THE STARS IN ACTION — JOY 181 



whole, Avhile the discovery of variation of brightness was the first 

 step in the study of the stars as individual entities. 



The total motion of a star may be resolved into two components, 

 one toward or away from the observer in the line of sight, called 

 radial velocity, and the other at right angles to it, known as proper 

 motion. Radial velocity of stars was thought indeterminable 60 

 years ago, but the use of the spectroscope — that beautiful optical 

 appliance which, by analyzing light and spreading it out into its va- 

 rious wave lengths and colors, has meant so much to astronomy and 

 physics in the last 50 years — has made it possible to measure the 

 velocities of the stars in the line of sight. Distance does not mat- 

 ter, provided only the star is bright enough to record its light rays on 

 the iDhotographic plate. The largest telescope can photograph the 

 spectra of stars as faint as the tenth or eleventh magnitude and de- 

 termine their velocities from single plates. 



Proper motion can be observed by noting the positions of stars as 

 compared with their positions observed 10, 50, or 100 years before. 

 The largest proper motion known is that of Barnard's star, a faint 

 star of the tenth magnitude, which moves among its neighbors at the 

 rate of 10 seconds of arc a year. This motion would carry it across 

 a space equal to the moon's apparent diameter in 180 years. Al- 

 though it seems slow, yet such is the extreme accuracy of modern 

 photographic methods that the motion could be detected on plates 

 separated by a time interval of only 24 hours. On account of their 

 great distances most stars have proper motions which are compara- 

 tively insignificant, as far as short periods of time are concerned. 

 The nearer stars have, on the average, the greater proper motion 

 because they mark out greater angular distances in the sky as they 

 move. 



With proper motion and line-of-sight velocity known, we can 

 calculate the total motion in space of those stars whose distance 

 can be determined. Space motions are the foundations of the 

 study of the structure and dynamical behavior of our universe. 

 They give a clue to the origin and relationships of stars which can 

 be found in no other way. For the nearer stars, at least, it ap- 

 pears that there is a tendency to move through space in groups at 

 certain speeds and in certain directions. Many such groups, com- 

 ]insed of a few hundreds or thousands of stars, are known among 

 our nearer neighbors. Smaller groups composed of three or four 

 stars, or even two stars, are numerous. These have the same motion 

 in space and revolve about each other in orbits, as the moon revolves 

 about tlie earth. They can usually be resolved by the telescope and 

 can be observed as they leisurely describe their orbits in periods 

 varying from a few years to many centuries. A number of the 

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