204 



KNOWLEDGE. 



[September 1, 1894. 



There is no branch of geology less advanced at the 

 present day than geological chemistry, to -which subject 

 this sketch of the genesis of gold reefs properly belongs. 

 At nearly every step we are groping in the dark, -with but 

 faint gleams of light from experimental science — the only 

 safe guide we can have — to show us the way that we should 

 attempt to follow. And whilst none can be more sensible 

 than myself of the numerous defects and shortcomings in 

 the theory here presented, I venture to hope that even the 

 mere attempt to put together the different hypotheses on 

 the subject may have its value, were it only in accentuating 

 the extent of our ignorance. 



WHAT IS A STAR CLUSTER? 



By A. C. E.ujYAED. 



ACCOEDING to the generally received nebular 

 hypothesis, our sim and the luminous stars have 

 been formed by the condensation of nebulous 

 masses. Kant, Sir William Herschel, La Place, 

 and the other earlier exponents of the nebular 

 hypothesis who lived before the great principle of the 

 conservation of energy had been propounded, assumed 

 that the nebular masses must, when originally distributed 

 in space, have been intensely heated to a far higher 

 temperature than the luminous stars which were evolved 

 from them. 



The great difficulty of conceiving of a hot nucleus re- 

 maining after ages of radiation into space from the vast 

 surface of a nebular mass does not seem to have occurred 

 to these earlier theorists, or, if it occurred to them, the 

 difficulty was swept on one side by assuming a still higher 

 temperature for the parent nebulous mass. But when the 

 mechanical equivalence of heat with other forms of energy 

 was demonstrated, it became evident that the heat of the 

 condensed nucleus might be derived from the motion of 

 the nebulous particles colliding with one another during 

 condensation. Thus a method of accounting for the great 

 heat and light of the stars was offered, and the popularity 

 of the nebular hypothesis was greatly enhanced. 



It seemed reasonable to suppose that we should find 

 large and small nebulous masses in all stages of con- 

 densation. The large and irregular nebulie were pointed 

 to as nebulous masses which were in the earliest stages of 

 condensation. Nebulous stars were supposed to be in an 

 intermediate stage, and ordinary stars were in a still later 

 stage, approaching a condition in which they would cease 

 to shine as incandescent bodies. But if the ordinary 

 assumptions of the nebular hypothesis were true, the 

 earlier stages of condensation would occupy a much longer 

 period than the final stages, and we might expect to find 

 a much greater number of oblate nebulous spheroids (such 

 as the hypothesis of La Place assumes) than of stars in 

 the later stages of condensation before their incandescent 

 condition had passed away. It could hardly be urged that 

 the stars and nebulous condensing masses were all so far 

 removed from us that they all equally appeared as stellar 

 points of light ; for incandescent spherical masses, com- 

 parable in diameter with the orbit of Nejitune, or even with 

 the orbits of Saturn or Jupiter, would in our larger tele- 

 scopes present very recognizable discs if they were situated 

 at distances from us ten or fifteen times as great as the 

 space which separates us from our nearest stellar neigh- 

 bours. 



While there are millions on millions of stellar points 

 of light to be observed in the heavens, the number of 

 spherical nebulous masses revealed by the telescope is com- 

 paratively few, a fact which may be reconciled with the 



nebular hypothesis by assuming that the condensing 

 masses only commence to be incandescent when they have 

 shi'unk to diameters of a few million miles, and that in the 

 earlier stages of incandescence the nebulous matter is cold 

 and dark, or only glows at a faint red heat, which is not 

 sufficiently bright to render the nebulous mass visible at a 

 distance. 



There are also a few nebulous rings and spirals 

 which shine with a faint nebulosity in the heavens, and a 

 great many nebulie of very irregular form generally sur- 

 rounding stars or associated with groups of stars, in a 

 manner which would seem to indicate that the nebulous 

 matter had issued from the stars rather than that it is 

 condensing about them, for frequently there are arms of 

 nebulosity or nebulous structures which appear to spring 

 from the place occupied by a star or group of stars within 

 the nebula. Such nebulfe would seem to present a closer 

 analogy with the solar corona than v/ith the fiery con- 

 densing mists conceived of by La Place. 



The form of the coronal structures about our sun indi- 

 cates that the coronal matter has issued from the sun, and 

 though we may, no doubt, assume that the matter which 

 is shot forth from the sun, as a general rule, returns to it 

 again, the brighter structures of the corona seem to indi- 

 cate by their form that they are composed of matter on 

 its outward course, that is, in its hot condition, as it is 

 shot upward from the sun. There are no coronal struc- 

 tures the form of which indicates a downward flow of 

 matter, and it seems, therefore, reasonable to assume either 

 that the coronal matter returns to the sun as a uniform 

 mist or that it returns in a comparatively non-luminous 

 form. 



There seems to be a very close analogy between the 

 irregular nebula and star clusters. Eecent photographs 

 indicate that most star clusters are nebulous, or contain 

 whisps of faint nebulosity, and the irregular nebulie are all 

 associated with groups or clusters of stars. Irregular 

 nebula?, as well as star clusters, are distributed along 

 the region of the Milky Way, and seem in some way to be 

 associated with it, while the smaller and regular nebulas 

 have a tendency to cluster in the poles of the JMilky Way. 



If the nebulous matter of the large and irregular nebulie 

 has been shot forth from stars, it seems to follow that 

 the nebulous matter of star clusters has had its origin in 

 the stars of the cluster rather than that the stars of the 

 cluster have condensed from the nebulous matter. 



Prof. George Darwiu pointed out some four or five years 

 ago that if two solid bodies were to collide with planetary 

 velocities, such a rapid evolution of gas would take place, 

 by reason of the heat developed at the region of contact, 

 that the bodies would rebound from one another almost as 

 if they were perfectly elastic bodies. If the moving bodies 

 were liquid or gaseous, no doubt a similar evolution of heat at 

 the region of contact would take place, causing an elastic 

 rebound, and it seems not improbable that within a short 

 period after such a collision the gaseous matter evolved at 

 the region of contact would be distributed in space between 

 the rebounding bodies, forming as it were a nebulous 

 ligature between them ; but it seems difficult to account on 

 this theory for a line of stars joined by nebulosity such as 

 we find in the Pleiades, or for a series of stars in a curving 

 line ligatured together by a band of nebulosity. Such 

 curving prominence-like forms as are shown in Fig. 1, or 

 the branching form shown in Fig. 5, seem rather to in- 

 dicate that the matter which is now luminous has been 

 shot out as a gaseous stream, and that the luminous 

 matter has subsequently aggregated into luminous 

 masses. 



The very beautiful pictures of the Hercules cluster which 



