420 
Conclusion 
[CH. XVII 
radioactive atoms to non-radioactive atoms is not only one of decreasing atomic 
number and decreasing complexity of structure; it is also one of decreasing 
capacity for liberating energy, the stellar atoms having far more capacity for 
liberating energy than radium or uranium, whereas of course the ordinary 
terrestrial atom has much less. 
The foregoing statements refer to a sort of average atom of which the star 
may be supposed to be composed. An actual star must be a mixture of 
a great number of kinds of atoms, including terrestrial atoms. These latter 
atoms are probably of little importance in the dynamics and physics of the 
star as a whole, but they have the special importance that, being the 
lightest atoms in the star, they float up to its surface and so determine its 
spectrum. 
Terrestrial chemistry, which deals only with these atoms, may properly be 
described as “surface-chemistry”; it must merge into a wider chemistry on 
passing inside a star. So also terrestrial physics is a mere “surface-physics.” 
Inside a star we are confronted at once with what appears to be the funda 
mental physical process of the universe, the wholesale transformation of matter 
into radiation; and of this terrestrial physics knows nothing. Again, the 
greater part of the matter of the universe exists in a state of high dissociation 
of which surface-physics has neither knowledge nor experience. Clearly our 
physics and chemistry are mere fragments of wider-reaching sciences. 
391 . In a quite different sense biology is a surface-science, since biology 
becomes meaningless in the interior of a star. Life implies duration in time, 
and there can be no life where atoms change their make-up millions of times 
per second. Life further implies mobility in space, and this restricts it to 
those small parts of the universe where the physical conditions, temperature in 
particular, permit of the existence of matter in the liquid state. This does not 
merely rule out the interior of a star; it rules out the whole star. Every known 
star pours out such an intense torrent of radiation as to make life quite im 
possible on its surface. The only possible opportunities for the existence of 
life would appear to be on a planet some distance from the star’s surface, this 
planet consisting necessarily only of “permanent” matter torn originally from 
the star’s surface and transplanted to a safe, but not too great, distance from 
its hot surface. Thus biological science may be described as planetary science, 
since nowhere else in the universe would it seem to have any meaning. 
On rare occasions only does one star pass so near to another that planets 
can be torn out and left to solidify in space, to form the cool ash on which 
alone life can exist. At a rough computation, about one star in a million may 
be surrounded by planets, but probably only a small fraction even of these 
planets are possible abodes of life. Thus life is perforce limited to an amazingly 
small corner of the universe; whether it even exists where it can we have no 
means of knowing.