136 
The Source of Stellar Energy [ch. iv 
We must suppose that one of the heavy atoms in a star’s interior first 
begins to change into radiation through one of its bound electrons falling 
into the nucleus, and coalescing with one of the nuclear protons so that both 
are annihilated. It is immaterial whether the whole atom changes into 
radiation at once or through a succession of comparatively slow changes. In 
either case the process of annihilation is likely to consist of a series of 
events in each of which a single proton and a single electron are annihilated 
simultaneously. 
As we have seen, the energy set free by the annihilation of a proton of 
mass M and an electron of mass m is (m +M)C 2 , which is equal to 0*0015 ergs. 
In accordance with general quantum principles each such annihilation must 
result in the production of a single quantum of radiant energy of frequency v 
given by 
hv — 0-0015 ergs, 
so that the frequency v is 2*3 x 10 23 and the wave-length is 1*3 x 10~ 13 cms. 
Each time a proton and electron are annihilated a splash of radiant energy 
of this wave-length and of total energy 0*0015 ergs is produced, and sets off 
to travel through the star until, after innumerable absorptions and re 
emissions, it reaches the star’s surface and wanders off into space. Except 
for being many thousands of times more powerful, each splash is similar to 
the splashes produced by radioactive material in the spinthariscope. The 
great energy of the splashes is to some extent counterbalanced by their rarity. 
In the sun, for instance, only about one atom in every 10 17 annihilates itself 
each hour. 
As this very high-frequency radiation travels through a star, it may be 
either scattered or absorbed when it meets an atom. Absorption can only be 
by complete quanta; the absorption of a quantum ejects an electron with a 
velocity representing kinetic energy of 0'0015 ergs, and so equal to 0*99999985 
times the velocity of light. When this electron strikes an atom a new quantum 
of radiation is emitted whose energy, and therefore also wave-length, is equal 
to that of the original radiation. The hardness of the radiation is thus 
unaffected by absorption and re-emission. The scattering of the radiation, on 
the other hand, is readily shewn to produce a softening of its quality, just as 
in the ordinary Compton effect, and a succession of such scatterings will 
increase the wave-length of the radiation until it becomes indistinguishable 
from ordinary temperature radiation. 
Newly generated radiation, in spite of its extreme hardness, will not 
penetrate far through the interior of a star without being changed in this way, 
so that we should expect the radiation emitted from the surface of a star to 
be ordinary temperature radiation, retaining no traces of its origin as radiation 
of extremely short wave-length. 
On the other hand, astronomical bodies exist which are transparent to 
ordinary light and so, à fortiori, must be transparent to this high-frequency