Professor Newcomb considers, from the numbers of the stars in all directions towards the Milky Way being approximately equal, that there cannot be much difference in our distance from it in various directions. It would follow that its plan is approximately circular or broadly elliptic. The existence of ring-nebulæ may be held to render such a form probable.
Sir Norman Lockyer gives facts which tend in the same direction. In an article in Nature of November 8th, 1900, he says: 'We find that the gaseous stars are not only confined to the Milky Way, but they are the most remote in every direction, in every galactic longitude; all of them have the smallest proper motion.' And again, referring to the hottest stars being equally remote on all sides of us, he says: 'It is because we are in the centre, because the solar system is in the centre, that the observed effect arises.' He also considers that the ring-nebula in Lyra nearly represents the form of our whole system; and he adds: 'We practically know that in our system the centre is the region of least disturbance, and therefore cooler conditions.'
These various facts and conclusions of some of the most eminent astronomers all point to one definite inference, that our position, or that of the solar system, is not very far from the centre of the vast ring of stars constituting the Milky Way, while the same facts imply a nearly circular form to this ring. Here, more than as regards our position in the plane of the Galaxy, there is no possibility of precise determination; but it is quite certain that if we were situated very far away from the centre, say, for instance, one-fourth of its diameter from one side of it and three-fourths from the other, the appearances would not be what they are, and we should easily detect the excentricity of our position. Even if we were one-third the diameter from one side and two-thirds from the other, it will, I think, be admitted that this also would have been ascertained by the various methods of research now available. We must, therefore, be somewhere between the actual centre and a circle whose radius is one-third of the distance to the Milky Way. But if we are about midway between these two positions, we shall only be one-sixth of the radius or one-twelfth of the diameter of the Milky Way from its exact centre; and if we form part of a cluster or group of stars slowly revolving around that centre, we should probably obtain all the advantages, if any, that may arise from a nearly central position in the entire star-system.
This question of our situation within the great circle of the Milky Way is of considerable importance from the point of view I am here suggesting, so that every fact bearing upon it should be noted; and there is one which has not, I think, been given the full weight due to it. It is generally admitted that the greater brilliancy of some parts of the Milky Way is no indication of nearness, because surfaces possess equal brilliancy from whatever distance they are seen. Thus each planet has its special brilliancy or reflective power, technically termed its 'albedo,' and this remains the same at all distances if the other conditions are similar. But notwithstanding this well-known fact, Sir John Herschel's remark that the greater brightness of the southern Milky Way 'conveys strongly the impression of greater proximity,' and therefore, that we are excentrically placed in its plane, has been adopted by many writers as if it were the statement of a fact, or at least a clearly expressed opinion, instead of being a mere 'impression,' and really a misleading one. I therefore wish to adduce a phenomenon which has a real bearing on the question. It is evident that, if the Milky Way were actually of uniform width throughout, then differences of apparent width would indicate differences of distance. In the parts nearer to us it would appear wider, where more remote, narrower; but in these opposite directions there would not necessarily be any differences in brightness. We should, however, expect that in the parts nearer to us the lucid stars, as well as those within any definite limits of magnitude, would be either more numerous or more wide apart on the average. No such difference as this, however, has been recorded; but there is a peculiar correspondence in the opposite portions of the Galaxy which is very suggestive. In the beautiful charts of the Nebulæ and Star Clusters by the late Mr. Sidney Waters, published by the Royal Astronomical Society and here reproduced by their permission (see end of volume), the Milky Way is delineated in its whole extent with great detail and from the best authorities. These charts show us that, in both hemispheres, it reaches its maximum extension on the right and left margins of the charts, where it is almost equal in extent; while in the centre of each chart, that is at its nearest points to the north and south poles respectively, it is at its narrowest portion; and, although this part in the southern hemisphere is brightest and most strongly defined, yet the actual extent, including the fainter portions, is, again, not very unequal in the opposite segments. Here we have a remarkable and significant symmetry in the proportions of the Milky Way, which, taken in connection with the nearly symmetrical scattering of the stars in all parts of the vast ring, is strongly suggestive of a nearly circular form and of our nearly central position within its plane. There is one other feature in this delineation of the Milky Way which is worthy of notice. It has been the universal practice to speak of it as being double through a considerable portion of its extent, and all the usual star-maps show the division greatly exaggerated, especially in the northern hemisphere; and this division was considered so important as to lead to the cloven-disc theory of its form, or that it consisted of two separate irregular rings, the nearer one partly hiding the more distant; while various spiral combinations were held by others to be the best way of explaining its complex appearance. But this newer map, reduced from a large one by Lord Rosse's astronomer, Dr. Boeddicker, who devoted five years to its delineation, shows us that there is no actual division in any portion of it in the northern hemisphere, but that everywhere, throughout its whole width, it consists of numerous intermingled streams and branches, varying greatly in luminosity, and with many faint or barely distinguishable extensions along its margins, yet forming one unmistakable nebulous belt; and the same general character applies to it in the southern hemisphere as delineated by Dr. Gould.
