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 transmission 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 equilibrium 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.