Optical depth in astrophysics refers to a specific level of transparency. Optical depth and actual depth, and respectively, can vary widely depending on the absorptivity of the astrophysical environment. Indeed, is able to show the relationship between these two quantities and can lead to a greater understanding of the structure inside a star. Optical depth is a measure of the extinction coefficient or absorptivity up to a specific 'depth' of a star's makeup. In some cases the Beer–Lambert law can be useful in finding .