PHYSICAL BACKGROUND OF EVOLUTION 19 



Richard Tolman overcomes the difficulty of heat death 

 by using the Lemaitre assertion that the repulsion of the 

 original explosion is opposed by the attraction of the group- 

 ing of matter in space, and concludes that the cosmos will 

 expand outward only until the attraction and repulsion 

 forces balance. Then, he thinks, the universe must fall back 

 in upon itself with increasing speed until another superatom 

 is formed— a cosmos expanding and contracting through 

 eternity. 



x\nother cosmos that is spectacularly self-sufficient and 

 endless has recently been proposed by Fred Hoyle and R. 

 A. Lyttle. They, too, were impressed by the new knowl- 

 edge that the greater part of the matter in the universe is 

 outside the stars. In their cosmos, space is filled with very 

 thin hydrogen which is gravitationally unstable and grad- 

 ually forms clouds that drift for billions of years, eventually 

 massing into nascent galaxies. They think that clots form 

 within the great gas masses, clots which pack denser and 

 denser under gravitational forces until atomic fires produce 

 the suns. Once formed, these suns undergo a varied devel- 

 opment, depending on how much gaseous cloud material is 

 left in the system. If a sun passes through great masses of 

 gas, it gathers, as Lyttle's mechanics shows, more and more 

 material which forces it to bum its hydrogen at a spend- 

 thrift rate. These are the steel blue giants of the heavens 

 burning out (turning their hydrogen into helium) in less 

 than a billion years. With no hydrogen left, such stars begin 

 to contract, spinning faster and faster and getting hotter 

 and hotter until fantastic temperatures are reached. When 

 free neutrons appear, there is a sudden formation of heavy 

 elements (iron, uranium, etc.). These nuclear reactions ab- 

 sorb energy and the star collapses, releasing so much gravi- 

 tational energy that the outer layers fly off in a tremendous 

 explosion, a stellar flare-up which astronomers call a super- 

 nova. After the great explosion a white dwarf remains— dim, 

 dense, and burned out. 



