22 
SURVEY OF THE PROBLEM 
same as that of our atmosphere. Take a slab of gas there 6 inches thick. 
Of the radiation falling on one side of the slab, f will be absorbed in the 
slab and only ^ transmitted. This seems a very high opacity if we compare 
it with the transmission of light through gas; but the experimental 
physicist knows well the difficulty of getting the softer kinds of X rays to 
pass through a few centimetres of air. 
In Table 3 we give a few examples of laboratory determinations of the 
absorption coefficients of X rays to compare with the value 100 found for 
Capella. 
Table 3. 
Mass Absorption Coefficients for X Rays*. 
Wave-length 
Air 
A1 
Cu 
Ag 
0-5 
0-5 
2-0 
19 
12 
10 
3-2 
14 
140 
82 
1-5 
9-2 
46 
52 
260 
2-0 
19 
110 
120 
580 
3-0 
53 
360 
350 
— 
5-0 
190 
1600 
— 
— 
The stellar value is thus of the same general order of magnitude as the 
laboratory determinations. 
19 . A closer study of the table shows that the relation of stellar 
opacity to terrestrial opacity is not so straightforward as it at first appears. 
It will be seen that the terrestrial opacity increases very rapidly with the 
wave-length; subject to certain discontinuities (illustrated in the column 
for Cu) the opacity varies approximately as the cube of the wave-length. 
Now the mean wave-length for Capella is considerably higher than any 
of those given in Table 3, and it is clear that if we took the values for, say, 
7 A the absorption coefficients would be higher than 100. 
Moreover, the rapid change with wave-length suggests that the opacity 
of a star ought to change very rapidly with its internal temperature, since 
the mean wave-length of the radiation is inversely proportional to the 
temperature. This does not agree with astronomical observation; the 
whole series of giant stars from type M to type A though differing widely 
in internal temperature is found to show a fairly constant opacity. 
There is thus some difference between stellar conditions and laboratory 
conditions which affects the absorption of X rays. It is not difficult to 
understand what this difference is. The primary dissimilarity is the 
exceedingly high temperature of the stellar material; and it is a natural 
(though fallacious) assumption that temperature will not much affect 
a process like absorption, which is performed by the atoms individually 
* From M. Siegbahn, The Spectroscopy of X Rays, Appendix, Table III.