Clusters Of Stars
( Originally Published Early 1900's )
WE have now to consider the clusters of stars whin, though seemingly nebulous in very small telescopes, become immediately resolved into individual stars on the application of a very slight additional amount of opti ' power. A select number of these are put together in the Appendix for the use of those readers of this book who, possessing telescopes, would wish to know whither to direct them profitably. It will suffice, therefore, to allude here to only a few of these clusters. 31 Tai VI. Cassiopeia is a somewhat conspicuous object and readily seen with a telescope of 2 inches aperture. Perhaps the best known of all the socalled globular clusters is 13 M. Herculis, that is to say, the 13th in Messier's Catalogue and in the constellation Hercules. This is commonly regarded as the finest of the globular clusters. Smyth called it "an extensive and magnificent mass of stars with the most compressed part densely compacted and wedged together under unknown laws of aggregation." Sir J. Herschel spoke of its thousands of stars and " hairy-looking curvilinear branches," which features the Earl of Rosse interpreted as indicative of a spiral tendency ; he also perceived several dark rifts in the cluster. Beautiful as it is one might even say magnificent yet J. P. Nichol goes a little too far in asserting that " perhaps no one ever saw it for the first time through a telescope without uttering a shout of wonder."
Before offering any further remarks on the larger clusters it will be convenient to explain the word "globular," and seemly to say something about the French astronomer Messier, whose name is so closely associated with these objects. " Globular," as a word, of course needs no explanation, but it was first applied to star clusters, I believe, by Sir W. Herschel, in order to convey to the mind the idea that, when looking at them, the eye is gazing not on a flat background sprinkled with stars, but on a veritable ball of stars. Without saying that all or even any of the clusters so called are truly such, yet undoubtedly an ordinary eye will readily appreciate them as balls of stars.
Messier was a Frenchman who dedicated himself about a century ago to the task of hunting for comets. In carrying out this work he was so far very successful that between 176o and 1798 he found no fewer than 13. He was, however, much bothered by constantly coming upon objects in his small telescope which, whilst they looked at first like comets, were only clusters and nebulae ; so in 1758 he thought to guard against being taken in any more by forming a permanent catalogue of nebulae, including clusters, by collecting together all that had been found by himself, La Caille, and Méchain. This catalogue was published (but whether for the first time or not I am not sure) in 1784, and is alike a monument of its author's shrewdness and of his industry, for it embraces, with scarcely an exception, the whole of the conspicuous clusters and nebulae visible in the latitude of Paris.
We will now resume our consideration of the clusters by mentioning a few more of them. Next after the cluster in Hercules comes perhaps 5 M. Librae, which, in the words of Webb, is a "beautiful assemblage of minute stars greatly compressed in the centre." Sir W. Herschel with his 4o-ft. reflector made out about 20o stars, though the middle of it was so compressed that it was impossible to individualise the components. Smyth says that :—"This superb object is a noble mass, refreshing to the senses after searching for faint objects, with outliers in all directions and a bright central blaze." Messier, however, "assured himself that it did not contain a single star," but this unsound statement was the unwise result of dogmatising, on the strength of a telescope 2 feet long.
80 M. Scorpii is a compressed globular cluster which Messier, who found it in 1780, described as resembling the nucleus of a comet ; and indeed its blazing centre and attenuated disc give it a very cometary aspect. Sir W. Herschel pronounced it to be the richest and most condensed mass of stars which the firmament can offer to the contemplation of astronomers, albeit that Messier had registered it as Nébuleuse sans étoiles. Near the centre of this object, or, as Webb suggested, " between it and us," there burst forth in 1860 a remarkable temporary star. Pogson had been familiar with the cluster because two variable stars, R and S Scorpii, were in the field with it, and he had frequently been in the habit of viewing them. On May 28, 1860, while seeking for these variables, his attention was arrested by the fact that a star of about the 7th magnitude had appeared in the place previously occupied by the cluster. He had seen the cluster as recently as May 9, and was positive that it had appeared exactly the same as usual without anything stellar about it. The same instrument and power had been employed on both occasions. A fortnight later, that is, on June 10, using a lower power, the stellar appearance had nearly vanished, but the cluster still shone with unusual brilliancy and a marked central condensation. Pogson's observations were fully confirmed by two German observers, E. Luther and Auwers. Pogson thus summed up the circumstances of this curious case : " It is therefore incontestably proved upon the evidence of 3 witnesses that between May 9 and June to  the cluster known as 80 Messier changed apparently from a pale cometary-looking object to a well defined star fully of the 7th magnitude, and then returned to its usual and original appearance. It seems to me absurd to attribute this phenomena to actual change in the cluster itself, but it is very strange if a new variable star,. the 3rd in the same field of view, should be situated between us and the centre of the cluster." At the time when this was written the incident thus narrated was unique, but the more recent case of Nova Andromedae appears to present various analogies to the case of 80 M. Scorpii in 1860. Schonfeld thought he saw some trace of the star in June, 1869, but, barring this, I am not aware of any further information being on record. There are many other globular clusters to be met with in the heavens, some which will be found referred to in the List in the Appendix, but z more only need be mentioned here. These are both in the southern hemisphere, and surpass, it would seem in the matter of size and brilliancy, anything visible in England.
