CHAPTERS ON THE STARS. 421 



Sums. 219 74 



To derive the best conclusions from these numbers we must com- 

 pare them with the mean star-density for the sky in general, and for 

 the regions near the galactic plane. Heis has 3,903 stars north of the 

 equator; Gould, 6,755 south of it. The area of each hemisphere is 

 20,626 square degrees. It will be convenient to express the various star- 

 densities in terms of 100 square degrees as the unit of area. Thus we 

 have the following star-densities according to the two authorities: 



Heis. Gould. 



Star-density of the entire hemisphere 19.0 32.7 



Star-density of the darker galactic regions 20.4 33.8 



Star-density of the bright-galactic regions 32.9 79.4 



The first two pairs of numbers lead to the remarkable and unexpect- 

 ed conclusion that the darker regions of the Milky Way are but slightly 

 richer in lucid stars than the average of the whole sky; certainly no 

 richer than is due to the general tendency of all the stars to crowd 

 toward the galactic plane. On the other hand, the bright areas are 



60 per cent, richer according to Heis, and more than 100 per cent, 

 richer according to Gould, than the darker areas seen among and 

 around them. The conclusion is that an important fraction of the lucid 

 stars which we see in the same areas with the agglomerations of the 

 Milky Way is really in those agglomerations and form part of them. 



A study quite similar to this has been made by Easton for the por- 

 tions of the Milky Way between Cygnus and Aquila, where the diversi- 

 ties of brightness are greatest. His count of the stars in the bright and 

 dark regions differs from that made above, principally by including all 

 the stars of the Durchmusterung, which we may suppose to extend to 

 about the ninth magnitude.! 



* A long narrow region between these limits. 



t Easton's work is given in detail in the 'Astronomische Nachrichten,' Vol. 137, and the 

 Astrophysical Journal/ Vol. I, No. 3. 



