Defining an exact surface for an object such as the Sun is made difficult by the diffuse nature of matter which constitutes the Sun at distances far from the stellar core. An often used definition for the surface of a star is based on the path that photons take. Inside a star, photons travel by random walk, constantly interacting with matter, and the surface of the star is defined as the point at which photons encounter little resistance from the matter in the stellar atmosphere, or in other words, when photons stream freely.