47 Toucani was described by Sir J. Herschel as a superb globular cluster " very visible to the naked eye and one of the finest objects in the heavens. It consists of a very condensed spherical mass of stars of a pale rose-colour concentrically enclosed in a much less condensed globe of white ones 15' or 20' in diameter." Herschel, in speaking of this cluster, made the very curious and significant remark that he could not remember a single elliptical nebula which is resolvable, all the resolvable clusters being more or less circular in form. He then goes on to add :—" Between these two characters then (ellipticity of form and difficulty of resolution) there undoubtedly exists some physical connection it deserves also to be noticed that in very elliptic nebulae which have a spherical centre (as in 65 M.) a resolvable or mottled character often distinguishes the central portion, while the branches exhibit nothing of the kind." This was written prior to the construction of Lord Rosse's great telescope, and therefore it is no reflection on Sir John's accuracy to point out that the " Crab Nebula " in Taurus is an exception to the above rule.
Respecting the cluster surrounding w Centauri, Sir John Herschel says that " it is visible to the naked eye as a dim, round, cometic object about equal to a star of 4 1/2 magnitude, though probably if concentrated in a single point the impression on the eye would be much greater. Viewed in a powerful telescope it appears as a globe of fully 20' in diameter, very gradually increasing in brightness to the centre, and composed of innumerable stars of the 13th and 15th magnitudes, the former probably being two or more of the latter closely juxtaposed."
This chapter may appropriately be concluded with a mention of some large clusters not specifically globular in form. 67 M. Cancri is a rich but loose cluster at the root of the Crab's southern claw. Smyth noted it as consisting principally of a mass of stars of the 9th and loth magnitudes, gathered somewhat in the form of a Phrygian cap, followed by a crescent of stragglers. W. Herschel saw above 20o stars at once in the field of view. This object precedes a Cancri by about 2°.
77 M. Ceti is a round stellar object near S in the constellation named. It is small, bright, and exactly on a line with 3 small stars, one preceding and 2 following; of which the nearest and largest is of the 9th magnitude.
Sir W. Herschel made this object a peg on which to hang the following remark : —" We may conclude that the profundity of the nearest part is at least of the 910th order." By this Sir William meant that this object is 910 times as far off as stars of the first magnitude ; but, to say the east of it, this is a highly imaginative thought one of a type which I think is too common and rather apt to make astronomy and astronomers look ridiculous in the minds of matter-of-fact people.
The cluster 11 M. Antinoi is an interesting cluster of uncommon form. Smyth likened it to a flight of wild ducks, a simile more appropriate than many of those met with in astronomical writings. There is an 8th magnitude star in the middle, and two outside its limits and preceding it. Smyth remarks :—" By all analogy these are decidedly between us and the cluster." This, however was not the opinion of Kirch, its discoverer, who, in 1681, described it as a small, obscure spot, with a star shining through and rendering it more luminous.
In the field with, and adjacent to, the star K Crucis there is a large and loose cluster, described by Sir John Herschel as one of the most beautiful objects of its class. It comprises more than Too stars from the 7th magnitude downwards, 8 of the more conspicuous of them being coloured various shades of red, green, and blue. This object was very carefully surveyed in 1872 by Russell at Sydney, who remarked that many of the stars had drifted (presumably in consequence of proper motion) in the 4o years which had elapsed since Sir John's drawing was made. Russell adds :—" The colours of this cluster are very beautiful, and fully justify Herschel's remark that it looks like a ' superb piece of fancy jewellery.' "
Story of The Stars:
The Stars In Poetry
Groups Of Stars
Clusters Of Stars
The Milky Way
The Application Of The Spectroscope To The Stars And Nebulae
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