386
DIFFUSE MATTER IN SPACE
262. Continuous absorption (as distinct from scattering) is produced
by the two causes studied in connection with the interior of a star.
(a) Ionisation of atoms.
(b) Switches of electron orbits at encounter with atoms.
To these we may possibly have to add a third cause not operative in the
stellar interior.
(c) Dissociation of molecules into atoms.
(a) Absorption by ionisation affects only radiation of frequency above
that corresponding to the ionisation potential. For absorption of yellow
light which contributes most to visual brightness the ionisation potential
would have to be as low as 2-2 volts. This is below any known ionisation
potential. An excited atom can be ionised by light of lower frequency but
the proportion of excited atoms must be extremely small.
( b) This must be extremely small owing to the rareness of encounters.
(c) It is unlikely that combination of atoms into molecules occurs in
interstellar space, because the atoms are ionised and their positive charges
tend to keep them apart when they meet; their chemical attraction is
given no chance. We may, however, provisionally examine what happens
if the combination is possible. The energy-density in space (10~ 12 ) corre
sponds to about 1 visual quantum in 3 cu. cm., or roughly 1 quantum
per molecule (or potential molecule) for our estimate of maximum
material density. By a synchronised effort the molecules could just
extinguish all the starlight lying about; after this they could do nothing
for 40 years—the time until the atoms suffered their next encounter and
had a chance of recombining. Extinction once in 40 years spread over
a light track means that a star distant 13 parsecs (40 light years) would
be dimmed in the ratio 1/e or 1 magnitude. This is a quite serious absorption.
But we have been dealing with extreme upper limits and cannot really
expect anything like so high an efficiency. It is fairly safe to conclude
that molecular absorption, if it occurs, is not great enough to produce
appreciable effects.
As regards obstruction by meteoric matter we have no evidence to
guide us. The meteoric matter encountered by the earth is generally
supposed to have originated in the solar system and there is no reason to
think that anything of the kind exists in interstellar space. We have the
impression (perhaps not too well founded) that the primordial state of
matter is gaseous, and that meteors or meteor dust must be the débris
of some former aggregation of matter. Obstruction by meteor dust is
rather more economical of mass than other forms of absorption or scatter
ing—a consideration often of importance ; and although we do not favour
the hypothesis of absorption from this cause it is not to be set aside alto
gether.