ASTRONOMY 5 



of a star by spectroscopic methods, developed at the Mount 

 Wilson Observatory. The method depends upon the discovery 

 that amongst the stars of the same spectral type (and therefore 

 of approximately the same temperature), differences in the 

 relative intensities of certain lines in the spectra can be detected 

 which can be utilised as criteria for the determination of 

 absolute or intrinsic magnitudes. In conjunction with a 

 knowledge of the apparent magnitude, the absolute magnitude 

 enables the distance of the star to be assigned. 



An important list of the parallaxes of i ,646 stars has recently 

 been published by Adams and his collaborators at Mount 

 Wilson {Astroph. Journ., 53, 13, 1921). The values of the 

 parallaxes obtained by this method are found to have probable 

 errors of about 20 per cent. On the face of it, this appears to 

 be a large error, but it must be compared with the errors to 

 which the direct trigonometrical determinations are liable 

 if it is to be properly appraised. The probable errors of the 

 most reliable trigonometrical determinations are not much less 

 than "•oi and therefore exceed 20 per cent, in the case of all 

 parallaxes less than '-05, i.e. for stars whose distances are less 

 than about 65 light years. For the nearest stars, therefore, 

 it appears that the trigonometrical method is the more reliable, 

 but for the more distant stars, the probable error of a spectro- 

 scopic determination becomes less than can be obtained by 

 the best photographic methods. It is for these stars, therefore, 

 that the spectroscopic method will prove of the greatest value. 



A careful examination reveals that these parallaxes are not 

 affected by systematic errors of any magnitude, and they can 

 therefore be used in stellar investigations without prejudice 

 and in conjunction with directly determined parallaxes. The 

 potency of the new method is sufficiently indicated by the list 

 of parallaxes now published — the output of a single observa- 

 tory. These have all been derived in the short period of about 

 five years, much of which time has been occupied in fixing the 

 bases of the method. The number of reliable trigonometrical 

 parallaxes cannot exceed about a couple of thousand, although 

 at present results are being published with great rapidity from 

 several observatories : many of the spectroscopic results refer, 

 of course, to stars whose parallaxes have also been trigono- 

 metrically determined, but the list includes, in addition, new 

 parallaxes for several hundred stars. It therefore provides a 

 very important contribution to our knowledge of stellar 

 distances, a knowledge which is of the utmost importance in 

 many investigations. 



The development of this new method, from which further 

 important results may be anticipated, together with the 

 perfection of photographic methods of determining parallaxes 



