Some Astronomical Aspects of Life in the 



Universe^ 



By Su-Shu Huang 



Institute for Advanced Study, Princeton, N.J.* 



[With 3 plates] 



Three different ways by which matter interacts are gravitational, 

 nuclear, and chemical. As our knowledge now stands, it appears that 

 the behavior of all matter in the universe — from shining stars to 

 exuberant life on the earth — may eventually be explained in terms of 

 these interactions. Indeed, the emergence of life in general, and on 

 earth in particular, is a net result of all three. 



All forms of life must rely for maintenance on a stellar source of 

 energy. Therefore, the nature and evolution of a star control the 

 emergence and development of life. There is no doubt now that 

 stars condense from gas and dust in the interstellar clouds, a newly 

 formed star's temperature being very low because the interstellar gas 

 is quite cool. As the star contracts, its temperature increases, and it 

 moves from the lower right-hand comer of the Hertzsprung-Russell 

 diagram toward the left side. Figure 1 shows the early evolutionary 

 tracks of stars of different masses, which can be roughly represented 

 by straight lines.^ 



Gravitational contraction stops when the internal temperature be- 

 comes high enough for thermonuclear reactions to begin to convert 

 hydrogen into helium. These reactions supply energy equal to that 

 radiated by the star, which therefore maintains an equilibrium con- 

 dition with constant luminosity for a long time. Such a state of affairs 

 corresponds to a star on the main sequence. 



The time of contraction to the main sequence depends on the mass, 

 as shown in table 1. The time scales given here are longer than usu- 



1 Reprinted by permission from Sky and Telescope, vol. 21, No. 6, June 1961, 

 ' On leave from Goddard Space Flight Center, National Aeronautics and Space Admin- 

 istration. 



* For a detailed description of a star of solar mass contracting to the main sequence, see 

 "Early Solar Evolution," Robert R. Brownlee and Arthur N. Cox, Slcy and Telescope, 

 May 1961, p. 252.— Ed. 



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