Airglow (or nightglow) is the glow of a planet's atmosphere. Earth has a faint airglow, from photoionization, from chemical reactions, and from the effects of cosmic rays. Because of this, none of the night sky is totally dark, even when starlight and the effects of the Sun on the interplanetary medium are taken into account. The auroras are some of the same thing, but the term aurora refers to a frequent, relatively bright glow near the poles whereas the term airglow refers to the effects that take place throughout the night sky. Airglow affects and limits observation from the ground, giving space telescopes an advantage, and motivating optics to mitigate the effects. OH airglow is emission from hydroxyl in the atmosphere.