opacity

Hold a hand in front of a flashlight and your skin glows red — some light fights through, most is blocked. Opacity is just a precise way of saying how hard it is for light to push through a material. Frosted glass has high opacity; clear air has low opacity. Inside a star, opacity measures how readily the hot gas absorbs and scatters the radiation trying to flow outward. It is the resistance the photon fights against on its long, stumbling crawl from the core to the surface.

Opacity inside a star comes from several effects working at once: free electrons batting photons aside, partly stripped atoms snatching photons to kick their electrons up an energy level or knock them loose entirely, and ions briefly grabbing passing light. How much each contributes depends sharply on the gas's temperature, density, and chemistry — especially its content of heavier elements, which are far better at blocking light than plain hydrogen and helium. Because of this, opacity is not a single number but a complicated function that astronomers compute in giant tables.

Opacity is one of the quiet master variables of a star. A high opacity dams up the radiation, forcing the temperature to fall more steeply with height; if it gets steep enough, the layer gives up on radiation and starts to convect. Opacity therefore helps decide which parts of a star are radiative and which are convective, how big and bright the star is, and how fast energy escapes. It is also one of the trickiest ingredients to get right, and revising opacity tables has, more than once, fixed stubborn mismatches between stellar models and observations.