14 



NATURE 



\Nov. I, 1888 



Herschel by their arrangement " chiefly in rows," by which 

 he gained some insight into the mechanical complexities 

 of such systems. Each row, he observed, while possessing 

 its own centre of attraction, will at the same time attract 

 all the others ; nay, " there must be somewhere in all the 

 rows together the seat of a preponderating clustering power 

 which will act upon all the stars in the neighbourhood " 

 (Phil. Trans., vol. civ. p. 269). Speculations, indeed, upon 

 the dynamical relations of " stars in rows," are still pre- 

 mature ; nor are they likely, for some time to come, to be 

 accounted as " of the order of the day." But the con- 

 tinual recurrence in the heavens of this mode of stellar 

 aggregation cannot fail to suggest the development of 

 plans of systemic dissolution and recomposition on too 

 grand a scale to be other than vaguely apprehended 

 by us. 



The more attentively clusters are studied, the more 

 intricate their construction appears to be. That which 

 challenged Herschel's notice is not singular in exhibiting 

 the federative union of a number of subordinate groups. 

 There is rarely evidence, in the conformation of irregular 

 clusters, of their being governed from a single focus of 

 attraction ; there are frequent indications of the simul- 

 taneous ascendancy of several. A cluster in Sagittarius 

 (" G. C." 4335) is distinctly bifid. It was remarked by 

 Sir John Herschel at Feldhausen as " divided by a broad, 

 vacant, straight band" ("Cape Observations," p. 116). 

 The fission (as in many nebu]3e),no longer in the inchoate 

 state of a *' dark lane," is complete. Admiral Smyth's 

 stellar "flight of wild ducks," in Sobieski's Shield (M 11), 

 is perhaps trifid. Father Secchi, at least, perceived in it 

 a three-lobed central vacuity {Atti dell' Accad. Pont., 

 t. vii. p. 75). Sir John Herschel, on the other hand, suc- 

 ceeded by the use of high powers, in breaking up •' this 

 glorious object " " into five or six distinct groups with 

 rifts or cracks between them" (Phil. Trans., vol. cxxiii. 

 p. 462). M. Helmert's measures of two hundred of its 

 components referred to a ninth magnitude star conspicuous 

 among them \" Publicationen der Hamburger Sternwarte," 

 No. I, 1874) will eventually afford the means of detecting 

 their relative displacements. Several of them appear to 

 be variable. 



The disruptive tendency indicated by the peculiarities 

 of their distribution is equally marked in " a reticulated 

 mass of small stars" in Cygnus (" G. €."4511), described 

 at Parsonstown as " a most gorgeous cluster, full of 

 holes." The figure published by Lord Rosse shows a 

 winding ribbon of stars inclosing three blank circular 

 spaces, of symmetrically diminishing diameters. 



Star-groupings of curiously definite forms are often met 

 with. A triangular swarm (" G. C." 5055) occurs in the tail 

 of Cetus ; a rectangular area in Vulpecula (" G. C." 4498) is 

 densely strewn with fine star-dust. Clusters shaped like 

 open fans are tolerably numerous. One situated in 

 Gemini would appear, according to Sir John Herschel, if 

 removed to a sufficient distance, " as a fan-shaped nebula 

 with a bright point like a star at the vertex." Another 

 specimen of an " acutangular " cluster (" G. C." 4902) is 

 bounded by " two principal lines of stars drawing to one" 

 {Phil. Trans., vol. cxxiii. pp. 476, 503). It is 2' in length, 

 and is to be found in the constellation Cepheus. An 

 oval annulus of stars in Cygnus, 4' across (" G. C." 4701), 

 centrally surrounds a ruddy ninth magnitude star. A 

 similar elliptical group, with a double substituted for the 

 red star, constitutes a quasi-nucleus for the great cluster 

 in Perseus (" G. C." 512). This superb object, like the still 

 richer group it immediately precedes, has probably galactic 

 affinities. The two together form a telescopic pageant 

 such as, in the wildest flights of imagination, Hipparchus 

 could little have dreamed would one day be unrolled 

 before the eyes of men, out of the "cloudy spot" in the 

 sword-handle of Perseus which he was the first (it is said) 

 to detect. Although the outliers of the two clusters can 

 be brought within the same field of view, they are believed 



to be really disconnected. The following, and more 

 considerable (knov\n as x Persei) was micrometrically in- 

 vestigated by Vogel in 1867-70, photographically by O. 

