THE COMPOSITION OF OUR UNIVERSE 



By Harrison Brown 



Institute for Nuclear Studies 

 The University of Chicago 



(With 1 plate] 



One of the more difficult fundamental problems which confront 

 science today is that of determining the chemical composition of the 

 matter of which our universe is made. Man, bound to the surface 

 of his planet, can see the billions of stars existing within the galaxy 

 of which his sun is a member, and the billions of galaxies extending 

 in all directions as far as his telescopes can penetrate; but he has 

 only the light that the stars emit with which to work. He knows 

 that a very large amount of matter is scattered throughout interstellar 

 space ; but he cannot sample it. He can see the other planets within 

 his solar system ; but he can study only the light that they reflect from 

 the sun. He is even prevented by the thick solid crust under his feet 

 from sampling the interior of his own planet. 



Nevertheless he has learned a great deal about the composition of 

 his universe from studies of what is available : Light from the stars 

 and planets, and the matter in the meteorites he finds and in the earth 

 at his feet. We can find significant regularities in the abundances of 

 elements on the surface of our earth. In 1917, W. D. Harkins made 

 the important discovery that elements of even atomic number are in 

 general more abundant than neighboring elements of odd atomic 

 number. But there were a number of exceptions to the rule and these 

 were attributed to the possibility that the surface of the earth is a 

 poor sample of cosmic matter. It was believed that if, in some manner, 

 a sample of the earth as a whole could be obtained, the exceptions 

 would be fewer in number. Soon thereafter many of the elements 

 were broken down into their component isotopes and it was found 

 that the rule could be more generally formulated : Nuclear species of 

 odd mass number are less abundant than neighboring nuclear species 

 of even mass number. 



Other generalizations could be made on the basis of the earth's 

 crust alone. It appeared that nuclear species of even atomic number 



• Reprinted by permission from Physics Today, vol. 3, No, 4, April 1950. 



197 



