CHAPTER XXVIII.

THE MILKY WAY.

The Milky Way forms a soft band of light round the whole heavens. In the Southern Hemisphere, as in the Northern Hemisphere, it is to be seen. In some parts the band narrows; in some parts it widens. Here it divides into two branches; there we find dark spaces in its midst. One such space in the south is so black and almost starless as to have been named the Coal-sack. All along, over the background of soft, dim light, lies a scattering of brighter stars shining on its surface. Much interest and curiosity have long been felt about this mysterious Milky Way. That it consists of innumerable suns, and that our sun is one among them, has been believed for a considerable time. But other questions arise. How many stars does the Milky Way contain? What is its shape? How far does it reach?

No harm in asking the questions, only we have to be satisfied in astronomy to ask many questions which can not yet receive answers. No harm for man to learn that the utmost reach of his intellect must fall short in any attempt to sound the depths of God’s universe, even as the arm of a child would fall short in seeking to sound the depths of the ocean over the side of a little boat. For the attempt has been made to sound the depths of our star-galaxy out of this little earth-boat.

The idea first occurred to the great Herschel, as we have already mentioned, and a grand idea it was--only a hopeless one. He turned his powerful telescope north, south, east, west. He counted the stars visible at one time in this, in that, in the other directions. He found a marked difference in the numbers. The portion of sky seen through his telescope was about one quarter the size of that covered by the moon. Sometimes he could merely perceive two or three bright points on a black background. At other times the field of his telescope was crowded. In the fuller portions of the Milky Way, he had four or five hundred stars under view at once. In one place he saw about one hundred and sixteen thousand stars pass before him in a single quarter of an hour.

Herschel took it for granted that the stars of the Milky Way, uncountable in numbers, are, as a rule, much the same in size--so that brightest stars would, as a rule, be nearest, and dimmest stars would, as a rule, be farthest off. Where he found stars clustering thickly, beyond his power to penetrate, he believed that the Milky Way reached very far in that direction. Where he found black space, unlighted by stars or lighted by few stars, he decided that he had found the borders of our galaxy in that direction.

Following these rules which he had laid down, he made a sort of rough sketch of what he supposed might be the shape of the Milky Way. He thought it was somewhat flat, extending to a good distance breadthways and a much greater distance lengthways, and he placed our sun not far from the middle. This imagined shape of the Milky Way is called “The Cloven-disk Theory.” To explain the appearance of the Milky Way in the sky, Herschel supposed it to be cloven or split through half of its length, with a black space between the two split parts.

It seems that Herschel did not hold strongly to this idea in later years, and doubts are now felt whether the rules on which he formed it have sufficient foundation.

For how do we know that the stars of the Milky Way are, as a rule, much the same in size? Certainly the planets of the Solar System are very far from being uniform, and the few stars whose weight can with any certainty be measured, seem to vary considerably. There is a great difference also between the large and small suns in many of the double-stars!

Again, how do we know that the bright stars are, as a rule, the nearest, and the dim stars the farthest off? Here, also, late discoveries make us doubtful. Look at Sirius and 61 Cygni--Sirius the most radiant star in the heavens, and 61 Cygni almost invisible to the naked eye. According to this rule, 61 Cygni ought to lie at an enormous distance beyond Sirius. Yet in actual fact, Sirius is the farthest away of the two.

The illustrious Herschel believed that he had penetrated, on one occasion, into the star-cluster on the sword-hand in the constellation of Perseus until he found himself among sidereal depths, from which the light could not have reached him in less than four thousand years.

The distance of those stars had not, and has not, been mathematically measured. Herschel judged of it by their dimness, by the strong power needed to make them visible, and by the rules which he had adopted as most likely true.

Again, when Herschel found black spaces in the heavens almost void of stars, and believed that he had reached the outside borders of the Milky Way, he may have been in the right, or he may have been mistaken. The limit might lie there, or thousands more of small stars might extend in that very direction, too far off for their little glimmer to be seen through the most powerful telescope.

[Illustration: A CLUSTER OF STARS IN PERSEUS.]

If this latter idea about the Milky Way being formed of a great many brilliant suns, and of vast numbers of lesser suns also, be true, astronomers will, in time, be able to prove its truth. For in that case, many faint telescopic stars being much nearer to us than bright stars of the greater magnitudes, it will be found possible to measure their distance. The journey of our earth round the sun must cause a seeming change in their position between summer and winter.

The theory also that some of the nebulæ are other outlying Milky Ways, or galaxies of stars, separated by tremendous distances from our own, is interesting, and was long held as almost certain, yet we have no distinct proof either one way or the other. Many of the nebulæ may be such gatherings of countless stars outside our own, or every nebula visible may be actually part and parcel of our galaxy.

