THE COEFFICIENT OF OPACITY
217
147. There can be little doubt that the principal process of absorption
in the stars is the photoelectric effect. A quantum of radiation is absorbed
and its energy is employed in removing an electron from the atom and
endowing it with kinetic energy. Other processes of absorption are known,
but these contribute relatively little to the stellar opacity. Ionisation
reduces the absorption because it leaves fewer electrons capable of per
forming the photoelectric process. The decrease may also be described—
but perhaps less accurately—as a saturation effect; the atoms cannot deal
efficiently with a very large quantity of radiation which removes their
electrons faster than they can be replaced. The ionisation is the sign of
overwork. We said less accurately because ionisation can also be caused
by collisions of atoms and electrons, so that in certain circumstances the
decrease of the coefficient might occur without implying great intensity
of the radiation; but, on the other hand, the principle of detailed balancing
in thermodynamical equilibrium permits us to deal with the photoelectric
effect and its converse as if these were the only processes occurring in the
stellar interior, so that the small absorption coefficient in the stars is
directly attributable to the intensity of the radiation.
The practical effect of ionisation on the absorption coefficient will be
seen from the following numerical results which have been calculated for
iron at the temperature and density of the centre of Capella*. If X rays
of similar wave-length but of ordinary laboratory intensity were directed
on iron in a terrestrial experiment they would be very strongly absorbed.
The absorption is mainly performed by the L group of 8 electrons. As
each quantum is absorbed, an L electron is expelled; but in terrestrial
experiments the L group is completed again by the falling in of an outer
electron or by capture of a wandering electron before the atom’s turn for
another absorption. The coefficient of absorption by the L electrons alone
is found experimentally to be 2950 c.G.s. units. Now let the intensity of
the X rays be raised to the actual strength in Capella. The L electrons will
be wrenched away almost immediately they take their places so that they
are not usually present in the atom. At any moment only 1 out of 1200
places for L electrons is filled; that is to say, instead of each atom having
eight L electrons, only 1 atom in 150 has even a solitary L electron. The
L absorption coefficient has therefore only J 200 if s laboratory value
and is reduced to 2-5. Actually more absorption in Capella is performed
by the two K electrons. In the laboratory these do relatively little work,
because most of the radiation has a frequency too low to operate the
K mechanism; the laboratory absorption coefficient is 8-3. But in
Capella about 70 per cent, of the K electrons are in their places at any
* Monthly Notices, 84, p. 113. Corresponding results for other elements are also
given.