THE STAKS IN ACTION JOY 189 



surface as a disk of considerable size, and with the aid of the tele- 

 scope we can examine the changes which are going on there. Sun 

 spots are indicators of the presence of disturbances low down in the 

 atmosphere of the sun. They are usually accompanied by faculse 

 which are areas of intense activity and high temperature. Even 

 the fine structure of the solar disk is not at rest but is continually 

 changing in the attempt to adjust itself to conditions within. The 

 prominences projecting from the solar atmosphere are perhaps the 

 most striking of all the phenomena of astronomy. At times they ai"e 

 thrown up to immense heights, as great as 500,000 miles, at speeds of 

 200 miles a second or more. Their rapid changes are most fascinat- 

 ing to the spectator, and their beautiful forms as seen in the deep 

 red of hydrogen Ha are beyond description. With the spectro-helio- 

 scope recently devised by Doctor Hale, they can be splendidly seen 

 even with a small telescope. 



When w^e classify the stars, we find some hot, some relatively cool ; 

 some large, some small; and some light, some heavy. It is natural 

 for us to ask the question as to what is the life history followed by 

 stars in the course of their existence. The answer is not as definite 

 as we might wish, because the process of change is too slow to be ob- 

 served in the few years of man's history. It seems most likely that 

 the solution will be found in connection wdth the density and mass 

 sequence which has been observed among the stars. Apparently 

 stars, in general, begin their visible careers under conditions of low 

 density and great mass, as w^ell as low temperature. As they con- 

 tract their density increases and they become hotter, until a maxi- 

 mum is reached. Then they gradually become cooler again while 

 continuing to decrease in size. Mass is lost throughout in the form 

 of radiation. It may be that no one star follows the course outlined 

 completely, but doubtless that is the direction of stellar progress. 

 The actual process taking place in a star depends largely on the 

 state of the atoms composing it. Very different behavior may be 

 exi^ected, depending on the extent to which the atoms are knocked 

 to pieces and their rings of electrons removed. 



We have not exhausted the story of the activities of the stars when 

 we have considered merely their movements in space, their light 

 variations, and the various disturbances which can be observed in 

 their atmospheres. Recent advances in the interpretation of the be- 

 havior of the atom have been applied to the conditions within the 

 stars. Eddington, by a process of masterly mathematical analysis, 

 has given us a picture of the atomic activities in the interior of stars. 

 He bases his conchisions on the known physical behavior of the atom 

 in the laboratory and tests them out in the high temperatures oi 

 the stars. It is true that we can see down into the stars only a feN\ 



