16 KNUT LUNDMARK, GLOBULAR CLUSTERS AND SPIRAL NEBUL^!. 



7T 3 =0". 000250, 

 7T 13 = 0", 000050, 



from vvhich follows an e' value about double the size of the one given above. 



A third way of examining the distance to globular clusters is attainable by 

 the data we possess regarding the absolute magnitude of different spectral classes. 

 In his treatise Charlier holds the view, that it is not certain that Shapley's negative 

 colour-indices in globular clusters pro ve that the stars in question are B stars. Even 

 if we could, with regard to the intiraate connection between spectral class and colour- 

 index for the earlier spectral classes, consider it justifiable to accept Shapley's 

 opinion, that his colour-classes b — b 9 correspond to the spectral types B — B 9 , we 

 shall, nevertheless, presume that they belong instead to the types A — A 9 . Charlier, 

 for A stars m < 6,o, gives M = + 0.47, and for parallax stars M = + 3.04. The weighted 

 mean of these values is + 0,57. If we suppose that the absolute magnitude has 

 on an average this value also in globular clusters, we obt.ain the following hypothe- 

 tical parallaxes: 





- 



Material 



M3 



< 0", 000082 



h—fo 



M 13 



122 



h—fa 



(M 11 



0,000216 



</o) 



The investigations concerning the total spectra of globular clusters have given 

 the result that the mean type is F. We presume that the stars in the above- 

 mentioned clusters, which were determined by Shapley, are on an average of the 

 type F, and now find from Charlier's data 39 for the absolute magnitude: 



M 3 <0" : ooo29o All colours. 



M 13 430 » 



(M 11 0,000752 » » ) 



If we use the formula given by Russell 171 , valid for clusters: 



M = t e + 2,i{Sp. — 2) (8) 



where Sp. means spectral index (.5 = 1, A =2 etc), and M the absolute magnitude, 

 we obtain: 



from b — / from all stars 



M3 <0",000083 <0",000263 



M 13 124 398 



(Mil 0,000219 0,000692) 



