Each layer in a star must further be checked for the conditions that determine whether the flow of energy is by radiation (photons) or by convection, the movement of hotter mass to cooler regions and cooler material into hotter regions. If a large temperature change occurs over a small distance in a star, then more energy is present than can be moved outward by the diffusion of photons. Thus, the matter must absorb the energy. It becomes hotter and expands, becoming lighter relative to its immediate surroundings. Lighter materials are buoyant and will rise, carrying energy in the form of heat or thermal energy outwards. Ultimately, when the matter has risen to a level where the temperature gradient is more gentle, radiation of light becomes effective in lowering the energy content of the material. The matter cools and contracts. Denser material loses its buoyancy and begins to sink to lower layers of the star. Transport of energy by convection is a very efficient means by which the star can move energy from the interior towards the surface where it can be radiated away.