60 



DISCOVERY 



THE GREAl NEBULA IN ORION. 

 iPhoio by Dr. Max Wolf.) 



and visible nebulae in thcLr vicinity. He drew atten- 

 tion to a bright nebula in Cygnus, " placed centrally 

 in a very fine lacuna, void of faint stars, which sur- 

 rounds the luminous cloud like a trench. The most 

 striking feature with regard to this object is that the 

 star-void halo encircling the nebula forms the end of 

 a long channel, running eastward from the western 

 nebulous clouds and their lacunas to a length of more 

 than two degrees " ; and he asked the question : "Is 

 there a dark mass following the path of the nebula, 

 absorbing the light of the fainter stars ? " Dr. Wolf 

 was gradually led to the opinion that these dark objects 

 were not vacant spaces, but were actually composed of 

 dark gaseous matter. 



More recently Professor Barnard has published a 

 catalogue of these objects, and there can be no doubt 

 that they are actually great patches of obscuring 

 material. Dr. Hubble, of the Mount Wilson Observa- 



tory, who has closely investigated these re- 

 gions of obscuration in recent years, believes 

 that the division in the MUky Way between 

 Aquila and Ophiuchus is apparent only and is 

 due to the presence of dark nebulosity. In a 

 recent paper the Rev. Father Hagen, of 

 Rome, has maintained that this dark nebu- 

 losity is much more extensive in all parts of' 

 the heavens than has hitherto been believed. 

 At all events the bright and dark nebula: are 

 closely akin. In some cases, as Professor 

 Russell remarks, " these regions of obscura- 

 tion merge into faintly luminous nebulosity," 

 and according to him, it is from these 

 dark nebulous masses that the most youth- 

 ful stars — the diffuse red giants — generally 

 spring. 



The view that stars are developed from 

 the bright nebulae was questioned as long 

 ago as 1902 by Professor T. J. J. See, who 

 suggested that " perhaps nebulosity may 

 be expelled from the stars and does not really 

 afford the basis out of which they are con- 

 stituted," and in 191S Professor Russell re- 

 marked on the probability that " the nebulae 

 as visible objects owe their existence to the 

 radiation of the stars and are their offspring 

 and not their parents." In a recent paper 

 on " Dark Nebulae " communicated in May 

 1922 to the National Academy of Sciences at 

 Washington, Professor Russell shows that 

 the normal nebula is the dark nebula — a 

 dark cloud in space — and that the bright 

 nebula is only a particular case. " The 

 transition from these dark nebulae to lumin- 

 ous nebulae in the vicinity of the stars," 

 says Professor Russell, ' ' appears to occur in 

 two ways. One is by simple reflection of the light of 

 the stars," while the second is "by the excitation of 

 gaseous emission." 



This would indicate that there are two sub-classes 

 of luminous gaseous nebulae — a conclusion which is 

 confirmed by actual observation. In 1914, a spectro- 

 scopic study by Dr. V. M. Slipher, of the Lowell Obser- 

 vatory, of the nebulae in the Pleiades surrounding the 

 bright stars Merope and Maia showed the spectra to be 

 continuous — identical with the spectra of the two 

 bright stars. The conclusion from this is that the 

 nebulosity is inherently dark and shines by reflected 

 light. Since then numerous other instances of this 

 have been detected. Recent work at Mount Wilson 

 has demonstrated the intimate relation between the 

 types of nebular spectra and of the stars involved, 

 and this relation, it is stated, in the most recent 

 " Report " of the director, " demands that one be 



