( 136 ) 



observed. The hypothesis maj' be indirectly tested by investigating 

 the brightness of the stars. To answer to a development as sketched 

 here the brightness of a star must first increase then decrease ; the 

 mean apparent brightness of stars, reduced to the same distances 

 from our solar system must vary with the spectral class in such a 

 way that a maximum is reached where the greatest brightness is 

 found while the apparent brightness decreases in the following stages 

 of development. 



^ 2. For these investigations we cannot make use of directly mea- 

 sured parallaxes as a general measure for the distance because of the 

 small number that have been determined. Another measure will 

 be found in the proper motions of the stars when we assume that 

 the real linear velocity is the same for different spectral classes. In 

 1892 W. H. S. MoNCK applied this method to the Bradley-stars in 

 the Draper Catalogue ^). He found that the proper motions of the 

 B stars were the smallest, then followed those of the A stars; much 

 larger are the mean proper motions of the F stars ^) which also con- 

 siderably surpasses that of the G, H and K stars and that of the 

 M stars. He thence concluded that these F stars (the 2^' type stars 

 which approach to the 1^"^ type) are nearest to us and therefore have 

 a smaller radiating power than the more yellow and redder stars 

 of the 2^^ type. "Researches on binary stars seem to establish that 

 this is not due to smaller average mass and it would therefore appear, 

 that these stars are of the dullest or least light-giving class — more 

 so not only than the Arcturian stars but than those of the type of 

 Antares or Betelgeux" (p. 878). This result does not agree with the 

 current opinion that the G, K and M stars have successively developed 

 from the F stars by contraction and cooling. 



It is, however, confirmed by a newly appeared investigation of 

 Ejnar Hertzsprung : Zur Strahlung der Sterne^), where Maury's 

 classification of the spectra has been folio w^ed. He finds for the 

 mean magnitude, reduced to the proper motion 0",01, the values 

 given in the following table where I have added the corresponding 

 proper motions belonging to the magnitude 4.0. 



Here also appears that for the magnitude 4,0 the pi'()})cr molion 

 is largest and hence the brightness smallest for the classes XII and 



1) Astronomy and Astrophysics XI p. 874. 



2) He constantly calls them incorrectly "Capellan stars" because in the Dr. Cat. 

 Capella is called F, Ihougli this star properly belongs to the sun and the G stars. 



•5) Zcitsclirift fiir wissenscliaftliclic Pbotograpbie Bd. III. S. 429. 



