CHAPTER IV 
THE SOURCE OF STELLAR ENERGY 
Inadequacy of Terrestrially-known Sources. 
99. We have seen that each square centimetre of the sun’s surface emits 
sufficient energy to drive an eight-horse-power engine continuously; the output 
from each square centimetre of an 0 or B type star, such as Plaskett’s star or 
V Puppis, which is at least 200 times as great, is sufficient to drive an express 
locomotive at full speed year after year and century after century for millions 
of years. Since the full implications of the doctrine of conservation of energy 
have been understood, efforts have been made to discover the origin of the 
energy which is poured out with such terrific profusion by the sun and stars. 
A priori there are two general possibilities open. Either the stream of 
energy liberated from a star’s surface may be continually fed to the star from 
outside, or it may be generated in the star’s interior, and driven out through 
its surface, as the only means of preventing an intolerable heating of the 
interior. An illustration of the former mode of liberation of energy is provided 
by a meteorite falling through the earth’s atmosphere, the energy of its 
radiation being provided by the impact of molecules of air on its surface; an 
illustration of the latter is provided by an ordinary coal fire. 
The only serious effort to explain the sun’s energy as being supplied from 
outside was that of Robert Mayer, who conceived solar energy as arising 
from a continuous fall of meteors into the solar atmosphere. But simple 
calculations shew that a mass of meteors equal in weight to the earth would 
barely suffice to maintain the sun’s radiation for a century, and that meteors 
sufficient to maintain the sun’s radiation for only 30 million years would 
double its mass. As it is quite impossible to admit that the sun’s mass can 
be increasing at such a rate, Mayer’s explanation has to be abandoned. As no 
other way can be imagined by which energy of comparable amount can be 
brought in from outside, we are driven to regard the sun’s generation of heat 
as taking place throughout its body. 
The essential datum of the problem is no longer the energy discharged by 
a square centimetre of a star’s surface, but the energy generated per gramme 
of its mass. As we have seen, the sun generates about 190 ergs per second 
for each gramme of its mass, while the corresponding figure for V Puppis is 
as high as 1100 ergs, and that for Pearce’s star is probably of the order of 
15,000. 
100. Let us fix our attention on the special case of the sun, which is more 
typical of general stellar radiation than the two extreme cases just mentioned.