Part 21

This method, founded on photometric principles, consisted in judging the penetrating power of his telescope by the brightness of the stars, and not, as formerly, by the number which they brought into view. He then studied by this new method the structure of the Milky-way and the probable distance of the clustering masses of which it is formed, concluding that the portion of the Galaxy traversing the constellation Orion is the nearest to us. This last result seems indicated by the fact that this portion of the Milky-way is the faintest and the most uniform of all the galactic belt.

More recently Otto Struve investigated the same subject, and arrived at very nearly similar conclusions, which may be briefly stated as follows: The galactic system is composed of a countless number of stars, spreading out on all sides along a very extended plane. These stars, which are very unevenly distributed, show a decided tendency to cluster together into individual groups of different sizes and forms, separated by comparatively vacant spaces. This layer where the stars congregate in such vast numbers may be conceived as a very irregular flat disk, sending many branches in various directions, and having a diameter eight or ten times its thickness. The size of this starry disk cannot be determined, since it is unfathomable in some directions, even when examined with the largest telescopes. The Sun, with its attending planets, is involved in this immense congregation of suns, of which it forms but a small particle, occupying a position at some distance from the principal plane of the Galaxy. According to Struve, this distance is approximately equal to 208,000 times the radius of the Earth's orbit. The Milky-way is mainly composed of star-clusters, two-thirds, perhaps, of the whole number visible in the heavens being involved in this great belt. In conclusion, our Sun is only one of the individual stars which constitute the galactic system, and each of these stars itself is a sun similar to our Sun. These individual suns are not independent, but are associated in groups varying in number from a few to several thousands, the Galaxy itself being nothing but an immense aggregation of such clusters, whose whole number of individual suns probably ranges between thirty and fifty millions. In this vast system our globe is so insignificant that it cannot even be regarded as one of its members. According to Dr. Gould, there are reasons to believe that our Sun is a member of a small, flattened, bifid cluster, composed of more than 400 stars, ranging between the first and seventh magnitude, its position in this small system being eccentric, but not very far from the galactic plane.

The study of the Milky-way, of which Plate XIII. is only a part, was undertaken to answer a friendly appeal made by Mr. A. Marth, in the Monthly Notices of the Royal Astronomical Society, in 1872. I take pleasure in offering him my thanks for the suggestion, and for the facility afforded me in this study by his "List of Co-ordinates of Stars within and near the Milky-way," which was published with it.

THE STAR-CLUSTERS

PLATE XIV

It is a well-known fact that the stars visible to the naked eye are very unequally distributed in the heavens, and that while they are loosely scattered in some regions, in others they are comparatively numerous, sometimes forming groups in which they appear quite close together.

In our northern sky are found a few such agglomerations of stars, which are familiar objects to all observers of celestial objects. In the constellation Coma Berenices, the stars are small, but quite condensed, and form a loosely scattered, faint group. In Taurus, the Hyades and the Pleiades, visible during our winter nights, are conspicuous and familiar objects which cannot fail to be recognized. In the last group, six stars may be easily detected by ordinary eyes on any clear night, but more can sometimes be seen; on rare occasions, when the sky was especially favorable, I have detected eleven clearly and suspected several others. The six stars ordinarily visible, are in order of decreasing brightness, as follows: Alcyone, Electra, Atlas, Maia, Taygeta and Merope. Glimpses of Celano and Pleione are sometimes obtained.

When the sky is examined with some attention on any clear, moonless night, small, hazy, luminous patches, having a cometary aspect, are visible here and there to the naked eye. In the constellation Cancer is found one of the most conspicuous, called Præsepe, which forms a small triangle with the two stars γ and δ. In Perseus, and involved in the Milky-way, is found another luminous cloud, situated in the sword-handle, and almost in a line with the two stars γ and δ of Cassiopeia's Chair. In the constellation Hercules, another nebulous mass of light, but fainter, is also visible between the stars η and ζ where it appears as a faint comet, in the depths of space. In Ophiuchus and Monoceros are likewise found hazy, luminous patches. In the southern sky, several such objects are also visible to the naked eye, being found in Sagittarius, in Canis Major and in Puppis; but the most conspicuous are those in Centaurus and Toucan. That in Centaurus involves the star ω in its pale diffused nebulosity, and that in Toucan is involved in the lesser Magellanic cloud.

When the telescope is directed to these nebulous objects, their hazy, ill-defined aspect disappears, and they are found to consist of individual stars of different magnitudes, which being more or less closely grouped together, apparently form a system of their own. These groups, which are so well adapted to give us an insight into the structure and the vastness of the stellar universe, are called Star-clusters.

Star-clusters are found of all degrees of aggregation, and while in some of them, such as in the Pleiades, in Præsepe and in Perseus, the stars are so loosely scattered that an opera glass, and even the naked eye, will resolve them; in others, such as in those situated in Hercules, Aquarius, Toucan and Centaurus, they are so greatly compressed that even in the largest telescopes they appear as a confused mass of blazing dust, in which comparatively few individual stars can be distinctly recognized. Although only about a dozen Star-clusters can be seen in the sky with the naked eye, yet nearly eleven hundred such objects visible through the telescope, have been catalogued by astronomers.

