STELLAR LABORATORIES DUNHAM 



269 



We are now ready to look at the actual situation in a stellar atmos- 

 phere. Figure 4 represents a thin slice through the middle of the 

 atmospheres of three different stars. The center one represents the 

 atmosphere of our sun. Light and heat are coming up in great 

 amounts from the depths of the sun below the bottom of the diagram. 





c 



livAW-* 





11 



\ 





C<' / ^-^'^ \ 





n<^ 



• 





STAR SLOWER PRESSURE. 



NO MORE EXCITED ATOMS 

 MOEE IONIZED ATOMS 

 MORE FREE ELECTRONS 

 SAME TYPE OF RADIATION 



SOLAR ATMOSPHERE 



V B R 



N 



IONIZED NORMW. rYriTrn 



ATOM 



9 ♦ 



ATOM ATOM 



STAR •• HIGHER TEMPERATURE 



MORE EXCITED ATOMS 

 MORE IONIZED ATOMS 

 MORE FREE ELECTRONS 

 RADIATION OF SHORTER WAVE LENGTH 



FiGDRB 4. — Atomic processes in stellar atmospheres 

 The diagram shows idealized cross sections of three stellar atmospheres to illustrate 

 the influence of temperature and pressure on the relative numbers of normal, excited, 

 and ionized calcium atoms and the resulting differences in the spectra of the light 

 which has come to us after passing through these atmospheres. The spectrum at 

 the lower part of the diagram shows the lines characteristic of each of the three 

 types of calcium atoms. Differences in the relative Intensities of these lines in the 

 spectra of various stars serve as measures of the numbers of atoms of each type 

 which are present in the atmospheres of these stars, and so make it possible to infer 

 the temperatures and pressures at the surfaces of the stars. The spectra in this 

 diagram are in the form of photographic negatives so that dark absorption lines 

 appear as white lines on a dark background. A, a normal atom ; B, an excited 

 atom ; C, an ionized atom ; D. an excited atom ; E, a doubly ionized atom, which has 

 lost two electrons ; F, a free electron ; O, a light dart colliding with a normal atom 

 and exciting one of its electrons ; Jf, two atoms which have just collided ; one of 

 them is left excited ; I, an excited atom which has returned to its normal condition, 

 emitting a light dart in a random direction ; J, an excited ionized atom which is 

 exciting another ionized atom by collision ; K, a light dart exciting an ionized atom. 



After working their way between the atoms in the atmosphere they 

 pass on beyond the top of the diagram, through the upper limits of 

 the atmosphere, and plunge off into outer space. 



At the high temperature of the solar atmosphere, all the atoms are 

 moving about quite rapidly and colliding frequently with one an- 



