From the Inside Out

Take another look at Fig. 3.1. Chap. 3 was largely concerned with the material body shown in the diagram, while this chapter is largely concerned with the flow of radiation, the second critical component in Eddington’s two-part star model. Recall that the material body is described according to the interactions between the protons, electrons and ions that make up the material mass of a star. In contrast, the radiative body is described according to the passage of radiation and energy through the material body. Neither component can exist for long without the other, and as we have seen, the material body of a star would collapse on a timescale of hours if its interior was not hot and supported by a pressure gradient. The radiative component would dissipate even more rapidly if it did not interact with a star’s material body. Traveling at the speed of light, a photon would take just 2 s to cross the Sun’s radius, and yet for all its haste, the passage of radiation through a star is a tortuous affair, and the escape time is stretched out to many hundreds of thousands of years. As Arthur Eddington descriptively put it in his 1920 address to the British Association, ‘The star is like a sieve, which can retain them [photons] only temporarily; they are turned aside, scattered, absorbed for a moment, and flung out again in a new direction. An element of energy may thread the maze for hundreds of years [this is an underestimate; see below] before it attains the freedom of outer space’.