Molecular hydrogen

Molecular hydrogen (H₂) is a molecule of two hydrogen atoms joined by a covalent bond. It is the most abundant molecule in molecular clouds and a major constituent of the interstellar medium.

H₂ formation on dust grains: The process by which hydrogen atoms meet on a dust-grain surface and leave as a bound H₂ molecule. Dust surfaces enable efficient molecule formation in cold clouds where gas-phase reactions are too slow.
CO–H₂ conversion factor: The empirical factor that converts carbon monoxide line intensity into an estimated molecular hydrogen column density. H₂ is difficult to observe directly in cold clouds, so CO emission commonly serves as its proxy.
Hydrogen atom: The simplest atom, consisting of one proton and one electron, with isotopes that may also contain neutrons. Two hydrogen atoms supply the nuclei and electrons that form H₂.
Molecular hydrogen cooling: The loss of thermal energy from gas through molecular hydrogen emission after collisional excitation. H₂ rotational and vibrational lines can cool warm cloud gas and influence gravitational collapse.
Atomic hydrogen: Neutral hydrogen consisting of individual atoms rather than bonded pairs. The atomic-to-molecular transition marks a key chemical boundary within interstellar clouds.
Lyman–Werner bands: Ultraviolet absorption bands of molecular hydrogen spanning roughly 11.2–13.6 electron volts. Photons in these bands excite H₂ and can dissociate it, limiting its survival near ultraviolet sources.
H₂ fluorescence: Light emitted by molecular hydrogen after ultraviolet excitation populates higher-energy states. Fluorescent line patterns identify irradiated molecular gas and constrain its physical conditions.
Covalent bond: A chemical bond formed when atoms share one or more pairs of electrons. The two atoms in H₂ are held together by a shared electron pair.
Jeans instability: The gravitational instability of a gas cloud when its mass exceeds the threshold set by pressure support. Cooling by H₂ lowers gas pressure and can help a cloud become gravitationally unstable.
Hydrogen deuteride: A diatomic molecule containing one hydrogen atom and one deuterium atom, written HD. Unlike H₂, HD has a permanent dipole and can emit cooling radiation through dipole transitions.

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