[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.