 Lohse in 1884 {Astr. Nack., No. 2650). The result of the 

 comparison of 172 stars was to show their complete im- 

 mobility in an interval certainly too short for the visible 

 development of such tardy movements as were alone likely 

 to be in progress. A rapid spectroscopic survey executed 

 by Vogel with the Berlin 9-inch refractor, March 30, 18; 6 

 (" Der Sternhaufen x Persei," p. 31), disclosed nothing 

 remarkable in the light of any of the clustered star^;, 

 although several of them have been called red, " pale 

 garnet," and even "-ruby." Their comparative brilliancy 

 suggests that this magnificent assemblage, as well as its 

 neighbour, may be less exorbitantly distant from the earth 

 than most other objects of its class. 



Red and double stars often— we are at a loss to imagine 

 for what reason — seem to dominate in clusters. Com- 

 pound objects must of course, through the chances of 

 optical juxtaposition, occur most freely where stars are 

 most crowded ; yet when they are marked out (as often 

 happens) both by superiority of lustre and by distinction 

 of place, some significance may be attached to their 

 presence. Thus, each of the oblique arms of a " cruciform " 

 group in Auriga (" G. C." 1 1 19), photographed at Paris on 

 January 28, 18S7, carries a pair of conjoined stars brighter 

 than the rest (Smyth, " Cycle," p. 140). A " superb cluster " 

 in Monoceros (" G. C." 1637), standing on a background of 

 sky " singularly dotted over with infinitely minute points," 

 has a double star in its most compressed part (J. Herschel, 

 Phil. Trans., vol. cxxiii. p. 386). The central star in 

 Praesepe is double ; and there are many examples of 

 more restricted groups gathered round a compound 

 luminary. 



Groups apparently ruled by a conspicuous ruddy star 

 are met with in the constellations of the Swan (" G. C" 

 4676) and Auriga (" G. C." 1067). Another in Cygnus 

 (" G. C." 4701 )has already been mentioned. 



The nebular affinities of stellar swarms are full of in- 

 terest, but have as yet been very imperfectly investigated. 

 The discoveries in the Pleiades, however, which may not 

 prove to be the only cluster involved in cosmical fog- 

 wreaths, show what can be done in this direction by the 

 aid of photography. But since nebulic thus situated are 

 likely to be of the last degree of faintness, the stars prob- 

 ably replacing their original more brilliant knots, their 

 existence can scarcely be made manifest otherwise than 

 by prolonged exposures of plates of the highest sensitive- 

 ness. Visual detections of the kind will always be rare. 

 Two rich clusters have nevertheless long been known to 

 include each a nebula of the planetary kind. One in Argo 

 (■' G. C." 1 801) has a central vacuity conspicuously occu- 

 pied by a nebulous disk 40" across ; the other (M 46), not 

 far from the head of Canis Major, displays well within its 

 borders a fine annular nebula (" G. C." 1565). It is difficult, 

 if not impossible, to believe either projected casually into 

 such a very remarkable position. 



The occurrence of clusters within clusters can just as 

 little be set down to the account of chance. In one such 

 instance, a large loose collection in Gemini (" G. C." 1490) 

 involves a neat group of " six or seven stars close together, 

 and well isolated from the rest" (Lord Rosse, Trans. R. 

 Dublin Soc, vol. ii. p. 56). The companion example 

 (" G. C." 1383) is found in the Milky Way, near Orion's 

 right arm. 



Researches into the mutual relations of clustered sta: s 

 are still in their infancy. They will demand for their 

 prosecution a reserve of' patience as inexhaustible as the 

 store of problems to be successively confronted. Before 

 these come to an end, the human race itself will perhaps 

 have become terrestrially extinct. But not, we may hope, 

 before much has been attained that is well worth waiting 

 and working for. 



A. M. Clerk E. 