Another feature, which is well shown to the eye by these more accurate maps, is the regular curvature of the central line of the Milky Way. We can judge of this almost sufficiently by the eye; but if, with a pair of compasses, we find the proper radius and centre of curvature, we shall see that the true circular curve is always in the very centre of the nebulous mass, and the same radius applied in the same manner to the opposite hemisphere gives a similar result. It will be noted that as the Milky Way is obliquely situated on these charts, the centre of the curve will be about in R.A. 0h. 40m. in the map of the southern hemisphere, and in R.A. 12h. 40m. in that of the northern hemisphere; while the radius of curvature will be about the length of the chord of eight hours of R.A. as measured on the margin of the maps. This great regularity of curve of the central line of the Galaxy strongly suggests rotation as the only means by which it could have originated and be maintained.
The Solar Cluster
Astronomers are now generally agreed that there is a cluster of stars of which our sun forms a part, though its exact dimensions, form, and limits are still under discussion. Sir William Herschel long ago arrived at the conclusion that the Milky Way 'consists of stars very differently scattered from those immediately around us.' Dr. Gould believed that there were about five hundred bright stars much nearer to us than the Milky Way, which he termed the solar cluster. And Miss Clerke observes that the actual existence of such a cluster is indicated by the fact that 'an enumeration of the stars in photometric order discloses a systematic excess of stars brighter than the 4th magnitude, making it certain that there is an actual condensation in the neighbourhood of the sun—that the average allowance of cubical space per star is smaller within a sphere enclosing him with a radius, say, of 140 light-years, than further away.'[5]
But the most interesting inquiry into this subject is that by Professor Kapteyn of Gröningen, one of the most painstaking students of the distribution of the stars. He founds his conclusions mainly on the proper motions of the stars, this being the best general indication of distance in the absence of actual determination of parallax. He made use of the proper motions and the spectra of more than two thousand stars, and he finds that a considerable body of stars having large proper motions, and also presenting the solar type of spectra, surround our sun in all directions, and show no increased density, as the more distant stars do, towards the Milky Way. He finds also that towards the centre of this cluster stars are far closer together than near its outer limits (he says there are ninety-eight times as many), that it is roughly spherical in shape, and that the maximum compression is, as nearly as can be ascertained, at the centre of the circle of the Milky Way, while the sun is at some distance away from this central point.[6]
It is a very suggestive fact that most of the stars belonging to this cluster have spectra of the solar type, which indicates that they are of the same general chemical constitution as our sun, and are also at about the same stage of evolution; and this may well have arisen from their origin in a great nebulous mass situated at or near the centre of the galactic plane, and probably revolving round their common centre of gravity.
As Kapteyn's result was based on materials which were not so full or reliable as those now available, Professor S. Newcomb has examined the question himself, using two recent lists of stars, one limited to those having proper motions of 10" a century, of which there are 295, and the other of nearly 1500 stars with 'appreciable proper motions.' They are situated in two zones, each about 5° in breadth and cutting across the Milky Way in different parts of its course. They afford, therefore, a good test of the distribution of these nearer stars with regard to the Galaxy. The result is, that on the average these stars are not more numerous in or near the Milky Way than elsewhere; and Professor Newcomb expresses himself on this point as follows:—'The conclusion is interesting and important. If we should blot out from the sky all the stars having no proper motion large enough to be detected, we should find remaining stars of all magnitudes; but they would be scattered almost uniformly over the sky, and show little or no tendency to crowd towards the Galaxy, unless, perhaps, in the region near 19h. of Right Ascension.'[7]