266 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 31 



collision with another atom. The atom is rushing about at the rate of 

 2 or 3 miles a second among other atoms in a star and frequently 

 collides with its neighbors. The violence of one of these collisions 

 may be just enough to raise the electron to a larger orbit. 



The second way in which an electron can be raised to a higher 

 orbit is by a collision with a dart of light. In the atmosphere of a 

 star great quantities of light are coming through from deep layers on 

 their way to the surface. As far as the atom is concerned this light 

 acts as if it were in the form of minute darts of energy, each with a 

 very definite rate of vibration and carrying a definite amount of 

 energy. Those vibrating rapidly have short wave lengths, carry a 

 large amount of energy, and appear blue to our eyes. Those vibrat- 

 ing more slowly have longer wave lengths, carry less energy, and 

 appear red. One of these light darts may strike an atom in its path, 

 and, if the atom can respond exactly to the rate of vibration of the 

 liglit dart, the energy which the light dart carries may raise one of 

 the electrons in the atom to a larger orbit farther from the nucleus. 

 Since the light dart is made of nothing but energy, and since it has 

 transferred all of this energy to the atom, it entirely disappears as 

 the result of striking the atom. 



Whether the atom was struck by another atom or whether it was 

 hit by a light dart, the result is the same, namely, an atom that is 

 wound up and has stored within it a definite amount of energy — 

 the power to do work of some kind. But this lasts only for an in- 

 stant. The electron must fall back. If undisturbed, the excited atom 

 hesitates for about a hundredth of a millionth of a second, and then 

 the electron falls to its normal place again. In doing so it unloads 

 the energy it borrowed by shooting off another light dart exactly like 

 the one which was absorbed. The only difference is that since the 

 atom has absolutely no idea of aiming, the light dart is sent out 

 entirely at random. 



There is another way in which the electron can fall back. While 

 the atom is wound up and hesitating, another atom may strike it 

 while traveling at a relatively moderate speed. If this happens the 

 atom may unload its stored energy directly to this second atom. The 

 colliding atom then bounces away faster than it struck, just as if it 

 had touched off a stick of dynamite, while the electron in the first 

 atom settles back into its normal orbit. 



We may now ask how atoms capable of passing through cycles 

 such as we have described are able to write code messages in stellar 

 spectra from which we may infer the conditions existing in the outer 

 parts of stars. To understand this we must first consider what hap- 

 pens when in the laboratory we artificially stir up an atom to produce 

 a spectral line. This may be done quite easily by means of an electric 



