CLUSTERS AND NEBULÆ

Even the most casual observer of the heavens cannot have failed to notice that in certain instances the stars are grouped so closely together as to form well-marked clusters. The most familiar example is the well-known group of the Pleiades, in the constellation Taurus, while quite close is the more scattered group of the Hyades. Another somewhat coarsely scattered group is that known as Coma Berenices, the Hair of Berenice, which lies beneath the handle of the Plough; and a fainter group is the cluster PrÆsepe, which lies in the inconspicuous constellation Cancer, between Gemini and Leo, appearing to the naked eye like a fairly bright, hazy patch, which the smallest telescope resolves into a cloud of faint stars.

The Pleiades form undoubtedly the most remarkable naked-eye group in the heavens. The six stars which are visible to average eyesight are Alcyone, 3rd magnitude; Maia, Electra, and Atlas, of the 4th; Merope, 4?; and Taygeta, 4½. While CelÆno, 5?; Pleione, 5½; and Asterope, 6, hang on the verge of visibility. With an opera-glass about thirty more may be counted, while photographs show between 2,000 and 3,000. It is probable that the fainter stars have no real connection with the cluster itself, which is merely seen upon a background of more distant star-dust. Modern photographs have shown that this cluster is involved in a great nebula, which stretches in curious wisps and straight lines from star to star, and surrounds the whole group. The Pleiades make a brilliant object for a small telescope with a low magnifying power, but are too scattered for an instrument of any size to be effective upon them. The finest of all irregular star-clusters is that known as the Sword-handle of Perseus. Midway between Perseus and the W of Cassiopeia, and directly in the line of the Galaxy, the eye discerns a small, hazy patch of light, of which even a 2 or 3 inch glass will make a beautiful object, while with a large aperture its splendour is extraordinary. It consists of two groups of stars which are both in the same field with a small instrument and low powers. Towards the edge of the field the stars are comparatively sparsely scattered; but towards the two centres of condensation the thickness of grouping steadily increases. Altogether there is no more impressive stellar object than this magnificent double cluster (Plate XXVIII., 2). Another very fine example of the irregular type of grouping is seen in M. 35, situated in the constellation Gemini, and forming an obtuse-angled triangle with the stars Mu and Eta Geminorum (Plate XXVIII., 1). There are many other similar groups fairly well within the reach of comparatively small instruments, and some of these are mentioned in the list of objects (Appendix II.).

Still more remarkable than the irregular clusters are those which condense into a more or less globular form. There are not very many objects of this class in the northern sky visible with a small telescope, but the beauty of those which are visible is very notable. The most splendid of all is the famous cluster M. 13 Herculis. (The M. in these cases refers to the catalogue of such objects drawn up by Messier, the French 'comet ferret,' to guide him in his labours.) M. 13 is situated almost on the line between Zeta and Eta Herculis, and at about two-thirds of the distance from Zeta towards Eta. It is faintly visible to the unaided eye when its place is known, and, when viewed with sufficient telescopic power, is a very fine object. Nichol's remark that 'perhaps no one ever saw it for the first time through a large telescope without uttering a shout of wonder' seems to be based on a somewhat extravagant estimate of the enthusiasm and demonstrativeness of the average star-gazer; but the cluster is a very noble object all the same, consisting, according to a count made on a negative taken in 1899, of no fewer than 5,482 stars, which condense towards the centre into a mass of great brilliancy. It takes a large aperture to resolve the centre of the cluster into stars, but even a 3-inch will show a number of twinkling points of light in the outlying streamers (Plate XXIX.). In the same constellation will also be found the cluster M. 92, similar to, but somewhat fainter than M. 13; and other globular clusters are noted in the Appendix. Most of these objects, however, can only be seen after a fashion with small instruments. Of the true nature and condition of these wonderful aggregations we are so far profoundly ignorant. The question of whether they are composed of small stars, situated at no very great distance from the earth, or of large bodies, which are rendered faint to our vision by immense distance, has been frequently discussed. Gore concludes that they are 'composed of stars of average size and mass, and that the faintness of the component stars is simply due to their immense distance from the earth.' If so, the true proportions of some of these clusters must be indeed phenomenal! A very remarkable feature to be noticed in connection with some of them is the high proportion of variable stars which they contain. Professor Bailey has found that in such clusters as M. 3 and M. 5 the proportion of variables is one in seven and one in eleven respectively, while several other groups show proportions ranging from one in eighteen up to one in sixty. As the general proportion of variables is somewhere about one in a hundred, these ratios are remarkable. They only characterize a certain number of clusters, however, and are absent in cases which seem strictly parallel to others where they exist.

We now pass from the star-clusters to the nebulÆ properly so called. Till after the middle of last century it was an open question whether there was any real distinction between the two classes of bodies. Herschel had suggested the existence of a 'shining fluid,' distributed through space, whose condensations gave rise to those objects known as nebulÆ; but it was freely maintained by many that the objects which could not be resolved into stars were irresolvable only because of their vast distance, and that the increase of telescopic power would result in the disclosure of their stellar nature. This view seemed to be confirmed when it was confidently announced that the great Rosse telescope had effected the resolution of the Orion Nebula, which was looked upon as being in some sort a test case. But the supposed proof of the stellar character of nebulÆ did not hold its ground for long, for in 1864 Sir William Huggins, on applying the spectroscope to the planetary nebula in Draco, found that its spectrum consisted merely of bright lines, one of which—the most conspicuous—was close to the position of a nitrogen line, but has proved to be distinct from it; while of the other two, one was unmistakably the F line of hydrogen and the other remains still unidentified. Thus it became immediately manifest that the nebula in Draco did not consist of distant stars, but was of gaseous constitution; and Sir William Herschel's idea of the existence of non-stellar matter in the universe was abundantly justified. Subsequent research has proved that multitudes of nebulÆ yield a bright-line spectrum, and are therefore gaseous. Of these, by far the most remarkable and interesting is the Great Nebula of Orion. The observer will readily distinguish even with the unaided eye that the middle star of the three that form the sword which hangs down from Orion's belt has a somewhat hazy appearance. A small telescope reveals the fact that the haziness is due to the presence of a great misty cloud of light, in shape something like a fish-mouth, and of a greenish colour. At the junction of the jaws lies the multiple star Theta Orionis, which with a 2- or 3-inch glass appears to consist of four stars—'the trapezium'—large instruments showing in addition two very faint stars.

