THE OUTSIDE OF A STAR 
363 
) 
rather than others? For intense radiation pressure the atom or ion must 
have a principal line in the region of the spectrum in which the stellar 
radiation is strong; it can then absorb the radiation strongly. A sub 
ordinate line is no use because very few atoms are in a state to absorb it 
at any one moment. Further, the principal line must be a long way from 
the head of its series ; that is to say, the stellar radiation must be able to 
excite the atom but not to ionise it. For at the low densities concerned 
an atom which once lost its electron would have small chance of re 
capturing one; and as it would meanwhile be unable to absorb, it would 
fall back into the photosphere. These conditions are well fulfilled by the 
H and K lines of Ca + which correspond to excitation of the odd electron 
from its normal 4 X orbit to two 4 2 orbits (which differ only slightly from 
one another and need not here be discriminated). 
253. In the highest part of the chromosphere only the H and K lines 
have been observed so that it is a fair approximation to take the material 
as constituted wholly of Ca + with the necessary complement of free 
electrons ; it is also assumed that the only processes are the transitions from 
4 X to 4 2 orbits and back again*. Let n x be the number of atoms at any 
moment in the normal state and n 2 the number in the excited state ; then 
the ratio 7q/n 2 is given by Einstein’s equation (36-3) 
There is no thermodynamical equilibrium, and this equating of direct 
and reverse transitions is only permissible because of the special postulates 
above which exclude any other transitions. 
Now I (v 12 ) is not the black-body intensity for temperature T e \ it is 
modified in two ways. Consider for simplicity atoms at the top of the 
chromosphere. The radiation travelling in directions in the inward hemi 
sphere is missing, hence the intensity is reduced to ^ the full intensity. 
Further, v 12 being in the midst of an absorption line, we must multiply 
by the ratio r between the intensity in the line and the intensity just 
outside the line. It will be possible to determine r by photometric measure 
ments of the intensity in the H and K lines in the observed spectrum. The 
equation accordingly is 
Using the values of the atomic constants found in (38-25) and (38-4), we 
* Actually certain other transitions are bound to occur. The necessary modification 
of the theory is discussed by Milne in Monthly Notices, 86, p. 8. 
%2 n \ I (^12) - ^21 n 2 a 21 n 21 (^12) 
(253-1). 
obtain 
(253-3).