THE CONSTITUTION AND EVOLUTION OF THE STAES 



By Henry Noeeis Russell 



The preceding lectures have, I hope, presented to you a picture — 

 drawn rather in outline, owing to the limitations of time — of the 

 stars as they are known at present, their dimensions, masses, densi- 

 ties, surface brightness, and the like. It remains to speak of what 

 has been done to correlate these facts into a theory of the constitution 

 of the stars, and their probable evolution and age. 



What makes this problem tractable, in spite of the limitations im- 

 posed by the remoteness of the stars in space, and our ephemeral du- 

 ration in time, is that we have to deal only with the simpler and 

 more general properties of matter. The vast variety of the forms of 

 rock and mountain depends upon the solidity of their materials ; the 

 still greater diversity of the forms of organic life is based on the 

 presence of chemical compounds of great complexity — and neither 

 of these conditions can exist at all in bodies as hot as even the 

 coolest stars. In the stars all matter must be gaseous, and the laws 

 of gases are among the simplest known to physics. Add to them the 

 still more general laws that govern gravitation, radiation, and the 

 structure of atoms, and we have the controlling factors in the evo- 

 lution of the stars. 



Considering a star, then, as a mass of gas, isolated in space, we 

 notice first that it must be in internal equilibrium under its own 

 gravitation. The weight of the overlying layers produces a pres- 

 sure, increasing steadily from the surface to the center, which must 

 at any point be balanced by the expansive tendency of the gas, arising 

 from its high temperature. The temperature, too, is greatest at 

 the center, and decreases toward the surface. Hence, heat must flow 

 continually through the star's substance, down the temperature 

 gradient, till it escapes by radiation at the surface. The supply of 

 heat must be kept up in some way; and one obvious process, as 

 Helmholtz suggested long ago, is the slow contraction of the star. 

 The work done by the gravitational forces in pulling the outer parts 

 of the star toward the center reappears as heat produced by the 

 compression, and maintains the star as a going concern. As the 



1 Reprinted by permission from the Rice Institute Pamphlet, Vol. IX, No. 2, April, 1922 

 This is the last of a series of three lectures delivered by Doctor Russell at the Rice 

 Institute. 



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