With greater telescopic power additional features begin to reveal themselves; the mist immediately above the trapezium assumes a roughly triangular shape, and is evidently much denser than the rest of the nebula, presenting a curdled appearance similar to that of the stretches of small cloud in a 'mackerel' sky; while from the upper jaw of the fish-mouth a great shadowy horn rises and stretches upward, until it gradually loses itself in the darkness of the background. This wonderful nebula appears to have been discovered in 1618, but was first really described and sketched by Huygens in 1656, since when it has been kept under the closest scrutiny, innumerable drawings of it having been made and compared from time to time with the view of detecting any traces of change. The finest drawings extant are those of Sir John Herschel and Mr. Lassell, and the elaborate one made with the help of the Rosse 6-foot mirror.

Drawing, however, at no time a satisfactory method of representing the shadowy and elusive forms of nebulÆ, has now been entirely superseded by the work of the sensitive plate. Common, Roberts, Pickering, and others have succeeded admirably in photographing the Great Nebula with exposures ranging from half an hour up to six hours. The extension of nebulous matter revealed by these photographs is enormous (Plate XXX.), so much so that many of the central features of the nebula with which the eye is familiar are quite masked and overpowered in the photographic print. The spectrum of the Orion Nebula exhibits indications of the presence of hydrogen and helium, as well as the characteristic green ray which marks the unknown substance named 'nebulium.'

The appearance of this 'tumultuous cloud, instinct with fire and nitre,' is always amazing. Sir Robert Ball considers it one of the three most remarkable objects visible in the northern heavens, the other two being Saturn and the Great Cluster in Hercules. But, beautiful and wonderful as both of these may be, the Orion Nebula conveys to the mind a sense of mystery which the others, in spite of their extraordinary features, never suggest. Absolutely staggering is the thought of the stupendous dimensions of the nebula. Professor Pickering considers its parallax to be so small as to indicate a distance of not less than 1,000 years light journey from our earth! It is almost impossible to realize the meaning of such a statement. When we look at this shining mist, we are seeing it, not as it is now, but as it was more than a hundred years before the Norman Conquest; were it blotted out of existence now, it would still shine to us and our descendants for another ten centuries in virtue of the rays of light which are already speeding across the vast gulf that separates our world from its curdled clouds of fire-mist, and the astronomers of A.D. 2906 might still be speculating on the nature and destiny of a thing which for ages had been non-existent! That an object should be visible at all at such a distance demands dimensions which are really incomprehensible; but the Orion Nebula is not only visible, it is conspicuous!

The rival of this famous nebula in point of visibility is the well-known spiral in the girdle of Andromeda. On a clear night it can easily be seen with the naked eye near the star Nu AndromedÆ, and may readily be, as it has often been, mistaken for a comet. Its discovery must, therefore, have been practically coincident with the beginnings of human observation of the heavens; but special mention of it does not occur before the tenth century of our era. A small telescope will show it fairly well, but it must be admitted that the first view is apt to produce a feeling of disappointment. The observer need not look for anything like the whirling streams of light which are revealed on modern long exposure photographs (Plate XXXI., 1). He will see what Simon Marius so aptly described under the simile of 'the light seen from a great distance through half-transparent horn plates'—a lens-shaped misty light, brightening very rapidly towards a nucleus which seems always on the point of coming to definition but is never defined, and again fading away without traceable boundary into obscurity on every side. The first step towards an explanation of the structure of this curious object was made by Bond in the middle of last century. With the 15-inch refractor of the Cambridge (U.S.A.) Observatory, he detected two dark rifts running lengthwise through the bright matter of the nebula; but it was not till 1887 and 1888 that its true form was revealed by Roberts's photographs. It was then seen to be a gigantic spiral or whirlpool, the rifts noticed by Bond being the lines of separation between the huge whorls of the spiral. Of course, small instruments are powerless to reveal anything of this wonderful structure; still there is an interest in being able to see, however imperfectly, an object which seems to present to our eyes the embodiment of that process by which some assume that our own system may have been shaped. So far as the powers of the best telescopes go, the Andromeda Nebula presents no appearance of stellar constitution. Its spectrum, according to Scheiner, is continuous, which would imply that in spite of appearances it is in reality composed of stars; but Sir William Huggins has seen also bright lines in it. Possibly it may represent a stage intermediate between the stellar and the gaseous.

Another remarkable example of a spiral nebula will be found in M. 51. It is situated in the constellation Canes Venatici, and may be easily picked up, being not far from the end star of the Plough-handle Eta UrsÆ Majoris. This strange object, 'gyre on gyre' of fire-mist, was one of the first spirals to have its true character demonstrated by the Rosse telescope. It is visible with moderate optical powers, but displays to them none of that marvellous structure which the great 6-foot mirror revealed for the first time, and which has been amply confirmed by subsequent photographic evidence (Plate XXXII.).

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.