DISCOVERY 



61 



considered a consequence of the other, or that both be 

 due to a common cause. It suggests that one source 

 of nebular hmiinosity may be found in some influence 

 emanating from associated stars that fall within cer- 

 tain ranges of spectral type and hence probably of 

 effective temperature." 



In the Orion nebula, Professor Russell points out, 

 the stars of the Trapezium appear to be the source of 

 excitation, but he concludes that " there is no reason 

 to believe that the luminous gas forms the whole or 

 even any large part of the matter present within the 

 region — only that it is selectively sensitive to the 

 incident excitation, and therefore gives out most of the 

 light, just as the gases (carbon compounds and nitro- 

 gen) do in the coma and tail of a comet." Bright 

 nebulae, then, are simply particular examples of the 

 normal dark nebulas which seem to constitute the 

 primeval chaos of which the stars are born. 



II. The Planetary Nebulae 



The great extended nebulae — bright and dark — 

 appear to be almost at rest in space. This is in marked 

 contrast with the planetary nebulae, whose average 

 speed as determined by the spectroscope is about 

 forty-eight mUes per second. These " planetaries," so 

 named by their discoverer. Sir William Herschel, 

 from their resemblance to planetary disks, form indeed 

 a small and select class, sharply differentiated from 

 the other types. Like the irregular and dark nebu- 

 losities, they congregate towards the galactic plane, 

 and they are truly gaseous, as Herschel believed them 

 to be. But their regular contour and rapid motion, as 

 well as their comparatively small number — only about 

 150 are known — mark them off as a class apart. 

 They are almost always distinguished by the presence 

 of a central star, so that from one point of view these 

 may be regarded as stellar rather than as nebular 

 objects. They are closely related to the rare type of 

 star known as Wolf-Rayet stars, which are charac- 

 terised by bright lines in their spectra. Indeed, 

 remarked Dr. \\'right, of the Lick Observatory, in 

 1914, " there is no escape from the conclusion that 

 the nuclei of planetary nebula; not only are closely 

 related to Wolf-Rayet stars, but in many cases they 

 are such stars." The parallaxes of six planetary 

 nebulae ha\"e been measured by Mr. Van Maanen, of 

 Mount \\'ilson, within the last few years, and he finds 

 the mean absolute magnitude of these stars to be 

 + 9-1, indicating them to be dwarfs. 



Dr. H. D. Curtis, of the Lick Observatory, has 

 recently suggested that the planetaries are to be 

 regarded as "a somewhat sporadic case in stellar 

 evolution, arising through some collision or cataclysm 

 and not to be regarded as cases typical of the general 

 run of stellar development." Considerable support is 



to be found for this view, not only in the rarity of the 

 planetaries and their enormous velocities, but also in 

 the connection between such nebula; and temporary 

 stars. Since the application of the spectroscope by 

 Huggins to the " blaze star " of 1866, it has been found 

 that the spectrum of a temporary star, "or nova," 

 passes through several well-marked stages. In every 

 case the nebular spectrum appears at a certain period 

 in the decline of temporary stars. The nebular stage, 

 however, seems to be only a transient one. In 1902 

 Pickering found the spectrum of Nova Persei to be 

 identical with that of a planetary nebula. Five 

 years later Hartmann, at Gottingcn, found it to be 

 essentially identical with that of a Wolf-Rayet star. 

 In July 19 14 Adams and Pease, at Mount Wilson, 

 found that Nova Aurigte and Nova Persei had changed 

 into Wolf-Rayet stars, and by April of the following 

 year a similar transformation had taken place in the 

 case of Nova Lacertae and Nova Geminerum No. 2. 

 In their paper announcing these changes, Adams and 



THE GREAT SPIRAI, NEBULA (X 51) IN CANES \TENATICI. 

 {Pholo by Dr. Max Wall) 



