330 



KNowLiincr:. 



September. 1912. 



corri'simiidinL^ to tin' siMcciitli iii;i^;nitii(li'. The 

 stars of that rank would not tlii'ii, on the 

 wliole, l)c fnrtluT off than those of th<- rank 

 ne.\t abovf thcni. Init would, on tlic wliolf, 

 possess only i-.-.V-r of their real light."" Vet. in 

 sjiitc of this quasi-recoj^nition of the fallacy of 

 the position that the star-ratio represents a space- 

 ratio, statistical studies of assumed stellar distrihn- 



S <0 )i \Z i.% 14 15 16 



Figure 35S. 



tion continue with very little regard for this fact. 



Admitting the extreme condensation of small stars 

 in certain galactic streams, we may possibly reach 

 some approximate estimate of the arrangement and 

 star-density of these streams. 



The superposition of distinct galactic rings is 

 strongly suggested b\- Kitchey's photograph of the 

 Milky Way in Cygnus, near N.G.C. 5960,+ where 

 the star density in the right-hand half of the plate 

 suddenly increases to about double that on the left. 

 The following counts are reduced to numbers of 

 stars per square degree : 



Right side. 

 f)5.i()() 



ordinary brightness ; but let us now considf.-r rather 

 the millions of insignificant stars — -stars which are 

 better re|)rcsented by the companion of Sirius than 

 b\- Its jjrincipal star. 



Let us first inake the supi)osition that we are 

 dealing with a spiral structure, equivalent to two 

 concentric rings ; the inner, at a distance of sixty 

 light-years, and of such section that its volume is 

 thirty thousand cubic light - \ears, is 

 supposed to consist chiefly of thirteenth 

 to fourteenth magnitude stars; while the 

 outer ring is at a distance of one hundred 

 and eighty light-years, with both section 

 and distance three times as great, or 

 has a volume of two hundred and 

 se\'enty thousand cubic light-years, and 

 is composed mainly of stars from the 

 '7 IS fifteenth to the sixteenth magnitude. 

 These values mav be approximately 

 included in a zone 10° wide for the 

 inner ring and 6° wide for the outer ring, or, con- 

 sidering that there is much irregularity in both 

 density, width and grouping, we may take for the 

 width an even ten degrees. 



The area covered by the rings constitutes nearly 

 one-eleventh of the entire sky, or nearly three 

 thousand six hundred square degrees, and contains 

 much the larger number of very faint stars : but as 

 far as the ninth magnitude, there is little extra 

 richness. Newcomb found seventv-seven thousand 

 three hundred and seventy stars to the ninth magni- 



-o ( iM\ 1^1 can 



9S,J()()I ^^"'""'^ 



numbers can 1 



Left side 

 4-5,600 1 ., 



29,500 •:.;'", 



31,300) -^"'""'^ 

 Let us see if these 

 represented by a hypothetical distribu- 

 tion of stars, according to sonic rational 

 conception. 



Miss Gierke sums up the consensus 

 of various investigators in the " con- 

 clusion that the main part of the annular 

 structure we call the Milky Way lies at 

 a distance from us intermediate between 

 the distances belonging to the tenth and 

 fourteenth stellar magnitudes." t 



If the 10-3-magnitude companion ot 

 Sirius,S which is a body somewhat more massive 

 than the Sun, but giving less than ji^jn of the Sun's 

 light, were to be transported to the distance which 

 I have assumed for the nearer streams of the 

 Milky Way, or had its parallax diminished from 

 0"-37 to 0"-05, it would fall below the average 

 brightness of the above definition. 



Hitherto, attention has been paid almost exclusiveh' 

 to a few thousand exceptional stars of more than 



Fir.uRK 35Q. 



tude north of the equator, or about one hundred and 

 fifty thousand stars for the entire sky, of which one 

 hundred and twenty thousand may ha\e been 

 between eighth and ninth magnitude. Our galactic 

 zone includes about one-tenth of these. The star- 

 ratio, X 3-85 [jer magnitude, has been found for the 

 first nine magnitudes in the galactic part of the sky, 

 and it continues to represent the rate of stellar 

 increase tolerablv well as far as the fourteenth 



'■^'- Miss Agnes M. Gierke, "The System of the St.irs," 1st Edition, page 314, 1>S90. 

 f G. W. Ritchey, Ycrkes Observatory Publications, Vol. II, ri.ite X.WII. 

 t Op. cit., page 366. 

 Magnitude given by E. C. Pickering, Annals Harvard Observatory. N'ol. .\I., page 26 1. 



