DISCOVERY 



39 



believed. Shaplcy's researches indicate that the 

 centre of gravity of the sidereal system is in the con- 

 stellation Sagittarius, amid the dense star-clouds of 

 that region, 60,000 light-years from the sun. The 

 stellar system itself is seen to be much vaster than 

 has been hitherto believed. " The study of the dis- 

 tances and structure of globular clusters has shown 

 that in volume the galactic system is more than a 

 hundred thousand times as large as we formerly 

 believed it to be " — at least 300,000 light-years in 

 diameter. " The phenomenon of the Milky Way is 

 largely an optical one. Although the existence of 

 local and occasionally very e.xtensive condensations 

 of Milky Way stars is not denied, the conception of a 

 narrow encircling ring is abandoned. The Milky Way 

 girdle is chiefly a matter of star-depth, and its long- 

 recognised weakness between longitudes 90^ and 180^ 

 is now taken to be a reflection of the eccentric portion 

 of the sun." The system is vastly more extended in 

 the galactic plane than in the direction of the poles. 

 " .\ thin central stratum of the galactic segment con- 

 tains every star that has been seen or has been photo- 

 graphed for our catalogues. This stratum of stars 

 apparently deviates less than two thousand light- 

 years from the galactic plane." In other words, 

 Shapley's researches show the sidereal system to be 

 a flattened disc, about 4,000 light-years in thickness 

 and 300,000 in diameter. We have reverted, there- 

 fore, in great measure to Herschel's standpoint and 

 to Herschel's conception of the vastness of interstellar 

 space. Grouped round about the sidereal system 

 are the subordinate clusters which Shapley believes 

 to be gradually falling into the larger system, which 

 is, in fact, an assemblage of absorbed clusters. He 

 considers the stars in the vicinity of the sun — including 

 Charlier's galaxy of B-type stars which that astronomer 

 thought to be co-extensive with the universe — to form 

 a local cluster — a striking confirmation of the theory 

 put forward years ago by B. A. Gould. The inter- 

 jjenetration of this cluster with the general galactic 

 stars gives rise to star-st'"eaming, which is, as Eddington 

 maintained ten years ago, a local phenomenon con- 

 fined to the stars in the vicinity of the sun. 



Such, then, is the vast universe which Dr. Shapley's 

 researches have revealed to man — a universe so vast 

 that the mind is staggered by its contemplation. Is 

 it the only universe ? During the past decade the 

 fascinating theory of " island universes " has been 

 revived in a new and plausible form. In November 

 191 1 Professor F. W. Very put forward the view 

 that the faint white nebula; were not actually gaseous, 

 but distant stellar systems — island universes, of which 

 the smallest and faintest was a million light-years 

 away. In 1912 Dr. V. M. Slipher commenced, at the 

 Lowell Observatorv, his measures of the radial velo- 



cities of the spiral nebula;. Grave doubts had already 

 been thrown by spectroscopic evidence on the supposed 

 gaseous nature of these nebulae — the spectrum of the 

 typical spiral being continuous. Slipher's investiga- 

 tions indicated that " the average velocity of the 

 spirals is about twenty-five times the average stellar 

 velocity." This differentiates them from the ordinary 

 nebuhe ; so does the peculiarity of their distribution. 

 Unlike the planetary and irregular nebula;, they avoid 

 the galactic regions, and are most numerous near the 

 galactic poles. These facts seemed to emphasise the 



Tllli ORIC.^T SriR.VL XEBUL.i IN .\NDROMED.\. 

 [From a photograph by Rilchey, ol the Yerkes Observatory) 



non-nebular, extra-galactic nature of the spirals, and 

 accordingly many of the most eminent astronomers — 

 such as Eddington and others — have been disposed to 

 look favourably on the " island universe " hypothesis. 

 The Messrs. Lindemann, in their recent preliminary 

 application of photo-electric photometry to astronomy, 

 have reached an estimate of the distance of the 

 Andromeda nebula, the greatest of the spirals. They 

 find it to be distant 350,000 parsecs, or a little 

 over 1,000,000 light-years, maintaining that there is 

 " nothing inconsistent in regarding this nebula as a 

 galaxy very similar to our own." 



The recent work of Shapley, however, would seem 

 to indicate that, whatever the spirals are, they are not 

 of equal status with the galactic system. He regards 