The stars composing the different clusters visible in the heavens vary greatly in number, and while in some clusters there are only a few, in others they are so numerous and crowded that it would be idle to try to count them, their number amounting to several thousands. It has been calculated by Herschel that some clusters are so closely condensed, that in an area not more than ⅒ part of that covered by the Moon, at least 5,000 stars are agglomerated.

When the group in the Pleiades is seen through the telescope it appears more important than it does to the naked eye, and several hundreds of stars are found in it. In a study of Tempel's nebula, which is involved in the Pleiades, I have mapped out 250 stars, mostly comprised within this nebula, with the telescope of 6⅓ inches aperture, which I have used for this study.

As a type of a loose, coarse cluster, that in Perseus is one of the finest of its class. It appears to the naked eye as a single object, but in the telescope it has two centres of condensation, around which cluster a great number of bright stars, forming various curves and festoons of great beauty. Among its components are found several yellow and red stars, which give a most beautiful contrast of colors in this gorgeous and sparkling region. In a study which I have made of this twin cluster, I have mapped out 664 stars belonging to it, among which are two yellow and five red stars.

While some clusters, like those just described, are very easily resolvable into stars with the smallest instruments, others yield with the greatest difficulty, even to the largest telescopes, in which their starry nature is barely suspected. Owing to this peculiarity, star-clusters are usually divided into two principal classes. In the first class are comprised all the clusters which have been plainly resolved into stars, and in the second all those which, although not plainly resolvable with the largest instruments now at our disposal, show a decided tendency to resolvability, and convey the impression that an increase of power in telescopes is the only thing needed to resolve them into stars. Of course this classification, which depends on the power of telescopes to decide the nature of these objects, is arbitrary, and a classification based on spectrum analysis is now substituted for it.

The star-clusters are also divided into globular and irregular clusters, according to their general form and appearance. The globular clusters, which are the most numerous, are usually well-defined objects, more or less circular in their general outlines. The rapid increase of brightness towards their centres, where the stars composing them are greatly condensed, readily conveys the impression that the general form of these sparkling masses is globular. The irregular clusters are not so rich in stars as the former. Usually their stars are less condensed towards the centre, and are, for the most part, so loosely and irregularly distributed, that it is impossible to recognize the outlines of these clusters or to decide where they terminate. The globular clusters are usually quite easily resolvable into stars, either partly or wholly, although some among them do not show the least traces of resolvability, even in the largest instruments. This may result from different causes, and may be attributed either to the minuteness of their components or to their great distance from the Earth, many star-clusters being at such immense distances that they are beyond our means of measurement.

As has been shown in the preceding section, the star-clusters are found in great number in the Galaxy; indeed, it is in this region and in its vicinity that the greater portion of them are found. In other regions, with the exception of the Magellanic clouds, where they are found in great number and in every stage of resolution, the clusters are few and scattered.

From a study made in June, 1877]

The star-cluster in the constellation Hercules, designated as No. 4,230 in Sir J. Herschel's catalogue, and which is represented on Plate XIV., is one of the brightest and most condensed in the northern hemisphere, although it is not so extended as several others, its angular diameter being only 7' or 8'. This object, which was discovered by Halley in 1714, is one of the most beautiful of its class in the heavens. According to Herschel, it is composed of thousands of stars between the tenth and fifteenth magnitudes. Undoubtedly the stars composing this group are very numerous, although those which can be distinctly seen as individual stars, and whose position can be determined, are not so many as a superficial look at the object would lead us to suppose. From a long study of this cluster, which I have made with instruments of various apertures, I have not been able to identify more stars than are represented on the plate, although the nebulosity of which this object mainly consists, and especially the region situated towards its centre, appeared at times granular and blazing with countless points of light, too faint and too flickering to be individually recognized. Towards its centre there is quite an extended region, whose luminous intensity is very great, and which irresistibly conveys the impression of the globular structure of this cluster. Besides several outlying appendages, formed by its nebulosity, the larger stars recognized in this cluster are scattered and distributed in such a way that they form various branches, corresponding with those formed by the irresolvable nebulosity. At least six or seven of these branches and wings are recognized, some of which are curved and bent in various ways, thus giving this object a distant resemblance to some crustacean forms. Although I have looked for it with care, I have failed to recognize the spiral structure attributed to this object by several observers. Among the six appendages which I have recognized, some are slightly curved; but their curves are sometimes in opposite directions, and two branches of the upper portion make so short a bend that they resemble a claw rather than a spiral wing. The spectrum of this cluster, like that of many objects of its class, is continuous, with the red end deficient.

A little to the north-east of this object is found the cluster No. 4,294, which, although smaller and less bright than the preceding, is still quite interesting. It appears as a distinctly globular cluster without wings, and much condensed towards its centre. The stars individually recognized in it, although less bright than those of the other cluster, are so very curiously distributed in curved lines that they give a peculiar appearance to this condensed region.

