Among other classes of nebulæ we can only mention the ring and the planetary. Of each of these, one good example can be seen, though, it must be admitted, not much more than seen, with very modest instrumental equipment. Midway between the two stars Beta and Gamma Lyræ, already referred to in connection with the variability of the former, the observer by a little fishing will find the famous Ring Nebula of Lyra. With low powers it appears simply as a hazy oval spot; but it bears magnifying moderately well, and its annular shape comes out fairly with a power of eighty on a 2½ inch, though it can scarcely be called a brilliant object with that aperture, or indeed with anything much under 8 inches. None the less, it is of great interest, the curious symmetry of this gaseous ring making it an almost unique object. It resembles nothing so much as those vortex rings which an expert smoker will sometimes send quivering through the air. Photographs show clearly a star within the ring, and this star has a very curious history, having been frequently visible in comparatively small telescopes, and again, within a year or two, invisible in much larger ones. Photography seems to have succeeded in persuading it to forgo these caprices, though it presents peculiarities of light which are still unexplained. The actinic plate reveals also very clearly that deficiency of light at the ends of the longer diameter of the ring which can be detected, though with more difficulty, by the eye. The class of annular nebulæ is not a large one, and none of its other members come within the effective range of small instruments.
Planetary nebulæ are so called because with ordinary powers they present somewhat of the appearance of a planet seen very dimly and considerably out of focus. The appearance of uniformity in their boundaries vanishes under higher telescopic power, and they appear to be generally decidedly elliptical; they yield a gaseous spectrum with strong evidence of the presence of 'nebulium,' the unknown substance which gives evidence of its presence in the spectrum of every true nebula, and has, so far (with one doubtful exception) been found nowhere else. The chief example of the class is that body in Draco which first yielded to Huggins the secret of the gaseous nature of the nebulæ. It lies nearly half-way between Polaris and Gamma Draconis, and is described by Webb as a 'very luminous disc, much like a considerable star out of focus.' It is by no means a striking object, but has its own interest as the first witness to the true nature of that great class of heavenly bodies to which it belongs.
The multitude of nebulous bodies scattered over the heavens may be judged from the fact that Professor Keeler, after partial surveys carried out by means of photography with the Crossley reflector, came to the conclusion that the number within the reach of that instrument (36-inch aperture) might be put down at not less than 120,000. It is a curious fact that the grouping of this great multitude seems to be fundamentally different from that of the stars. Where stars are densely scattered, nebulæ are comparatively scarce; where nebulæ abound, the stars are less thickly sown. So much is this the case, that, when Herschel in his historic 'sweeps' of the heavens came across a notably starless region, he used to call out to his assistant to 'prepare for nebulæ.' The idea of a physical connection between the two classes of bodies is thus underlined in a manner which, as Herbert Spencer saw so early as 1854, is quite unmistakable.
There remain one or two questions of which the very shortest notice must suffice—not because they are unimportant, but because their importance is such that any attempt at adequate discussion of them is impossible in our limited space. One of these inevitably rises to the mind in presence of the myriads of the heavenly host—the familiar question which was so pleasingly suggested to our growing minds by the nursery rhyme of our childhood. To the question, What is a star? it has now become possible to give an answer which is satisfactory so far as it goes, though it is in a very rudimentary stage as regards details.
The spectroscope has taught us that the stars consist of incandescent solid bodies, or of masses of incandescent gas so large and dense as not to be transparent; and further, that they are surrounded by atmospheres consisting of gases cooler than themselves. The nature of the substances incandescent in the individual bodies has also to some extent been learned. The result has been to show that, while there is considerable variety in the chemical constitution and condition of the stars, at least five different types being recognised, each capable of more minute subdivision, the stars are, in the main, composed of elements similar to those existing in the sun; and, in Professor Newcomb's words, 'as the sun contains most of the elements found on the earth and few or no others, we may say that earth and stars seem to be all made out of like matter.' It is, of course, impossible to say what unknown elements may exist in the stars; but at least it is certain that many substances quite familiar to us, such as iron, magnesium, calcium, hydrogen, oxygen, and carbon, are present in their constitution. Indeed, our own sun, in spite of its overwhelming importance to ourselves is to be regarded, relatively to the stellar multitudes, as merely one star among many; nor, so far as can be judged, can it be considered by any means a star of the first class. There can be no doubt that, if removed to the average distance of first magnitude stars—thirty-three years light journey—our sun would be merely a common-place member of the heavenly host, far outshone by many of its fellow-suns. In all probability it would shine as about a fifth magnitude star, with suspicions of variability in its light.
