CHAPTERS ON THE STARS. 



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to the star the spectroscope could give us no information as to the mo- 

 tion. The motion to or from the earth would be invariable. To show 

 the result of the orbit being seen obliquely, let E be the earth 

 and A S be the plane of the orbit seen edgewise. Drop the per- 

 pendicular A M upon the line of "sight. Then, while the star is 

 moving from S to A the spectroscope will measure the motion as 

 if it took place from S to M. Since S M is less than A S, the 

 measured velocity will always be less than the actual velocity, ex- 

 cept in the rare case when the plane of the orbit is directed toward 

 the earth. Since the spectroscope can give us no information as to 

 the inclination under which we see the orbit, it follows that the actual 

 orbital velocities of the spectroscopic binaries must remain unknown. 

 We can only say that they cannot be less, but may be greater to any 

 extent than that shown by our measures. 



Fig. 4. The Mills Spectrograph of the Lick Observatory. 



If the components of a binary system do not differ greatly in bright- 

 ness, its character may be detected without actually measuring the radial 

 velocities. Since the motion is shown by a displacement of the spectral 

 lines and since, in any binary system, the two components must always 

 move in opposite directions, it follows that the displacements of the 

 spectral lines of the two stars will be in opposite directions. Hence, 

 when one of the stars, say A, is moving toward us, and the other, say 

 D, from us, all the spectral lines will appear double, the lines made by 

 A being displaced toward the blue end of the spectrum and those by 

 B toward the red end. After half a revolution the motion will be re- 

 versed and the lines will again be double; only the lines of star A will 

 now be on the red side of the others. Between these two phases will 



