14 The Evolution of the Universe 



temperature reaches about five million degrees, a series of nuclear 

 reactions occur which convert hydrogen to helium with the produc- 

 tion of large amounts of energy. This will heat the gases to a white 

 luminosity. Thus the "cold jewels" we see in the sky are, in reality, 

 extremely hot incandescent stars. Their heat radiates into space and 

 is thus dissipated. When sufficient hydrogen is converted to helium, 

 the star becomes unstable and contracts until its core may reach 

 a temperature of 100 million degrees or more. Under these condi- 

 tions, another set of nuclear reactions occur and helium is converted 

 to carbon. When the amount of helium is sufficiently reduced, the 

 star again becomes unstable and contracts still further. As higher 

 core temperatures are reached, new nuclear reactions occur and 

 heavier elements up to iron and its relatives are produced. Finally, 

 when a star has exhausted all its fuel, it may explode or become 

 what is called a white dwarf, or both. White dwarfs are unstable, if 

 large, and will break up and lose matter until they reach the small 

 size necessary for stability. In this fashion, the star begins as a ball 

 of hydrogen aggregated from the gas cloud of the galaxy and ends 

 as a small remnant composed chiefly of heavier elements. In this 

 automatic, evolutionary process, a large proportion of the star's 

 total matter is returned to the gas cloud of the galaxy but in a more 

 complex chemical form than at the beginning. 



Not all the stars in a galaxy form simultaneously. Star formation 

 goes on continuously. Later generation stars have, in their original 

 makeup, not only hydrogen but also some of the elements formed 

 in earlier, now-exploded stars. Under these conditions, the nuclear 

 reactions in the star may make different elements. Furthermore, 

 stars may follow different evolutionary paths, each producing a 

 different complement of elements during the life of the star. Ac- 

 cording to Fowler and the Burbidges, enough such element-making 

 processes are known, or theoretically possible, to account for the 

 formation of all the elements. From this they deduce that our sun, 

 possessing all of the elements possible at its temperatures, is a 

 third generation star. 



THE SOLAR SYSTEM 



The sun, composed chiefly of hydrogen and helium, is quite an 

 ordinary star compared with others in the universe. Hoyle (1950) 

 recognized that the earth and other planets of the solar system 

 are "freaks" in the astronomical sense because they contain a high 

 proportion of heavier elements such as iron and nickel. Many 



