ASTRONOMY 



uniform brightness but in others mottled. They 

 thus bear a superficial resemblance to planets. 

 Recent photographic and spectrographic work, 

 however, leaves little doubt that these nebulae are 

 to be regarded as shells of matter. Their varying 

 appearance may be ascribed to differences in the 

 thickness of the shell. When it is thin, the resul- 

 tant appearance is that of a ring, such as that of 

 the well-known ring-nebula in Lyra, but when the 

 interior space is more completely filled, the disc 

 presents a more uniform degree of brilliancy. 



These nebulas also are closely associated with 

 stars, for in nearly every case a stellar point of 

 condensation is found at the centre. 



Although these two types of nebula are so 

 different in form, the spectra are similar. They 

 are bright-line spectra, proving that the matter 

 which emits the light exists in the form of gas. 

 The spectral lines are those characteristic of 

 hydrogen and of an unknown element to which 

 the name ' nebulium ' has been given. The most 

 conspicuous lines ascribed to nebulium are two 

 green ones, and hence these gaseous nebulas are 

 often called green nebulas, in contradistinction to 

 the white nebulas whose spectrum is continuous. 



The opinion was held till recently that these 

 gaseous nebulas stand at the beginning of the path 

 of stellar evolution. It was supposed that they 

 immediately preceded a class of stars, very few 



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