This is a list of molecules that have been detected in the interstellar medium, grouped by the number of component atoms. The chemical formula is listed for each detected compound, along with any ionized form that has also been observed.
Detection
The molecules listed below were detected by spectroscopy. Their spectral features are generated by transitions of component electrons between different energy levels, or by rotational or vibrational spectra. Detection usually occurs in radio, microwave, or infrared portions of the spectrum. The first such molecule to be detected in the interstellar medium was the methylidyne radical (CH) in 1937.
Interstellar molecules are formed by chemical reactions within very sparse interstellar or circumstellar clouds of dust and gas. Usually this occurs when a molecule becomes ionized, often as the result of an interaction with a cosmic ray. This positively-charged molecule then draws in a nearby reactant by electrostatic attraction of the neutral molecule's electrons. Molecules can also be generated by reactions between neutral atoms and molecules, although this process is generally slower.
A particularly large and rich region for detecting interstellar molecules is Sagittarius B2 (Sgr B2). This giant molecular cloud lies near the center of the Milky Way galaxy and is a frequent target for new searches. About half of the molecules listed below were first found near Sgr B2, and nearly every other molecule has since been detected in this feature.
Molecules
The following tables list molecules that have been detected in the interstellar medium, grouped by the number of component atoms. If there is no entry in the Molecule column, only the ionized form has been detected. For molecules where no designation was given in the scientific literature, that field is left empty.
