RECENT ADVANCES IN SCIENCE 359 



The Causes Underlying the Spectral Differences of the Stars. — 

 A revolutionary theory as to the causes which underlie the 

 spectral differences of the stars has been suggested by Prof. 

 C. D. Perrine in the Astrophysical Journal, 47, 289, 191 8. The 

 usually accepted theory is that the spectral type' is conditioned 

 mainly by the stage in its evolution at which a star has arrived ; 

 its mass is also concerned in a lesser measure, for a star of 

 small mass will not attain so high a temperature as a star of 

 large mass. Prof. Perrine 's theory is that a large part of the 

 characteristics of spectral class among the stars generally are 

 due to external causes depending upon location in the stellar 

 system. 



The theory was suggested by the discovery that, in almost 

 all instances at present known, double stars in which the fainter 

 component is of the earlier spectral type are distant from the 

 sun, and therefore from the centre of the stellar system, whilst 

 those in which the fainter component is of later spectral type 

 are comparatively near. Prof. Perrine considers that the differ- 

 ence is due to a difference in the density of cosmical matter 

 in the two regions, and he suggests that the surface conditions 

 of a star are largely determined by the sweeping up of cosmical 

 matter. In this connection emphasis is laid on the phenomena 

 attending the outburst of Nova Persei, which reached the type 

 B stage at about the time of its maximum brightness. The 

 preference of the B type stars for the Milky Way can be ex- 

 plained by supposing that they have passed through stages 

 similar to Nova Persei. Thus in regions where there is much 

 cosmic matter, the direction of spectral change is towards the 

 nebulae, whilst in regions where there is not much cosmic 

 matter, the effect of radiation will predominate and the direc- 

 tion of spectral change will be towards the later types. 



The theory will need to be examined in all its bearings 

 before it will gain general acceptance in preference to the 

 evolutionary theory, which is now supported by a very large 

 amount of evidence. 



The following is a selection of the more important papers, 

 etc., recently published : 



The Solar System. — Veronnet, A., Constitution Physique 

 ' du Soleil et des Etoiles, £tudiee au moyen de la formule des 

 gaz reels, Bull. Astr. 35, 10 1, 191 8. 



