528 WRIGHT— RECENT SPECTROSCOPIC OBSERVATIONS. 



object (N. G. C. No. 40), which has a central condensation of ex- 

 ceptional strength, Fig. 6. The somewhat lumpy look of its con- 

 tinuous spectrum is due to the presence of emission bands. It will 

 be noted that the positions of these bands do not correspond with 

 those of the narrow nebular lines. It is a somewhat peculiar fact 

 that the spectrum of a nebula and its nucleus differ markedly from 

 each other. The photograph was made to show the narrow nebular 

 lines, and the continuous spectrum is overexposed, but another 

 record, Fig. 7, enlarged by means of a cylindrical lens, is more 

 legible. The dots above the spectrum indicate the positions of the 

 brighter emission bands of the Class O or Wolf-Rayet stars. The 

 correspondence is complete, and the neucleus of the nebula is to be 

 regarded as a star of that group. Fig. 8 shows the spectrum of 

 another system of which the nucleus is extremely bright. The nar- 

 row lines belong to the nebula, while the hazy bands constitute the 

 spectrum of the nucleus. The object was originally catalogued as a 

 Class O star, and the surrounding nebulosity was found later. It 

 is Campbell's so called hydrogen envelope star, in reality a planetary 

 nebula. 



Examples such as these might be multiplied, but there is no occa- 

 sion for the repetition. Summarizing the results for all of the 

 nebulae observed, we have: 



1. Of the forty-seven nebulae examined thirty have nuclei suffi- 

 ciently bright for their spectra to be photographed. 



2. Of these thirty nuclei fifteen are Class O stars, and all show 

 spectra indicative of high temperature. 



I believe that upon this showing all of the nebular nuclei are 

 to be regarded as belonging to the same general group as the Class 

 O stars. When we recall that throughout the sky only one star 

 in several thousand is known to belong to that group, it is difficult 

 to escape the conclusion that there is an intimate connection between 

 them and the nebulae. The table summarises the argument in favor 

 of such a connection ; there are : 



1. Nebulae without nuclei. 



2. Nebulae with nuclei. The nuclei are in all instances stars of 



very high temperature, and in half of the cases show Class O 

 bands. 



