28 HUMAN BIOLOGY 



The Sun has certainly been shining, with nearly its 

 present brightness, throughout geological time. During 

 this interval it must have dissipated into space an amount 

 of energy vastly exceeding the whole initial store of 

 potential energy of all kinds (gravitational, chemical, etc.) 

 which can be attributed to it in virtue of known properties 

 of matter. 



All investigators are therefore now agreed that some 

 otherwise unknown, and enormously great, source of energy 

 exists in the interior of the Sun and of the stars, where 

 it is being gradually transformed into heat and radiated 

 from their surfaces. It is agreed, too, that this energy 

 must come in some way from the disintegration of atoms, 

 though the details of the process are still in debate. According 

 to the theory of relativity, all energy possesses mass, and 

 the Sun cannot radiate heat, i.e. energy, without diminishing 

 in mass. To a single pound of mass corresponds heat enough 

 to raise 20,000,000 tons of rock to a temperatuie of 2000°c. 

 and convert it into incandescent lava; yet, measured in 

 this way, the Sun's total radiation corresponds to a loss 

 of mass of 4,600,000 tons per second. This is a truly startling 

 figure, and might raise alarm concerning the future, were 

 it not that calculation shows that if consumed steadily 

 at this rate, the Sun would last for 15,000,000,000,000 years. 



To what degree the Sun's actual hfe as a luminary 

 approaches this figure depends upon the as yet unanswerable 

 question how great a portion of its mass is capable of trans- 

 formation into energy. It appears probable, in view of 

 certain properties of double stars, to be discussed below, 

 that a large part of the mass is transformable, and that 

 the fife of a star is thousands of billions of years in length. 

 It is well established, both by Eddington's theory and by 

 observation, that the more massive stars are the brighter. 

 Ages ago, when the sun was more massive, it was doubtless 

 brighter and hotter; long hence, when it has lost mass 

 perceptibly, it will be fainter. It should, so far as can at 

 present be estimated, continue to supply light and heat 

 enough to maintain life on the Earth, at its present distance, 

 for tens or even hundreds of billions of years. The past 

 duration of terrestrial life appears theiefore to be but a 



