THE STRUCTURE AND ROTATION OF THE 



GALAXY ' 



Bv J. S. Plaskett 



I. THE STRUCTURE OF THE GALAXY 



For uncounted generations thoughtful men, philosophers and 

 mathematicians, physicists and astronomers, as well as many other 

 students, have been puzzling over the nature of our surroundings in 

 space. They have not been satisfied, as I am afraid many of us are 

 satisfied, with their merely terrestrial surroundings but have tried to 

 penetrate the almost inconceivable distances that separate the mem- 

 bers of our stellar system, to trace the evolution and unravel some 

 of the mysteries of the structure and constitution of the universe. 

 The word " universe ", however, is so broad and inclusive in its 

 meaning, it implies something so vast and incomprehensible to our 

 finite minds, as to be quite beyond the possibility of useful definition 

 or discussion in a lecture like this. The term " universe " has fre- 

 quently been used, and indeed is still used, I believe improperly, as 

 referring to the particular system of stars of which our sun is one 

 insignificant member among thousands of millions of others. A 

 much better designation, however, is " galaxy ", and hereafter we will 

 refer to the system in which we are situated as the galaxy. Although 

 it is now almost universally believed that there are countless other 

 stellar systems extending to distances beyond the capacity of the most 

 powerful telescope to penetrate, all of which may justl}^ be included 

 in the universe, yet it is generally recognized that our system is the 

 largest and most complex of any known. The galaxy indeed is more 

 than sufTiciently large and complex, with its structure and motions 

 still only partially determined, as to occupy fully our attention for 

 one evening. 



It seems hardly necessary to state in this twentieth century that 

 the fundamental problem in the background of all modern astronom- 

 ical research is the problem of the constitution, tlie structure, and 

 the motions of the galaxy. While undoubtedly some practical ap- 

 plications of astronomy to the determination of time and to naviga- 

 tion and surveying have little to do with the structure of the galaxy, 



^ A lecture delivered before the Astrononrical Society of the Pacific at San Francisco on 

 Apr. 20, 1932. Reprinted by permission from Publications of the Astronomical Society 

 of the Pacific, vol. 44, no. 259, June 1932. 



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