There remains to be noted the fact that the sun is not to be regarded as a fixed centre, its fixity being only relative to the members of its own system. With all its planets and comets it is sweeping continually through space with a velocity of more than 1,000,000 miles in the twenty-four hours. This remarkable fact was first suspected by Sir William Herschel, who also, with that insight which was characteristic of his wonderful genius, saw, and was able roughly to apply, the method which would either confirm or disprove the suspicion.
The principle which lies at the bottom of the determination is in itself simple enough, though its application is complicated in such a manner as to render the investigation a very difficult one. A wayfarer passing up the centre of a street lighted on both sides by lamps will see that the lamps in front of him appear to open out and separate from one another as he advances, while those that he is leaving behind him have an opposite motion, appearing to close in upon one another. Now, with regard to the solar motion, if the case were absolutely simple, the same effect would be produced upon the stars among which we are moving; that is to say, were the stars absolutely fixed, and our system alone in motion among them, there would appear to be a general thinning out or retreating of the stars from the point towards which the sun is moving, and a corresponding crowding together of them towards the point, directly opposite in the heavens, from which it is receding. In actual fact the case is not by any means so simple, for the stars are not fixed; they have motions of their own, some of them enormously greater than the motion of the sun. Thus the apparent motion caused by the advance of our system is masked to a great extent by the real motion of the stars. It is plain, however, that the perspective effect of the sun's motion must really be contained in the total motion of each star, or, in other words, that each star, along with its own real motion, must have an apparent motion which is common to all, and results from our movement through space. If this common element can be disentangled from the individual element, the proper motion of each star, then the materials for the solution of the problem will be secured. It has been found possible to effect this disentanglement, and the results of all those who have attempted the problem are, all things considered, in remarkably close agreement.
Herschel's application of his principle led him to the conclusion that there was a tendency among the stars to widen out from the constellation Hercules, and to crowd together towards the opposite constellation of Argo Navis in the southern hemisphere, and the point which he fixed upon as the apex of the sun's path was near the star Lambda Herculis. Subsequent discussions of the problem have confirmed, to a great extent, his rough estimate, which was derived from a comparatively small number of stars. So far as general direction was concerned, he was entirely right; the conclusion which he reached as to the exact point towards which the motion is directed has, however, been slightly modified by the discussion of a much larger number of stars, and it is now considered that the apex of the solar journey 'is in the general direction of the constellation Lyra, and perhaps near the star Vega, the brightest of that constellation' (Newcomb, 'The Stars,' p. 91). There are but few stars more beautiful and interesting than Vega; to its own intrinsic interest must now be added that arising from the fact that each successive night we look upon it we have swept more than 1,000,000 miles nearer to its brilliant globe, and that with every year we have lessened, by some 400,000,000 miles, the distance that divides us from it. There can surely be no thought more amazing than this! It seems to gather up and bring to a focus all the other impressions of the vastness of celestial distances and periods. So swift and ceaseless a motion, and yet the gulfs that sever us from our neighbours in space are so huge that a millennium of such inconceivable travelling makes no perceptible change upon the face of the heavens! There rise other thoughts to the mind. Towards what goal may our world and its companions be voyaging under the sway of the mighty ruler of the system, and at the irresistible summons of those far-off orbs which distance reduces to the mere twinkling points of light that in man's earliest childlike thought were but lamps hung out by the Creator to brighten the midnight sky for his favourite children? What strange chances may be awaiting sun and planet alike in those depths of space towards which we are rushing with such frightful speed? Such questions remain unanswered and unanswerable. We are as ignorant of the end of our journey, and of the haps that may attend it, as we are helpless in the grasp of the forces that compel and control it.