62 



DISCOVERY 



Pease • remarked that "this identity of spectrum, 

 taken in connection with the well-known agreement of 

 distribution relative to the Milky Way of Novae and 

 Wolf-Rayet stars, makes it probable that at least a 

 portion of the latter are temporary stars in the later 

 stages of their history." On the hypothesis of tem- 

 porary stars due to Professor Seeliger, these novae are 

 caused by the rush of feebly-luminous stars through 

 masses of invisible nebulosity, and the nebular and 

 Wolf-Rayet stages may be successive stages in the 

 passage and emergence of stars into and out of such 

 nebulosity. Be this as it may, there is much to be 

 said for the suggestion that the planetar}- nebula; are 

 " the wrecks of ancient novie." 



III. The Spiral Nebulae 



The problem of the spu'al nebula; has bulked very 

 largely in recent astronomical thought. The greatest 

 of the spirals is the great nebula in Andromeda, which 

 was known before the invention of the telescope, but 

 its spiral form was not discerned until after the applica- 

 tion of photograph}' ; so the recognition of the spirals 

 as a distinct class is due to the great Rosse reflector, 

 and the classic instance was that of the famous object 

 (M 51) in Canes Venatici. 



About twenty years ago, after Keeler's startling 

 announcement concerning the profusion of these 

 objects, the prevalent view was that, in the spirals, 

 suns and systems were to be seen forming under the 

 eye of the observer. Indeed, the famous planetisimal 

 hypothesis of evolution was based on the assumed 

 gaseous nature of the spiral nebula;. About a decade 

 ago, however, opinion veered round to the view that 

 the spirals were not true nebuke at all, but irresolvable 

 clusters at vast distances from our galactic system. 

 In 1911 Professor F. W. Very revived the "island 

 universe " theory, which derived considerable support 

 from the numerous differences between the spirals 

 and the other members of the nebular family. The 

 average velocity of the spirals is about twenty-five 

 times the average stellar velocity, and Dr. Shapley 

 has remarked that as a class — " apparently regardless 

 of the gravitational attraction of the galactic system " 

 — they are moving away from the stellar system. 

 Further, the discovery in 1917, on photographs of 

 spiral nebulas, of a number of faint temporary stars 

 gave plausibility to the idea that such might be the 

 novae of distant galactic systems. 



Thus the available evidence about five or six years 

 ago seemed to point to the view that the spirals were 

 not true nebulae at all, but external universes which 

 are too far away for the component stars to be separ- 

 ately discerned. This view seemed highly probable 

 and indeed likely to prevail ; but more recently 

 the work of Van Maanen, Lampland, Shapley, and 



others has thrown grave doubts upon its validity. 

 Mr. Van Maanen and Mr. Lampland have succeeded 

 in measuring the motions of internal points in the 

 nebula Messier loi. Dr. Shapley truly remarks that 

 these measures " would indicate rotational velocities 

 greater than the velocity of light, if that spiral is held 

 to be even one-fifth as large as our galaxy now appears 

 to be." Finally, Dr. Shapley points out that " if in 

 real dimensions spiral nebuke were analogous to our 

 galactic system, the absolute magnitude of the novae in 

 spirals would far transcend any luminosity with which 

 we are acquainted, and would be at direct variance 

 with present results on intrinsic stellar brightness." 



Accordingly, we seem compelled to regard the 

 spirals as nebulous objects, closely connected with 

 our stellar system. Shapley has suggested that " the 

 spirals represent the failure to form stars from the 

 original condensing nebulosity through the presence 

 of too much material." On this hypothesis the 

 spirals are composed of the unused material driven 

 away towards the poles of the galaxy by the radiation- 

 pressure of the stars, and would be, in a sense, analo- 

 gous to the cometic and meteoric matter in the Solar 

 System. This is not out of harmony with Dr. See's 

 ideas that " if repulsive forces are everywhere at work 

 expelling dust from the stars for the formation of 

 nebula;, it is evident that as it is repelled by the stars, 

 it will tend to gather especially in vacant regions or 

 spaces remote from the stars and should accumulate 

 with maximum density near the poles of the Milky 

 Way." ' On Dr. See's view, however, the nebulae are 

 not waste material, but true nebulae, the progenitors 

 of suns and systems. According to Dr. Jeans, the 

 spirals are undoubtedly such, and the " condensa- 

 tions " which are visible in them are the stars of the 

 future. These enormous nebulce, he justly maintains, 

 cannot develop into Solar Systems, but into " streams 

 of stars." Indeed, Dr. Jeans contends that the process 

 of evolution is actually taking place and that its rate 

 is to be calculated by the internal motions which 

 Vain Maanen, Lampland, and others have measured. 

 In the case of the spiral M loi, at a distance of 5,000 

 light-years, Dr. Jeans finds that " this mass is, under 

 the influence of increasing rotation, breaking up into 

 streams of stars. Each star is of mass comparable 

 with our sun, and the stars are generated at the rate 

 of one every few hundred years." 



The alleged extraordinary profusion of the spirals 

 has long constituted a formidable problem. They have, 

 for instance, been estimated to number from eight 

 hundred thousand to a milhon. In 1919, however, 

 Mr. J. H. Reynolds, the English astronomer, put 

 forward certain objections to the view that all the small 

 I This view finds support in the theory of Professor Russell 

 outlined in his paper on dark nebula. 



