THE STARS AND NEBULAE 261 



A. S. Eddington, of the University of Cambridge (Stellar 

 Movements, p. 255) : 



The problems on which dynamics would be expected to throw 

 some light are numerous. Why have the stars in the early stages 

 very small velocities? Why do these velocities afterwards in- 

 crease? In particualr, how do the stars acquire the velocities at 

 right angles to the original plane of distribution, which cause the 

 latest types to be distributed in a nearly spherical form ? How are 

 the two star-streams to be explained? What is the meaning of 

 the third steam, Drift O? Can the partial conforming to Max- 

 well's law be accounted for? What prevents the collapse of the 

 Milky Way? 



Some of these problems seem to be at present quite insoluble. 

 Indeed, it must be admitted that very little progress has been 

 made in the application of dynamics to stellar problems. What 

 has been accomplished is rather of the nature of preparatory 

 work. It has been shown that stellar dynamics is a different 

 study from gas-dynamics, and, indeed, from the theory of any 

 type of system that has yet been investigated. A regular progres- 

 sion may be traced through rigid dynamics, hydro-dynamics, gas- 

 dynamics to stellar dynamics. In the first all the particles move 

 in a connected manner ; in the second there is continuity between 

 the motions of contiguous particles; in the third the adjacent 

 particles act on one another by collision, so that, although there 

 is no mathematical continuity, a kind of physical continuity re- 

 mains; in the last the adjacent particles are entirely independent. 

 A new type of dynamical system has therefore to be considered, 

 and it is probably necessary first to work out the results in simple 

 cases and to become familiar with the general properties, before 

 attempting to solve the complex problems which the actual stellar 

 universe presents. This has been the mode of development in the 

 other branches of dynamics. 



STAR STRUCTURE AND CHARACTERISTICS 



Granting to all stars the same chemical constitution 

 in kind and ratio, I hold that their temperature, density, 

 velocity, color, and brightness are all functioned on the 

 mass, and, further, that their spectra are ordinarily 

 reliable criteria of their masses. By this I do not mean 

 that there is any exactly commensurate relationship or 

 correspondence between all of these characteristics, but 

 merely that, the bigger the star the hotter its tempera- 

 ture, the thinner its density, the slower its velocity, the 



