MOUNT WILSON OBSERVATORY. 201 



Spectroscopic Determinations of Luminosity and Parallax. 

 Investigations dealing with the spectroscopic method of deriving parallaxes 

 have been carried on along three principal lines; first, the continuation of 

 the determination of the absolute magnitudes and parallaxes of the stars under 

 observation with the stellar spectrographs; second, an extension of the method 

 to the B-type stars; third, a special study of the giant stars of the K type of 

 spectrum. The work has been in charge of Adams, Joy, and Sanford, with 

 Stromberg taking part in the computations relating to the K-type stars. 



Determinations of absolute magnitude for stars of types A, F, G, K, and M 

 have made rapid progress during the year and more than 900 stars, in addition 

 to those already published, are now upon our lists with values of the parallax 

 obtained in this way. For the stars of types F to M we have used the 

 methods of reduction described in connection with the catalogue of 1,646 

 stars, and for the A-type stars the method derived last year, to which refer- 

 ence was made in the annual report. The stars added during the year include 

 numerous stars of large proper motion, a considerable number of visual 

 binaries which are comprised in Russell's list of dynamical parallaxes and 

 have been observed for this reason, and many stars from the catalogue of 

 Boss. It is evident that the effect of selection, which is very marked in the 

 catalogue of 1,646 stars and makes it far from representative of stars in 

 general, will become much less effective in future lists. In the earlier stages 

 of the investigation of absolute magnitude by spectroscopic methods, the 

 attempt was made to select stars of exceptionally high or low luminosity 

 in order to calibrate the reduction curves over a wide range. With the com- 

 pletion of the reduction system, however, such a selection becomes no longer 

 necessary. 



The extension of the method of spectroscopic parallax determinations to the 

 B-type stars depends, as in the case of those of type A, upon the small dis- 

 persion in absolute magnitude shown by the stars of this spectral class. By 

 the aid of individual parallaxes obtained directly from trigonometric measure- 

 ments and from moving clusters, and the use of mean parallaxes derived from 

 the numerous statistical investigations on the helium stars, it has become 

 possible to construct a simple reduction system which gives the absolute 

 magnitude of a B-type star when its spectral class is known accurately. The 

 method is in all respects analogous to that used forthe A stars, of which it forms 

 a direct continuation. The stars with sharp lines in their spectra are found to 

 be brighter than those with diffuse lines in all cases except possibly the very 

 early B stars, where the results are uncertain. 



The application of the method to 300 stars gives individual values in good 

 agreement with those derived from trigonometric measures or moving clusters. 

 The absolute magnitudes also show an excellent degree of correlation with the 

 reduced proper motion, 0.2 m-j-log /x, the curve being very nearly a straight 

 line, except in the case of the stars with extremely small proper motions. 



The relatively small dispersion in absolute magnitude among the giant 

 K-type stars given in the catalogue of 1,646 stars might be explained either 

 as an inherent characteristic of these stars or as a result of failure of the lines 

 used in the spectroscopic method to serve as adequate criteria for distin- 

 guishing differences of magnitude among such stars. The latter explanation 

 has been advocated by van Rhijn. For the purpose of distinguishing between 

 these alternatives, a special investigation has been made, using as material 



