ionizedgas

Ionized gas, or plasma, is a state of matter in which a significant fraction of atoms have lost electrons, creating positive ions and free electrons. Because of these free charges, the gas conducts electricity and responds collectively to electromagnetic fields. Ionized gas can be fully ionized, with most atoms stripped of electrons, or partially ionized, with a mix of ions and neutral atoms.

Ionization occurs when energy input exceeds binding energies: by photons (photoionization) from hot stars or active galactic nuclei; by collisions in high-temperature environments; or by energetic particles. Common astrophysical contexts include H II regions around young, hot stars, planetary nebulae, and the hot plasma of the interstellar and circumgalactic medium; in laboratories, ionized gas is produced in tokamaks, inertial confinement fusion devices, or discharge plasmas.

Plasma properties include high electrical conductivity, long-range electric and magnetic interactions, Debye shielding, and plasma frequency; the behavior depends on temperature, density, and ionization fraction. Emission and cooling occur through line radiation from ions, recombination, bremsstrahlung, and other processes.

Observationally, ionized gas emits characteristic spectral lines such as H-alpha, [O III], [N II], and He II, which are used to study star-forming regions, galactic halos, and the ionized component of the interstellar and intergalactic media. The presence of ionized gas reveals energetic radiation fields and thermal histories.