112
RADIATIVE EQUILIBRIUM
At 10 million degrees the frequency RT/h corresponds to wave-length
14-3 A, so that 69 per cent, of the weight is between 6 and 2 A. For
elements of moderate atomic weight this corresponds to L radiation. It
is this region that we must search particularly for sources of stellar
opacity.
When dealing with sources of continuous absorption which operate
chiefly in the above stretch of spectrum, we shall often drop the distinction
between absorption and opacity for approximate treatment. The correc
tion necessary to reduce k x to k 2 will be referred to as Rosseland’s correction.
Table 8.
Weights for Calculating Opacity. 80
x — hv/BT
Weight at x
Weight 0 to a;
0
•000
•0000
i
•244
•0016
1
•921
•0121
H
1-872
•038
2
2-897
•084
2i
3-806
•150
3
4-467
•230
4
4-864
•413
5
4-270
•591
6
3-229
•736
7
2-194
•840
8
1-376
•908
9
•810
•949
10
•454
•973
oo
•000
1-000
80. Consider two frequencies v' and v" , the first in a strong absorption
line or band and the second in a region of little absorption. If we were
considering the outer atmosphere of a star the absorption would cause the
radiation to be lacking in the constituent v . But it is not so in the interior
of the star, where thermodynamic equilibrium is nearly perfect. Strong
absorption is compensated by strong emission, and v' and v" are present
in the ordinary proportions indicated by Planck’s Law. The difference is
that whereas the v" radiation is on the whole flowing outwards the v'
radiation is practically brought to a standstill.
If, for example, v" is on the average absorbed and re-emitted once in
100 cm. path and v once in 1 cm. path, the difference of intensity of the
outward and inward streams of v" corresponds to the difference of tempera
ture of the points from which these streams were emitted, viz. the tempera
ture drop in 200 cm. The difference of intensity of the outward and inward