STELLAR LABOEATORIES DUNHAM 275 



atmosphere each second. This gives us a scale of miles on the plate, 

 and we can see just hoAv the solar atmosphere fades away in its upper 

 regions. It turns out that most of the ordinary absorption spectrum 

 is given by a layer not much more than 100 miles thick. But above 

 this is a vast outer atmosphere known as the chromosphere in which 

 the various elements reach up to different heights. Calcium and 

 hydrogen have been traced as high as 10,000 miles. It is probable 

 that it is the pressure of the intense light from the solar surface be- 

 low which supports these atoms, since the gas pressure at this level 

 would be entirely inadequate. A study of the chromosphere is par- 

 ticularly interesting because it is one of the few places in the uni- 

 verse where atoms can be examined while acting each one for itself, 

 without disturbance by its neighbors. The pressure is in fact so 

 very low that an atom probably travels thousands of miles without 

 striking another atom, whereas the particles of air under ordinary 

 conditions are experiencing thousands of collisions in every inch 

 they move. 



So far I have said nothing about the insides of stars. We have 

 been discussing only the merest outer skin, which is kept in its 

 present brilliant state by what is going on below. When we inquire 

 into the conditions within a star, the methods which are useful for 

 determining temperatures and pressures at the surface fail us and 

 we are thrown back on what Eddington calls an " analytical boring 

 machine." If we give the problem to a mathematician, telling him 

 that he may assume the material to act as a perfect gas and ask him 

 to apply the elementary principles of physics, he will come back 

 with a large part of the answer. 



It turns out that the temperature at the center of the sun is about 

 30,000,000° C, that the pressure is 200 million tons on every square 

 inch of area, and that the material is crowded until it has 28 times 

 the density of water. Under these conditions the atoms are scarcely 

 recognizable. They have had all but their last few electrons torn 

 away, and are rushing about with velocities which would carry them 

 from California to New York and back in a second if their directions 

 were not changed a million times in the interval. 



All this is interesting enough, but there is one thing still more 

 remarkable about the stars. They are sending into outer space tre- 

 mendous quantities of heat and light, and they have been doing it for 

 a long time in the past. Geologists tell us that the earth must have 

 been here for at least a thousand million years. But there are various 

 astronomical arguments which lead us to believe that the stars have 

 ages even a thousand times as great as this. 



No source of energy with which we are familiar could provide so 

 much heat for so long a time. Simple cooling would last only a short 



102992—32 19 



