CHAPTERS ON THE STARS. 25 



The nebula has a nucleus with a star exactly in the center. This 

 is very conspicuous on a photograph, hut barely if at all visible with a 

 36-inch reflector. 



Another curious class of nebulas are designated as planetary, on 

 account of their form. These consist of minute, round disks of light, 

 having somewhat the appearance of a planet. The appellation was 

 suggested by this appearance. These- objects are for the most part 

 faint and difficult. 



It is impossible to estimate the number of nebulas in the heavens. 

 New ones have been from time to time discovered, located and de- 

 scribed by many observers during the last thirty years. Among these 

 Lewis Swift is worthy of special mention. On photographing the sky 

 near the galactic pole with the Crossley reflector, Keeler found no less 

 than seven of these objects in a space of about one-half a square degree. 

 He therefore estimates the whole number in the heavens capable of be- 

 ing photographed at several hundred thousand. It may be assumed that 

 only a moderate fraction of these are visible to the eye, even aided by 

 the largest telescopes. 



Among the most singular of these objects are large diffused nebulas, 

 sometimes extending through a region of several degrees. A number 

 of these were discovered by Herschel. Barnard, W. H. Pickering and 

 others have photographed these for us. One of the most remarkable of 

 them winds around in the constellation Orion in such a way that at 

 first sight one might be disposed to inquire whether the impression on 

 the photographic plate might not have been the result of some defect 

 in the apparatus or some reflection of the light of the neighboring stars, 

 which is so apt to occur in these delicate photographic operations. But 

 its existence happens to be completely confirmed by independent testi- 

 mony. It was first detected by W. H. Pickering and afterwards inde- 

 pendently by Barnard. 



A curious fact connected with the distribution of nebulas over the 

 sky is that it is in a certain sense the reverse of that of the stars. The 

 latter are, as we shall hereafter show in detail, vastly more numerous 

 in the regions near the Milky Way and fewer in number near the poles 

 of that belt. But the reverse is the case with the nebulas proper. They 

 are least numerous in the Milky Way and increase in number as we go 

 from it in either direction. Precisely what this signifies one would not 

 at the present time be able to say. Perhaps the most obvious sugges- 

 tion would be that in these two opposite nebulous regions the nebulas 

 have not yet condensed into stars. This, however, would be a purely 

 speculative explanation. 



On the other hand, star-clusters are more numerous in the galactic 

 region. This, however, is little more than saying that in the regions 

 where the stars are so much more numerous than elsewhere many of 



