The process by which an atom or molecule absorbs a photon with sufficient energy to remove an electron, creating a positively charged ion and a free electron. This process is crucial in determining the ionization state of gas in space.
Compound of Greek 'photos' (light) and 'ionization,' from Greek 'ion' (going) referring to moving particles. The term emerged in early 20th century physics as scientists understood how light could strip electrons from atoms.
Photoionization is like cosmic chemistry where starlight literally tears atoms apart! A single ultraviolet photon from a hot star can rip an electron away from a hydrogen atom millions of miles away, creating the glowing nebulae we see as stellar nurseries and planetary nebulae.
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