A little to the north of γ Centauri may be found the great ω Centauri cluster, No. 3,531, already referred to above. This magnificent object, which appears as a blazing globe 20' in diameter, is, according to Herschel, the richest in the sky, and is resolved into a countless number of stars from the twelfth to the fifteenth magnitude, which are greatly compressed towards the centre. The larger stars are so arranged as to form a sort of net-work, with two dark spaces in the middle.

The great globular cluster No. 52, involved in the lesser Magellanic cloud, in the constellation Toucan, is a beautiful and remarkable object. It is composed of three distinct, concentric layers of stars, varying in brightness and in degree of condensation in each layer. The central mass, which is the largest and most brilliant, is composed of an immense number of stars greatly compressed, whose reddish color gives to this blazing circle a splendid appearance. Around the sparkling centre is a broad circle, composed of less compressed stars, this circle being itself involved in another circular layer, where the stars are fainter and more scattered and gradually fade away.

Many other great globular clusters are found in various parts of the heavens, among which may be mentioned the cluster No. 4,678, in Aquarius. This object is composed of several thousand stars of the fifteenth magnitude, greatly condensed towards the centre, and, as remarked by Sir J. Herschel, since the brightness of this cluster does not exceed that of a star of the sixth magnitude, it follows that in this case several thousand stars of the fifteenth magnitude equal only a star of the sixth magnitude. In the constellation Serpens the globular clusters No. 4,083 and No. 4,118 are both conspicuous objects, also No. 4,687 in Capricornus. In Scutum Sobieskii the cluster No. 4,437 is one of the most remarkable of this region. The stars composing it, which are quite large and easily made out separately, form various figures, in which the square predominates.

Among the loose irregular clusters, some are very remarkable for the curious arrangement of their stars. In the constellation Gemini the cluster No. 1,360, which is visible to the naked eye, is a magnificent object seen through the telescope, in which its sparkling stars form curves and festoons of great elegance. The cluster No. 1,467, of the same constellation, is remarkable for its triangular form. In the constellation Ara the cluster No. 4,233, composed of loosely scattered stars, forming various lines and curves, is enclosed on three sides by nearly straight single lines of stars. In Scorpio the cluster No. 4,224 is still more curious, being composed of a continuous ring of loosely scattered stars, inside of which is a round, loose cluster, which is divided into four parts by a dark cross-shaped gap, in which no stars are visible.

Among the 1,034 objects which are now classified as clusters more or less resolvable, 565 have been absolutely resolved into stars, and 469 have been only partly resolved, but are considered as belonging to this class of objects. In Sir J. Herschel's catalogue there are 102 clusters which are Considered as being globular; among them 30 have been positively resolved into stars.

The agglomeration of thousands of stars into a globular cluster cannot be conceived, of course, to be simply the result of chance. This globular form seems clearly to indicate the existence of some bond of union, some general attractive force acting between the different members of these systems, which keeps them together, and condenses them towards the centre. Herschel regards the loose, irregular clusters as systems in a less advanced stage of condensation, but gradually concentrating by their mutual attraction into the globular form. Although the stars of some globular clusters appear very close together, they are not necessarily so, and may be separated by great intervals of space. It has been shown that the clusters are agglomeration of suns, and that our Sun itself is a member of a cluster composed of several hundreds of suns, although, from our point of observation, these do not seem very close together. So far as known, the nearest star to us is a Centauri, but its distance from the Earth equals 221,000 times the distance of the Sun from our globe, a distance which cannot be traversed by light in less than three years and five months. It seems very probable that if the suns composing the globular clusters appear so near together, it is because, in the first place, they are at immense distances from us, and in the second, because they appear nearly in a line with other suns, which are at a still greater distance from us, and on which they accordingly are nearly projected. If one should imagine himself placed at the centre of the cluster in Hercules, for instance, the stars, which from our Earth seems to be so closely grouped, would then quite likely appear very loosely scattered around him in the sky, and would resemble the fixed stars as seen from our terrestrial station.

Judging by their loose and irregular distribution, the easily resolvable clusters would appear, in general, to be the nearer to us. It is probable that the globular clusters do not possess, to a very great degree, the regular form which they ordinarily present to us. It seems rather more natural to infer that they are irregular, and composed of many wings and branches, such as are observed in the cluster in Hercules; but as these appendages would necessarily be much poorer in stars than the central portions, they would be likely to become invisible at a great distance, and therefore the object would appear more or less globular; the globular form being simply given by the close grouping of the stars in the central portion. It would seem, then, that in general, the most loosely scattered and irregular clusters are the nearest to us, while the smallest globular clusters and those resolvable with most difficulty are the most distant.

In accordance with the theory that the clusters are composed of stars, the spectrum of these objects is in general continuous; although, in many cases, the red end of the spectrum is either very faint or altogether wanting. Many objects presenting in a very high degree the principal characteristics exhibited by the true star-clusters, namely, a circular or oval mass, whose luminous intensity is greatly condensed toward the centre, have not yielded, however, to the resolving power of the largest telescopes, although their continuous spectrum is in close agreement with their general resemblance to the star-clusters. Although such objects may remain irresolvable forever, yet it is highly probable that they do not materially differ from the resolvable and partly resolvable clusters, except by their enormous distance from us, which probably reaches the extreme boundary of our visible universe.

THE NEBULÆ