COMPOSITION OF THE SUN KUSSELL 205 



atom, are equally easy to produce. The atom may exist in a multi- 

 tude of different states, each with its own store of internal energy. 

 We may picture the atom, if we will, by imagining the electrons 

 revolving about the nucleus in orbits, and the different states by sup- 

 posing that some of the outer orbits differ in size, shape, and incli- 

 nation to one another. (This picture, though imperfect, is as good 

 as the familiar one of "rays" of light.) Whatever picture (or 

 mathematical formula) we adopt for the energy states, there is no 

 doubt of their existence. 



An atom which changes from a state heavily loaded with energy 

 to one less powerfully " excited " must release its energy, and it does 

 so by giving out light. The number of vibrations per second in this 

 radiation — of kilocycles, to use a word now familiar to all radio 

 listeners — is exactly proportional to the amount of energj^ released. 

 No one knows why — if we did, we would be a good deal nearer to 

 understanding the nature of things than most of us ever hope to be — 

 but the fact is again firmly established. 



Knowing this, it is possible, by a study of the spectra, to map out 

 with very great accuracy the various energy states of a given atom 

 and the transitions between them which cause the emission, or, when 

 reversed, the absorption of a spectral line. 



The properties of these atomic states or spectroscopic terms, as 

 they are also called, are governed by a complicated and precise sys- 

 tem of rules, which are now fully understood. The work of Som- 

 merfeld, Lande, Pauli, Hund, and others has led to a theory which 

 interprets, and, what is more, predicts, the structure of even the 

 most complicated spectra, and enables us to specify exactly what 

 occurs when each one of the hundreds or even thousands of lines is 

 emitted or absorbed. 



An atom, left to itself, will settle down into its normal state, with 

 the smallest possible content of energy. From this state it may pass 

 by absorption of light into any one of a number of others, producing 

 certain spectral lines. Each atom is doing only one thing at a time ; 

 but among the billions of atoms in the smallest perceptible quantity 

 of gas, all these transitions are happening at once. Those which are 

 most likely to occur give the strongest lines. 



An atom in an exited state of higher energy content absorbs a 

 number of other lines, quite different from the first, and so on for 

 each of the numerous exited states, till the whole complicated spec- 

 trum is built up. 



Now the relative numbers of atoms in the various energy states 

 depend on the temperature of the gas. In a cool gas practically 

 all of them are in the normal state ; but, with rising temperature, an 

 ever-increasing though usually small proportion will be found at any 

 moment in each of the exited states. 



