The structure of a star will also be affected by how easily photons can pass through the material in any given layer. If a layer absorbs photons, then it will be heated with an increase in pressure that will expand the layer. Alternatively, if a layer is transparent and allows photons to readily escape, then that layer will be cooler with a lower gas pressure that will allow gravity to compress its material. Astronomers actually use the reciprocal of transparency, or opacity, to measure the ability of photons to pass through material. A low opacity means high transparency; a high opacity means a low transparency. The opacity of the material in a given layer depends on the chemical composition (some elements absorb light more easily than other elements), the temperature, and the density.