INTRODUCTION. 



The problein of the motion of the stars is essentially different from that of 

 the motion of the members of our planetary system. In the latter case the con- 

 stants of integrations are known for each individual planet from many years or 

 many centuries of observations, and the position and the velocity of each planet or 

 satelHte may be computed in advance for long periods with an accuracy generally 

 surpassing the errors of observation. 



With regard to the stars, matters stand otherwise. The coordinates and the 

 velocities of the stars are very roughly known, and even if this accuracy could be 

 multiplied ten thousand or a million times the possibility of treating the motions 

 of the stars according to the methods of classical mechanics would be as remote 

 as it is at present. 



More precisely expressed the problem resolves itself into two distinct parts. 

 We may consider the motion of each individual star as governed by the attraction 

 of the wohle stellar universe (the Milky Way) just as we consider the motion of an 

 element of a fluid body as determined by the attraction of the whole fluid mass. 

 This is a problem an rational mechanics and it may be treated as such, as soon as 

 the form and the density distribution of the Galaxy are known with some certainty. 

 For the present our knowledge of these points is somewhat hazy, the various teo- 

 ries given often contradicting one another, but there is no doubt that the energetic 

 researches that are being made into these questions will soon result in a conception 

 of the construction of the Galaxy at least in its main features which will be suffi- 

 ciently definite to enable us to attack and solve the problem of the attraction of 

 the Galaxy on the individual stars, as far as such a solution is at present needed. 



The other side of the problem, however, vis. the influence of one individual 

 star on another at more or less close passages is left untouched by this method. 

 The stars of our stellar universe may be comparatively few in number, so that one 

 star may be able to describe a whole circumference in the Galaxy without inter- 

 fering notably with the others; but in the course of time, in billions or trillions 

 of years, these passages of the stars are able not only totally to deflect the path 

 of an individual star, but to bring order into the originally haphazard distribution 

 of the coordinates and the velocities of the members of the stellar universe. 



Lands Universitets Årsskrift. N, F. Avd. 2. Bd 28. 1 



