The Milky Way 



measured. The spectra of the stars are analogous 

 to the solar spectrum that is to say, composed 

 of a continuous series of colours crossed by 

 dark lines, characteristic of the different elements 

 contained in the absorptive atmosphere of these 

 stars; now these lines move slightly, either 

 towards the violet or the red, according as the 

 object and the observer are approaching or 

 receding from one another; it is possible to 

 determine, from the magnitude of these dis- 

 placements, the radial velocity of the star 

 relatively to the earth. The velocity due to 

 the earth's annual and diurnal motions can 

 be easily deducted, and the actual motions of 

 the stars, relatively to the sun, determined by 

 combining their tangential and radial velocities. 

 These motions take place in all possible 

 directions, at speeds varying from ten to fifty 

 kilometres per second ; but they are not directed 

 in haphazard fashion. 



Let us first consider the case of the sun. 

 If this orb were fixed in position the motions 

 of the other stars, relatively to it, would follow 

 all possible directions without preference for 

 any, and would be of all magnitudes; this is 



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