2.
Irregular Star Clusters. Photographed by E. E. Barnard.
1. Messier 35 in Gemini. 2. Double Cluster in Perseus.
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, 4⅓; and Taygeta, 4½. While Celæno, 5⅓; 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.
Cluster M. 13 Herculis. Photographed by Mr. W. E. Wilson.
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.