OIII

O III denotes doubly ionized oxygen, the chemical species O2+. In spectroscopic notation, the roman numeral indicates the ionization stage: I is neutral, II is singly ionized, III is doubly ionized. O III is commonly studied in astronomy because its emission lines are strong in many gaseous nebulae and star-forming regions.

The most prominent features associated with O III are the forbidden emission lines at 4959 and 5007 angstroms, often among the brightest lines in optical spectra of ionized gas. A weaker auroral line at 4363 angstroms is also observed. These lines originate from collisionally excited transitions within the oxygen ion in low-density environments, where metastable states emit photons as electrons drop to lower energy levels.

O III lines are powerful diagnostic tools. The ratio involving the auroral 4363 line relative to the 4959 and 5007 lines is sensitive to electron temperature, while other line ratios provide information on electron density. Together, these diagnostics allow estimates of the gas metallicity, especially the abundance of oxygen, and the overall ionization state. They are routinely used to study H II regions, planetary nebulae, and the interstellar and intergalactic medium in galaxies.

In practice, [O III] lines help characterize the physical conditions and chemical evolution of astronomical sources. They are observable in the optical for many nearby objects and shift into the near-infrared for distant galaxies, enabling studies of star formation and galaxy growth across cosmic time.