MgII

MgII refers to singly ionized magnesium, Mg+, a common ion in many astrophysical and laboratory plasmas. In Mg II the electronic configuration is [Ne]3s1, and the ground term is 2S1/2. The first and second ionization energies are about 7.646 eV and 15.035 eV, respectively. The most prominent spectral feature associated with Mg II is the resonance doublet at 2796.35 Å and 2803.53 Å, produced by transitions from the 3p 2P upper levels to the ground 3s 2S1/2 state. The lines arise from the fine-structure components 2P3/2 → 2S1/2 and 2P1/2 → 2S1/2. The oscillator strengths are approximately f(2796) ≈ 0.612 and f(2803) ≈ 0.305, leading to a 2:1 strength ratio in optically thin gas; in saturated absorbers the observed depths approach parity.

Mg II lines have broad utility in astronomy. The Mg II doublet is a primary tracer of cool, photoionized gas in the circumgalactic medium and galactic halos, used to study gas flows, accretion, and chemical evolution around galaxies across cosmic time. Mg II absorption systems in quasar and galaxy spectra help map the distribution and kinematics of circumgalactic material and often accompany other metal lines and Lyman-alpha features. In solar and stellar contexts, the Mg II h and k lines near 280 nm are formed in the chromosphere and serve as diagnostics of magnetic activity, temperature structure, and dynamic processes, accessible via ultraviolet spectroscopy from space-based observatories.

Magnesium is an alpha-element produced in massive stars and distributed by core-collapse supernovae; Mg II thus traces moderately ionized gas and contributes to understanding chemical enrichment in various astrophysical environments.