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CHAPTER V 
RADIATIVE EQUILIBRIUM 
68 . Energy in the form of radiant heat and light is continually flowing 
from the surface of a star into space. The surface layers of material cannot 
continue to provide this energy for long unless their heat is replenished 
from below. We are thus led to consider the process of transfer of energy 
from the interior to the surface. 
There are two modes of transfer of heat in material in static equilibrium, 
viz. conduction and radiation. In both the net flow is in the direction of 
the temperature gradient from high to low temperature. In both this flow 
is the resultant of streams of energy in both directions ; the stream from 
the high-temperature region is rather more intense than the stream from 
the low-temperature region, and the difference constitutes the net flow. 
In conduction molecules of the hotter region transmit their energy by 
diffusion and collision to surrounding regions ; in radiation the hot material 
emits aether waves which are absorbed in the surrounding regions. In 
both cases this transmission is largely neutralised by a similar trans 
mission from the surrounding regions, and the resultant transfer depends 
on the slight preponderance of the flow from the hotter region. 
A third mode of transfer is possible if the limitation to static equi 
librium is abandoned. There may be a system of ascending and descending 
currents in the star by which the material is kept stirred. Heat-energy 
is then carried from one region to another by actual movement of the 
matter carrying it—as in the lower part of our own atmosphere. If matter 
through proximity to the surface loses more heat than can be replaced 
by radiation and conduction so that it cools below the normal temperature 
at its level, it will sink and be replaced by fresh unexposed material from 
below. This mode of transfer is called convection. 
It was recognised early that the conductivity of matter is much too 
small to pass the necessary quantity of heat through the star. In the first 
investigations of the stellar interior the importance of transfer by radiation 
was not realised. Accordingly a system of convection currents was as 
sumed to be present, and processes of transfer other than convection were 
considered negligible. The star was said to be in convective (or adiabatic) 
equilibrium. 69 
69. On the hypothesis of convective equilibrium a definite relation 
between the pressure, density and temperature at different levels can be 
found. Consider an ascending current of material. By hypothesis there