65 



A comparison of this kind, made for about 300 

 faint stars near the pole and about 300 additional 

 stars in two other regions, shows clearly that there 

 is a gradual change in color with brightness. The 

 fainter stars, on the average, are redder than the 

 brighter ones. 



But while this result is just what would be 

 expected on the basis of Kapteyn's reasoning, other 

 possible explanations must not be overlooked. 

 The increased redness of the fainter stars may be 

 due to a gradually increasing preponderance of 

 late spectral types (old stars) with decreasing 

 brightness; it may be (as Kapteyn pointed out in 

 his papers) a consequence of absolute luminosity, 

 which for the faint stars is less, on the average, 

 than for the brighter objects; or it may be due to 

 scattering of light in space. Perhaps all of these 

 possibilities enter in some degree. We shall soon 

 see how the study of stellar spectra bears upon 

 this problem. 



STELLAR MOTIONS. 



The extension of Kapteyn's study of star- 

 streaming involves the measurement of the ve- 

 locity, toward or away from the earth, of a great 

 number of stars. These velocities are determined 

 by means of the spectrograph shown attached to 

 the telescope tube (Fig. 39). Side by side on the 

 photographic plate, the spectrum of a star and the 

 standard lines in the spectrum of iron or titanium 

 are recorded. If the titanium or iron lines in the 

 star are shifted toward the red, with reference to 



