It will be well for the student to take an imaginary journey from the sun forth into space, along the plane in which extends that vast aggregation of stars which we term the Milky Way. Let him suppose that his journey could be made with something like the speed of light, or, say, at the rate of about two hundred thousand miles a second. It is fit that the imagination, which is free to go through all things, should essay such excursions. On the fancied outgoing, the observer would pass the interval between the sun and the earth in about eight minutes. It would require some hours before he attained to the outer limit of the solar system. On his direct way he would pass the orbits of the several planets. Some would have their courses on one side or the other of his path; we should say above or below, but for the fact that we leave these terms behind in the celestial realm. On the margin of the solar system the sun would appear shrunken to the state where it was hardly greater than the more brilliant of the other fixed stars. The onward path would then lead through a void which it would require years to traverse. Gradually the sun which happened to lie most directly in his path would grow larger; with nearer approach, it would disclose its planets. Supposing that the way led through this solar system, there would doubtless be revealed planets and satellites in their order somewhat resembling those of our own solar family, yet there would doubtless be many surprises in the view. Arriving near the first sun to be visited, though the heavens would have changed their shape, all the existing constellations having altered with the change in the point of view, there would still be one familiar element in that the new-found planets would be near by, and the nearest fixed stars far away in the firmament.
With the speed of light a stellar voyage could be taken along the path of the Milky Way, which would endure for thousands of years. Through all the course the journeyer would perceive the same vast girdle of stars, faint because they were far away, which gives the dim light of our galaxy. At no point is it probable that he would find the separate suns much more aggregated or greatly farther apart than they are in that part of the Milky Way which our sun now occupies. Looking forth on either side of the "galactic plane," there would be the same scattering of stars which we now behold when we gaze at right angles to the way we are supposing the spirit to traverse.
As the form of the Milky Way is irregular, the mass, indeed, having certain curious divisions and branches, it well might be that the supposed path would occasionally pass on one or the other side of the vast star layer. In such positions the eye would look forth into an empty firmament, except that there might be in the far away, tens of thousands of years perhaps at the rate that light travels away from the observer, other galaxies or Milky Ways essentially like that which he was traversing. At some point the journeyer would attain the margin of our star stratum, whence again he would look forth into the unpeopled heavens, though even there he might discern other remote star groups separated from his own by great void intervals.
The revelations of the telescope show us certain features in the constitution and movements of the fixed stars which now demand our attention. In the first place, it is plain that not all of these bodies are in the same physical condition. Though the greater part of these distant luminous masses are evidently in the state of aggregation displayed by our own sun, many of them retain more or less of that vaporous, it may be dustlike, character which we suppose to have been the ancient state of all the matter in the universe. Some of these masses appear as faint, almost indistinguishable clouds, which even to the greatest telescope and the best-trained vision show no distinct features of structure. In other cases the nebulous appearance is hardly more than a mist about a tolerably distinct central star. Yet again, and most beautifully in the great nebula of the constellation of Orion, the cloudy mass, though hardly visible to the naked eye, shows a division into many separate parts, the whole appearing as if in process of concentration about many distinct centres.
The nebulas are reasonably believed by many astronomers to be examples of the ancient condition of the physical universe, masses of matter which for some reason as yet unknown have not progressed in their consolidation to the point where they have taken on the characteristics of suns and their attendant planets.
Many of the fixed stars, the incomplete list of which now amounts to several hundred, are curiously variable in the amount of light which they send out to the earth. Sometimes these variations are apparently irregular, but in the greater number of cases they have fixed periods, the star waxing and waning at intervals varying from a few months to a few years. Although some of the sudden flashings forth of stars from apparent small size to near the greatest brilliancy may be due to catastrophes such as might be brought about by the sudden falling in of masses of matter upon the luminous spheres, it is more likely that the changes which we observe are due to the fact that two suns revolving around a common centre are in different stages of extinction. It may well be that one of these orbs, presumably the smaller, has so far lost temperature that it has ceased to glow. If in its revolution it regularly comes between the earth and its luminous companion, the effect would be to give about such a change in the amount of light as we observe.
The supposition that a bright sun and a relatively dark sun might revolve around a common centre of gravity may at first sight seem improbable. The fact is, however, that imperfect as our observations on the stars really are, we know many instances in which this kind of revolution of one star about another takes place. In some cases these stars are of the same brilliancy, but in others one of the lights is much brighter than the other. From this condition to the state where one of the stars is so nearly dark as to be invisible, the transition is but slight. In a word, the evidence goes to show that while we see only the luminous orbs of space, the dark bodies which people the heavens are perhaps as numerous as those which send us light, and therefore appear as stars.
Besides the greater spheres of space, there is a vast host of lesser bodies, the meteorites and comets, which appear to be in part members of our solar system, and perhaps of other similar systems, and in part wanderers in the vast realm which intervenes between the solar systems. Of these we will first consider the meteors, of which we know by far the most; though even of them, as we shall see, our knowledge is limited.
From time to time on any starry night, and particularly in certain periods of the year, we may behold, at the distance of fifty or more miles above the surface of the earth, what are commonly called "shooting stars." The most of these flashing meteors are evidently very small, probably not larger than tiny sand grains, possibly no greater than the fragments which would be termed dust. They enter the air at a speed of about thirty miles a second. They are so small that they burn to vapour in the very great heat arising from their friction on the air, and do not attain the surface of the earth. These are so numerous that, on the average, some hundreds of thousands probably strike the earth's atmosphere each day. From time to time larger bodies fall—bodies which are of sufficient bulk not to be burned up in the air, but which descend to the ground. These may be from the smallest size which may be observed to masses of many hundred pounds in weight. These are far less numerous than the dust meteorites; it is probable, however, that several hundred fragments each year attain the earth's surface. They come from various directions of space, and there is as yet no means of determining whether they were formed in some manner within our planetary system or whether they wander to us from remoter realms. We know that they are in part composed of metallic iron commingled with nickel and carbon (sometimes as very small diamonds) in a way rarely if ever found on the surface of our sphere, and having a structure substantially unknown in its deposits. In part they are composed of materials which somewhat resemble certain lavas. It is possible that these fragments of iron and stone which constitute the meteorites have been thrown into the planetary spaces by the volcanic eruption of our own and other planets. If hurled forth with a sufficient energy, the fragments would escape from the control of the attraction of the sphere whence they came, and would become independent wanderers in space, moving around the sun in varied orbits until they were again drawn in by some of the greater planets.