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.