INTERSTELLAR SPACE ABBOT 217 



longer ones, such as the red. Plence, if there are in space minute dust 

 particles, we should expect that not only would distant stars be made 

 fainter but their lijjht would be somewhat redder than if they were 

 near. This effect is known as " Rayleigh scattering." 



In 1930 Trumpler found evidence in favor of selective reddening 

 of the stars in space. His results have led to a considerable amount 

 of controversy in the astronomical literature of 1931-32, but the pre- 

 vailing opinion now seems to be that the reddening is actually pro- 

 duced by dust in space. Trumpler concluded that this dust causes a 

 general absorption of starlight weakening the total light received by 

 us from distant stars in proportion to their distance and amounting 

 to 0.7 stellar magnitude per 3,000 light-years. He also observed in 

 addition a selective absorption for blue and violet rays amounting to 

 0.3 magnitude per 3,500 light-years, which, by eliminating the blue 

 and violet more than the red, produces a marked reddening of the 

 more distant stars. Elvey, Stebbins, Huffer, Van de Kamp, Miss 

 AVilliams, and others have found a large amount of supporting obser- 

 vational evidence. Schonberg and Gleissberg have worked out the 

 theory of the problem of Rayleigh scattering as applied to the sup- 

 posed interstellar dust. On the other hand, the Harvard group, 

 notably Gerasimovic and Miss Payne, have found evidence which 

 they regard as being against the hypothesis of Rayleigh scattering, 

 (jpik has tried to reconcile the observations with the theory of scat- 

 tering, and his results are of great importance. 



I think it will be generally agreed upon that the more distant 

 B-type stars (the bluish stars, naturally very rich in blue and violet 

 rays) are actually redder than the nearer ones of the same class. 

 There can be no doubt that the hypothesis of Kayleigh scattering 

 would provide the most attractive explanation of this observation. 



There is a great deal of independent evidence that there is an 

 absorbing layer of matter in and near the galactic plane. Hubble 

 lias found a well-defined region of avoidance of extragalactic nebulae, 

 which follows roughly the outlines of the Milky Way. Stebbins and 

 Huffer have recently shown that the red B stars — that is, those which 

 may have been weakened as regards their blue and violet rays by 

 their long path through interstellar haze — are concentrated in this 

 region of avoidance. Scares and Elvey have both found indications 

 that space-reddening is related to the regions of obscuration in the 

 Milky Way. Van de Kamp has found from statistical considerations 

 that the existence of a relatively thin absorbing layer is probable. 

 Vyssotsky and Miss Williams, and also Stebbins, have found support- 

 ing evidence from the colors of globular clusters. 



Not one of the explanations suggested for the various observations 

 made which indicate the existence of interstellar haze is fully satis- 



