( l-t3 ) 



ing light is less, the cm^Iolu' of the I'adiated light will be redder. 



Therefore it is bejond doubt that a redder colour corresponds at 

 any rate with a less degree of railiance per unit of surface. 



Then only one explanation remains: tke K and M stars {the redder 

 2"^ type stars like Arcturus and the d''^ iyp^i) possess on an average 

 a much larger surface and volume than the other 2"^ type stars of 

 the classes F and G. This result is at variance with the usual 

 representation of stellar evolution according to which the redder K 

 and later the M stars are developed from the yellow-white F and G 

 stars by further contraction and cooling. 



§ 6. A further examination of the constitution of these stars shows 

 us that it is improbable that they should possess a very small 

 density; the low temperature, the strongly absorbing vapours point 

 to a stage of high condensation. These circumstances lead to expect 

 greater (with regard to the F and G stars) rather than less density. 

 From the larger volumes it then follows tliat the K and M stars 

 have much larger masses than the F's and (rs. This result is the 

 more remarkable in connection with the coiu-iusion derived above 

 about their greater mean velocity. If the stars of our stellar system 

 form a group in the sen-e that theii' velocities withiti the group 

 depend on their mutual attraetiou, \ve may expect that on an 

 average the velocities will be the greater as the masses are smaller. 

 No diftleulty from this ari-os for the Orion stars with small speed, 

 because the same circumsttinces which allow us to ascribe to them 

 a mass equal to that of the A, F and G stars, enable us likewise 

 to ascribe to them a larger mass. The K stars which have both 

 a greater mass and a greater \elocity are characterized by this 

 thesis as belonging to a separate group, wliicii through whatever 

 reason must originally have been endowed \vith a greater velocity, 

 Arcturus with its immeasurably small parallax and large proper 

 motion is therefore through its enormously great linear velocity and 

 extraordinary luminosity an exaggerated type of this entire class, of 

 which it is the brightest representative. Therefore it would be worth 

 while to in\ estigate separately the systematic motions of the K stars 

 which hitherto have been classed without distinction with the F and 

 G stars as 2"'^ type. 



If this result with regard to the greater masses of the K and M 

 stars should not be confirmed, the only remaining possibility is the 

 supposition that the density of these star is extremely small. In this 

 case their masses might be equal to that of other stars and they 

 may represent stages of evolution of the same bodies. Where 



