An Etymological Dictionary of Astronomy and Astrophysics

A molecule consisting of two hydrogen atoms (H₂) which is the most abundant molecule in the Universe. Molecular hydrogen plays a fundamental role in many astrophysical contexts. It is found in all regions where
→ self-shielding against the ultraviolet photons, responsible for its → photodissociation, is sufficiently large. Containing two identical hydrogen atoms, H₂ is highly symmetric. Due to this property, the molecule has no → dipole moment and all → rotation-vibrational transitions within the electronic → ground state are → quadrupolar with low → spontaneous emission  → Einstein coefficient values. The molecule exists in two almost independent states, namely → orthohydrogen and → parahydrogen. H₂ may be excited through several mechanisms, including: 1)
→ far ultraviolet (FUV) induced → optical pumping and
→ collisional excitation in → photodissociation regions (PDRs) associated with → star formation; 2)
→ hard X-rays penetrating and heating regions within → molecular clouds, which in turn excite H₂ via collisions with electrons or hydrogen atoms; and 3)
collisional excitation of H₂ due to acceleration produced by interstellar → shock waves. H₂ is thought to be chiefly produced via surface reactions on → interstellar grains, but the exact formation mechanism is not fully understood.

See also: → molecular; → hydrogen.