254 CARNEGIE INSTITUTION OF WASHINGTON. 



Extended nebulae with emission spectra are 



N. G. C. 281 N. G. C. 2024 



I. C. 59. 63 N. G. C. 2237 



N. G. C. 1491 N. G. C. 2359 



N. G. C. 1499 N. G. C. 5128 



N. G. C. 1624 N. G. C. 6357 



I. C. 405 N. G. C. 7635 



I. C. 423 



N. G. C. 7635 appears to be a planetary involved in an extended 

 nebula. There is some suggestion that this is a case of actual collision. 

 I. C. 434, the Bay Nebula south of f Orionis, gives an emission spec- 

 trum. This fact was previously determined by Max Wolf (Astro- 

 nomische Nachrichten, v. 180, 152), who unfortunately concealed the 

 matter by giving a wrong catalogue number — N. G. C. 2023, a nebula 

 which has a continuous spectrum. 



The extended nebulae giving continuous spectra are 



N. G. C. 1333 N. G. C. 2247 



I. C. 348 I. C. 4592 



N. G. C. 1579 I. C. 4601 



I. C. 2087 I. C. 4603 



N. G. C. 1788 I. C. 4605 



I. C. 2118 N. G. C. 6726-7 



N. G. C. 2023 N. G. C. 6914 



N. G. C. 2183 N. G. C. 7129 



I. C. 448 I. C. 5146 



I. C. 447 



and the uncatalogued objects 



= 4'' H"- 6 = +28° 2' (1920) 



4 22.1 +24 32 



6 45.5 + 1 Brightest part of the great "spiral" in Orion. 



6 3.1 +18 42 



Nebulosity about B. D. -12° 1771 

 Nebulosity about n- Scorpii. 



The spectroscopic observations of stars involved in nebulosity re- 

 ferred to on page 273 establish a sequence in the types of the stars 

 in galactic nebulae as follows: 



1. Small planetaries Stars involved, probably Wolf-Rayet. 



2. Large planetaries Stars involved, between Wolf-Rayet and Oe5. 



3. Extended emission nebulae Stars involved, OeS and BO. 



4. Extended continuous-spectrum nebulae. . . .Stars involved, Bl to A3. 



The overlap between groups is extremely small and is confined to 

 nebulae showing peculiar spectra. 



The intimate relation between the types of nebular spectra and of 

 the stars involved demands that one be considered a consequence of 

 the other, or that both be due to a common cause. It suggests that 

 one source of nebular luminosity may be found in some influence 

 emanating from associated stars that fall within certain ranges of 

 spectral type, and hence probably of effective temperature. On this 

 basis the earliest and hottest starts alone would excise bright-line 

 nebular luminosity. 



Near a critical type, Bl or a fractional subdivision earlier, the bright 

 lines fade rapidly and a continuous or absorption spectrum quickly 



