GALAXIES; — SHAPLEY 135 



the stellar neighbors of the sun and the nearer parts of the Milky Way. 

 The old but unproved concept that the spiral nebulae and their rela- 

 tives were external galaxies, coordinate with our own Milkey Way 

 system, gradually became established. The dimensions of the galaxy 

 and of the universe approached clarification, chiefly through the power 

 of the telescopes of American observatories and the vision of European 

 and American theoreticians. 



In clarifying some of the earlier puzzles, however, the astronomers 

 only succeeded in opening vaster vistas for exploration, interpretation, 

 and wonderment. The net gain has been considerable. It is no longer 

 believed that the severe difficulties of certain astronomical enterprises 

 have definitely blocked the progress of inquiry. A hundred years ago 

 a distinguished scientist (not an astronomer) gloated a bit over the 

 pronouncement that one thing would certainly forever remain un- 

 known, namely, the chemical nature of the stars ! It was not many 

 years before the spectroscope began to betray him. And at the time 

 of her death in 1941 Dr. Annie Cannon had classified more than half 

 a million stars on the basis of the chemistry of their surfaces. A 

 great deal is now known of the chemical constitution of a galaxy of 

 a billion stars at a distance of 10 million light-years. An elementary 

 astronomical student can quickly learn, with the use of modern equip- 

 ment, about the hydrogen, calcium, iron, magnesium, helium, carbon, 

 and the like in stiirs that have never actually been seen except through 

 use of t]ie photographic plate. 



The moral of that bad ancient pronouncement about stellar chem- 

 istry is that it is not wise to be discouraged with the difficulties arising 

 from our awkward location in the galaxy. Eventually all the answers 

 to all the questions you could now ask about Milky Way structure may 

 be known. And, of course, we would then be wise enough to ask other 

 (questions that you could not answer, nor could we. Here are some of 

 the current questions, and, for some of them, preliminary answers. 



1. Are the sun and its planets in the middle of our dis';'oidal galaxy? 

 They certainly are not. There are many lines of evidence which indi- 

 cate that the center is far away in the direction of the region where the 

 constellations of Sagittarius, Ophiuchus, and Scorpio come together, 

 30 degrees or a little more south of the celestial equator in the thick of 

 the bright star clouds along the Milky Way. My early study of the 

 globular star clusters (a reproduction of an important one, 47 Tucanae, 

 is shown in pi. 2) was instrumental in showing the observer that he 

 is well out toward the rim of the wheel -shaped galaxy. There may be 

 some "subcenters" in other parts of the ^^.lilky Way, in far south Carina, 

 for instance, and in Cygnus. But those conglomerations of stars ap- 

 pear to be important local structures within the great galaxy that has 

 its massive nucleus in the Sagittarius direction. 



