List of interstellar and circumstellar molecules

A catalog of molecules detected in interstellar space or in material surrounding stars, from simple diatomic species to complex organic compounds.

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Interstellar medium: The gas, dust, cosmic rays, and magnetic fields occupying the space between stars. Its clouds and diffuse regions host many cataloged molecular species.
Gas-phase ion chemistry: Chemical reactions among ions and neutral particles in a gas. Ion–molecule reactions build many species in cold interstellar clouds.
Molecular hydrogen: The diatomic molecule H₂, consisting of two hydrogen atoms. It is the dominant molecular species in many interstellar clouds.
Molecular line: A discrete spectral feature produced by a transition between molecular energy levels. Line frequencies identify species, while line strengths and shapes reveal their conditions.
Spectral line confusion: The blending of numerous spectral features that makes individual lines difficult to identify. Crowded spectra can make assignments of complex interstellar molecules ambiguous.
Circumstellar envelope: Gas and dust expelled by a star, forming an extended shell around it. These envelopes are a major source of molecules around evolved stars.
Grain-surface chemistry: Chemical reactions occurring on the surfaces of dust grains. Dust grains enable molecules to form when gas-phase reactions are inefficient.
Carbon monoxide: A diatomic molecule composed of one carbon atom and one oxygen atom. Its bright emission lines make it a standard tracer of molecular gas.
Column density: The number of particles of a specified species along a line of sight per unit area. Observed molecular lines can be used to estimate how much of a species lies along a sightline.
Molecular spectroscopy database: A curated collection of laboratory-measured molecular spectral data. Reliable identifications depend on accurate reference frequencies and transition strengths.