DOUBLE AND PLANETAKY NEBULAE. 



In figs. 23 and 24 they are unequal, and also partly overlay 

 each other. 



Fig. 23. Fig. 24. 



These double nebulae are generally circular in their apparent, 

 and therefore probably globular in their real form. In some cases 

 they are resolvable clusters. 



That such pairs of clusters are physically connected does not 

 admit of a reasonable doubt, and it is highly probable that, like 

 the binary stars, they move round each other, or round a common 

 centre of attraction, although the apparent motion attending such 

 revolution is rendered so slow by their immense distance that it 

 can only be ascertained after the lapse of ages. 



83. Planetary Nebula.- This class of objects derive their name 

 from their close resemblance to planetary discs. They are in 

 general either circular or very slightly oval. In some cases the 

 disc is sharply defined, in others it is hazy and nebulous at the 

 edges. In some the disc shows a uniform surface, and in some 

 it has an appearance which Sir J. Herschel describes by the term 

 curdled. 



There is no reason to doubt that the constitution of these objects 

 is the same as that of other nebulae, and that they are in fact 

 clusters of stars which by mutual proximity and vast distance are 

 reduced to the form of planetary discs. 



Nebulae of this class, which are not numerous, present some 

 remarkable peculiarities of appearance and colour. It has been 

 already observed that, although the companion of a red individual 

 of a double star appears blue or green, it is not certain that this is 

 its real colour, the optical effect of the strong red of its near 

 neighbour being such as would render a white star apparently 

 blue or green, and no example of any single blue or green star 



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