STELLAR LABORATOEIES DUNHAM 



265 



our purposes it will be entirely satisfactory to think of the simpler 

 and older model with electrons revolving around a central nucleus. 



To make things simpler still, we shall consider only one kind of 

 atom, the calcium atom, because it illustrates so well some of the 

 things I wish to describe. A calcium atom on the model we are using 

 consists of a central positive nucleus which holds by electrostatic at- 

 traction 20 negative electrons. Eighteen of these revolve about the 

 nucleus in orbits relatively close to the nucleus and do not take any 

 part in giving out the light which we see coming from the stars. 



♦ 



*" 



Figure 1. — Atomic transformations 

 A, a rapidly moving atom colliding witii a normal atom. B, after the collision the 

 first atom recedes more slowly than It approached, having transferred part of its 

 energy to the second atom which is now in an excited state, i. e., one of its electrons 

 now occupies a larger orbit. C, a light dart colliding with a normal atom able to 

 absorb the exact amount of energy carried by the light dart. D, an excited atom 

 which has stored the energy resulting from a collision either with another atom or 

 with a liglit dart. E, the electron which occupied an excited orbit has fallen back 

 to its normal position, and the energy which It had stored at D now appears in the 

 form of a light dart traveling in a random direction. F, a slowly moving atom 

 colliding with an excited atom. O, after the collision, the first atom Is recoiling 

 more rapidly than it approached, having talcen up energy from the excited atom 

 which is left in its normal state. An atom may become excited in two ways and an 

 excited atom may unload its energy in two ways. Accordingly, atomic transforma- 

 t''"^- may follow any one of four cycles ; 



A-.B->D-»r-G 



A-.B-»D-.E 



C-D-*E 



C-.D-»F-»Q 



In Figure 1 the nucleus and these 18 inner electrons are all repre- 

 sented by a large dot. But the two outer electrons are very im- 

 portant. These are shown as smaller dots. 



Normally these two outer electrons circulate close to the nucleus. 

 But they may also circulate in a number of larger orbits, of definite 

 sizes. Intermediate positions do not occur. Now it takes energy or 

 work to lift an electron from a smaller orbit to one of these larger 

 or " excited " orbits, and this lifting of the electron can be done in 

 either of the two ways illustrated in Figure 1- The first is by a 



