This gives a reasonably good snapshot picture of a star’s structure. The corresponding picture of its mechanism is obtained by thinking of the nuclei as α-ray particles, of the free electrons as β-ray particles, and of the radiation as γ-rays (although in most stars the main bulk of the radiation has the wave-length of X-rays). All these thread their way through the star, and, precisely as in laboratory work, the β-rays are more penetrating than the α-rays, and the γ-rays are more penetrating than either. THE TRANSPORT OF ENERGY IN A STAR. We have seen how the heat of a gas is merely the energy of its molecular motion. Conduction of heat in a gas is usually studied by regarding each molecule as a carrier of energy; when it collides with a second molecule the energy of the two colliding molecules is redistributed between them, and in this way heat is transported from hotter to cooler regions. Each molecule has a power of transport which is jointly proportional to its energy of motion, its speed of motion, and its “free-path”—the distance it travels between successive collisions. In the interior of a star, there are three distinct types of carrier in action—atoms (or bare nuclei), free electrons, and radiation. We can compare their relative capacities as carriers by multiplying up the energy, speeds and free-paths of each. For this purpose the “free-path” of radiation may be taken to be the distance the radiation travels before 37 per cent. of it has been absorbed, since it can be shewn that this is the average distance it carries its energy. On carrying out the calculation, the carrying capacity of both nuclei and electrons is found to be insignificant in comparison with that of the radiation. The nuclei and electrons may have the greater amount of energy to carry, but owing to their feebler penetrating powers, the distance over which they carry it, their free-path, is far less than that of the radiation. Their speed of transport is also less, since radiation transports its energy with the velocity of light. In this way it comes about that practically the whole transport of energy from the interior of a star to its surface is by the vehicle of radiation. This general principle was first clearly stated by Sampson in 1894. He also shewed how the temperature of any small fragment of a star’s interior must be determined by the condition that it receives just as much radiation as it emits, but his detailed applications were vitiated through his using an erroneous law of radiation. Twelve years later, Schwarzschild independently advanced the same idea, and expressed it in mathematical equations of “radiative equilibrium” which have formed the basis of every subsequent discussion of the problem.