( 141 ) 



radial velocities. While Campbei.l found 19.9 kilometres for the 

 velocity of the solar motion, and 34 kilometres for the mean velocity 

 of all the stars, Frost and Adams dcM-ivcd from the radial velocities 

 of 20 Orion stars measured by them, after having applied the correction 

 for the solar motion : 7.0 kilometres as mean value '), hence for the 

 actual mean speed in space 14 kilometres, whence follows the ratio 

 0.7 for 2xlq. Hence the Orion stars are the particularly slow ones and 

 the Arcturian stars (class XV) are those which move witii the greatest 

 speed. 



§ 5. When we look at the values of g'4.0 or those of jtj.o or 

 //o.io, derived from ttiem, we find, as we proceed in the series of 

 development from the earliest Orion stars to the Capella or solar 

 type G, that the brightness constantly decreases. That q was larger for 

 the 2'^ type as a wiiole than for the tirst (the Orion stars included) 

 has long been known ; some time ago Kapteyn derived from 

 the entire Bradley -Draper material that on an average the 2"^ type 

 stars (F G K) are 2,7 times as near and hence 7 times as faint as 

 the l^t type stars (A and B). This result perfectly agrees with the 

 ordinary theory of evolution according to which the 2'^' type arises 

 from the 1*^ type through contraction and cooling. 



A look at the subdivisions shows us first of all that the Orion 

 stars greatly surpass the A stars in brightness, and also that among 

 the Orion stars those which represent the earliest stage greatly 

 surpass again in brightness those of the later stages. As compared 

 with the solar type G the Sirius stars are 12 times, the stars which 

 form the transition to the Orion stars 38 times and lastly the « Orionis 

 type 250 times as bright. This result is in good harmony with the 

 hypothesis that one star goes successively through the different con- 

 ditions from class I to class XIV ; we then must accept that the 

 density becomes less as we come to the lower classes. Whether the 

 temperature of the Orion stars is higher than that of the Sirius stars 

 or lower cannot be derived from this result ; even in the latter case 

 it may be that the larger surface more than counterbalances the 

 effect of smaller radiation. This must be decided by photometric 

 measurements of the spectra. As the Wolf-Rayet stars follow next 

 to class I, an investigation of their proper motion, promised by 

 Kapteyn, will be of special interest. 



Past the G stars, the solar type of the series, the brightness again 

 increases. The values obtained here for q confirm in this respect the 

 results of MoNCK and Hertzsprung. 



1) Publications Yerkes Observatory. Vol. II. p. 105. 



