PROBLEMS OF MODERN SCIENCE 



of this challenge are for the most part irrelevant 

 when the problem of the origin of a single star, 

 or even of binary or multiple stars, is under dis- 

 cussion. However, in the absence of exact know- 

 ledge regarding the forms and constitution of the 

 nebulae, speculation regarding the relations existing 

 between them and the stars was of little value, and 

 it is only within the past few years that observational 

 material has been obtained which really affords 

 an adequate basis for fruitful theorising. 



The nebulae are classified according to shape 

 as irregular, planetary, and spiral. 



The irregular, as the name implies, present 

 no simple geometrical outline. They often cover 

 large areas in the sky, while condensation in the 

 neighbourhood of particular stars and wisps of 

 nebulous matter joining star to star indicate with- 

 out question the existence of a relation between 

 the nebular and stellar forms. The best-known 

 and most typical nebulas of this class are the great 

 nebula in Orion, whose brightest portion envelops 

 the quadruple star Orionis, and the nebula 

 surrounding the Pleiades. To this class also be- 

 long the dark nebulae whose existence is deduced 

 from the apparent obscuration of the light of the 

 stars in certain parts of the Milky Way. 



The planetary nebulae are so named because 

 they exhibit a regular circular or oval outline 

 enclosing a disc which in some cases is of fairly 

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