marvellous collection, it awakens in me a feeling of solemnity. I see there the remains of mighty extinct animals which once roamed over this earth; also objects which have been dredged from the bottom of the sea at a depth of some miles; there I can examine crystals which have required incalculable ages for their formation; and there I look at meteorites which have travelled from the heavens above down on to the earth beneath. Such sights, and the reflections they awaken, bring before us in an imposing manner the phenomena of our earth, and the extent and interest of its past history. Oliver Wendell Holmes said that the only way to see the British Museum was to take lodgings close by when you were a boy, and to stay in the Museum from nine to five every day until you were an old man; then you would begin to have some notion of what this Institution contains. Think what millions of British Museums would be required were the universe to be adequately illustrated: one museum for the earth, another for Mars, another for Venus—but it would be useless attempting to enumerate them!
Most of us must be content with acquiring the merest shred of information with regard even to our own earth. Perhaps a schoolboy will think it fortunate that we are so ignorant with respect to the celestial bodies. What an awful vista of lessons to be learned would open before his view, if only we had a competent knowledge of the other globes which surround us in space! I should like to illustrate the extent of the universe by following this reflection a little further. I shall just ask you to join with me in making a little calculation as to the extent of the lessons you would have to learn if astronomers should succeed in discovering some of the things they want to know.