OUR RELATION TO THE MILKY WAY
We now approach what may be termed the very heart of the subject of our inquiry, the determination of how we are actually situated within this vast but finite universe, and how that position is likely to affect our globe as being the theatre of the development of life up to its highest forms.
We begin with our relation to the Milky Way (which we have fully described in our fourth chapter), because it is by far the most important feature in the whole heavens. Sir John Herschel termed it 'the ground-plane of the sidereal system'; and the more it is studied the more we become convinced that the whole of the stellar universe—stars, clusters of stars, and nebulæ—are in some way connected with it, and are probably dependent on it or controlled by it. Not only does it contain a greater number of stars of the higher magnitudes than any other part of the heavens of equal extent, but it also comprises a great preponderance of star-clusters, and a great extent of diffused nebulous matter, besides the innumerable myriads of minute stars which produce its characteristic cloud-like appearance. It is also the region of those strange outbursts forming new stars; while gaseous stars of enormous bulk—some probably a thousand or even ten thousand times that of our sun, and of intense heat and brilliancy—are more abundant there than in any other part of the heavens. It is now almost certain that these enormous stars and the myriads of minute stars just visible with the largest telescopes, are actually intermingled, and together constitute its essential features; in which case the fainter stars are really small and cannot be far apart, forming, as it were, the first aggregations of the nebulous substratum, and perhaps supplying the fuel which keeps up the intense brilliancy of the giant suns. If this is so, then the Galaxy must be the theatre of operation of vast forces, and of continuous combinations of matter, which escape our notice owing to its enormous distance from us. Among its millions of minute telescopic stars, hundreds or thousands may appear or disappear yearly without being perceived by us, till the photographic charts are completed and can be minutely scrutinised at short intervals. As undoubted changes have occurred in many of the larger nebulæ during the last fifty years, we may anticipate that analogous changes will soon be noted in the stars and the nebulous masses of the Milky Way. Dr. Isaac Roberts has even observed changes in nebulæ after such a short interval as eight years.
The Milky Way a Great Circle
Notwithstanding all its irregularities, its divisions, and its diverging branches, astronomers are generally agreed that the Milky Way forms a great circle in the heavens. Sir John Herschel, whose knowledge of it was unrivalled, stated that its course 'conforms, as nearly as the indefiniteness of its boundary will allow it to be fixed, to that of a great circle'; and he gives the Right Ascension and Declination of the points where it crosses the equinoctial, in figures which define those points as being exactly opposite each other. He also defines its northern and southern poles by other figures, so as to show that they are the poles of a great circle. And after referring to Struve's view that it was not a great circle, he says, 'I retain my own opinion.' Professor Newcomb says that its position 'is nearly always near a great circle of the sphere'; and again he says: 'that we are in the galactic plane itself seems to be shown in two ways: (1) the equality in the counts of stars on the two sides of this plane all the way to its poles; and (2) the fact that the central line of the Galaxy is a great circle, which it would not be if we viewed it from one side of its central plane' (The Stars, p. 317). Miss Clerke, in her History of Astronomy, speaks of 'our situation in the galactic plane' as one of the undisputed facts of astronomy; while Sir Norman Lockyer, in a lecture delivered in 1899, said, 'the middle line of the Milky Way is really not distinguishable from a great circle,' and again in the same lecture—'but the recent work, chiefly of Gould in Argentina, has shown that it practically is a great circle.'[4]
About this fact, then, there can be no dispute. A great circle is a circle dividing the celestial sphere into two equal portions, as seen from the earth, and therefore the plane of this circle must pass through the earth. Of course the whole thing is on such a vast scale, the Milky Way varying from ten to thirty degrees wide, that the plane of its circular course cannot be determined with minute accuracy. But this is of little importance. When carefully laid down on a chart, as in that of Mr. Sidney Waters (see end of volume), we can see that its central line does follow a very even circular course, conforming 'as nearly as may be' to a great circle. We are therefore certainly well within the space that would be enclosed if its northern and southern margins were connected together across the vast intervening abyss, and in all probability not far from the central plane of that enclosed space.
The Form of the Milky Way and our
Position on its Plane
Although the Galaxy forms a great circle in the heavens from our point of view, it by no means follows that it is circular in plan. Being unequal in width and irregular in outline, it might be elliptic or even angular in shape without being at all obviously so to us. If we were standing in an open plain or field two or three miles in diameter, and bounded in every direction by woods of very irregular height and density and great diversity of tint, we should find it difficult to judge of the shape of the field, which might be either a true circle, an oval, a hexagon, or quite irregular in outline, without our being able to detect the exact shape unless some parts were very much nearer to us than others. Again, just as the woods bounding the field might be either a narrow belt of nearly uniform width, or might in some places be only a few yards wide and in others stretch out for miles, so there have been many opinions as to the width of the Milky Way in the direction of its plane, that is, in the direction in which we look towards it. Lately, however, as the result of long-continued observation and study, astronomers are fairly well agreed as to its general form and extent, as will be seen by the following statements of fact and reasoning.
Miss Clerke, after giving the various views of many astronomers—and as the historian of modern astronomy her opinion has much weight—considers that the most probable view of it is, that it is really very much what it seems to us—an immense ring with streaming appendages extending from the main body in all directions, producing the very complex effect we see. The belief seems to be now spreading that the whole universe of stars is spherical or spheroidal, the Milky Way being its equator, and therefore in all probability circular or nearly so in plan; and it is also held that it must be rotating—perhaps very slowly—as nothing else can be supposed to have led to the formation of such a vast ring, or can preserve it when formed.