Much attention has of late been paid to the arrangement of stars in the sky. The more the matter is looked into, the more plainly it is seen that stars are neither regular in size nor regular in distribution. They are not merely scattered carelessly, as it were, here, there, and anywhere, but certain laws and plans of arrangement seem to have been followed which astronomers are only now beginning dimly to perceive.

Stars are not flung broadcast through the heavens, each one alone and independent of the rest. They are placed often, as we have already seen, in pairs, in triplets, in quartets, in clusters. Also, the great masses of them in the heavens seem to be more or less arranged in streams, and sprays, and spirals. So remarkable are the numbers and forms of many of these streams that the idea has been suggested, with regard to the Milky Way, whether it also may not be a vast stream of stars, like a mighty river, collecting into itself hundreds of lesser streams.

The contemplation of the heavens affords no spectacle so grand and so eloquent as that of a cluster of stars. Most of them lie at such a distance that the most powerful telescopes still show them to us like star-dust. “Their distance from us is such that they are beyond, not only all our means of measurement,” says Newcomb, “but beyond all our powers of estimation. Minute as they appear, there is nothing that we know of to prevent our supposing each of them to be the center of a group of planets as extensive as our own, and each planet to be as full of inhabitants as this one. We may thus think of them as little colonies on the outskirts of creation itself, and as we see all the suns which give them light condensed into one little speck, we might be led to think of the inhabitants of the various systems as holding intercourse with each other. Yet, were we transported to one of these distant clusters, and stationed on a planet circling one of the suns which compose it, instead of finding the neighboring suns in close proximity, we should see a firmament of stars around us, such as we see from the earth. Probably it would be a brighter firmament, in which so many stars would glow, with more than the splendor of Sirius, as to make the night far brighter than ours; but the inhabitants of the neighboring worlds would as completely elude telescopic vision as the inhabitants of Mars do here. Consequently, to the inhabitants of every planet in the cluster, the question of the plurality of worlds might be as insolvable as it is to us.”

These are clusters of stars of regular form in which attraction appears to mark its secular stamp. Our mind, accustomed to order in the cosmos, anxious for harmony in the organization of things, is satisfied with these agglomerations of suns, with these distant universes, which realize in their entirety an aspect approaching the spherical form. More extraordinary, more marvelous still, are the clusters of stars which appear organized in spirals.

In considering stars of the first six magnitudes only--stars visible to the naked eye--a somewhat larger number is found in the Southern Hemisphere than in the Northern Hemisphere. In both hemispheres there are regions densely crowded with stars, and regions by comparison almost empty.

It has been long questioned whether the number of bright stars is or is not greater in the Milky Way than in other parts of the sky. Careful calculations have at length been made. It appears that the whole of the Milky Way--that zone of soft light passing round the earth--covers, if we leave out the Coal-sack and other such gaps, between one-tenth and one-eleventh of the whole heavens.

The entire number of naked-eye stars, or stars of the first six magnitudes, does not exceed six thousand; and of these, eleven hundred and fifteen lie scattered along the bed of the Milky Way stream. If the brighter stars were scattered over all the sky as thickly as throughout the Milky Way, their number would amount to twelve thousand instead of only six thousand. This shows us that the higher-magnitude stars really are collected along the Milky Way in greater numbers than elsewhere, and is an argument used by those who believe the Milky Way to be a mighty stream of streams of stars.

In the dark spaces of the Milky Way, on the contrary, bright stars are so few that if they were scattered in the same manner over all the sky, their present number of six thousand would come down to twelve hundred and forty. This would be a serious loss.

We know in the sky 1,034 clusters of stars and more than 11,000 nebulæ. The former are composed of associated stars; the latter may be divided into two classes: First, nebulæ which the ever-increasing progress of optics will one day resolve into stars, or which in any case are composed of stars, although their distances may be too great to enable us to prove it; second, nebulæ properly so called, of which spectrum analysis demonstrates their gaseous constitution. Here is an instructive fact. The clusters of stars present the same general distribution as the telescopic stars--they are more numerous in the plane of the Milky Way, while it is the contrary which is presented by the nebulæ properly so called; they are rare, thinly spread in the Milky Way, and thickly scattered to the north as well as to the south of this zone up to its poles. The constitution of the Milky Way--not nebulous, but stellar--is a very significant fact. The nebulæ properly so called are distributed, in a sense, contrary to the stars, being more numerous towards the poles of the Milky Way and in regions poor in stars, as if they had absorbed the matter of which the stars are formed.