interstellar molecules

artwork of carbon molecules in space. Image: NASA / Jenny Mottar

More than 140 different varieties of molecules have been found in the interstellar medium, ranging in complexity from molecular hydrogen (H2), which is also by far the commonest molecule in space, through other familiar ones, such as water, hydrogen cyanide (HCN), nitrous oxide or 'laughing gas' (N2O), and ethanol (CH3CH2OH), to esoteric carbon-chains known as cyano-polyynes, the biggest known of which is HC11N. In addition, there is strong evidence for even larger aromatic (carbon-ringed) molecules called polycyclic aromatic hydrocarbons, or PAHs. Some molecules, such as HCO+, were not even known on Earth at the time of their detection in space.


Because chemical bonds holding molecules together are sensitive to high-energy radiation and high temperatures, molecules are found in cool astronomical environments such as the dark interior of dense interstellar clouds and in expanding envelopes around dying red giants (and also in comets and planetary atmospheres). Most of the molecular material in our galaxy and elsewhere occurs in giant molecular clouds (GMCs). Within these, the greatest concentration and diversity of molecules are found in pockets, known as hot cores, near certain recently formed luminous stars. Hot cores are very compact (fractions of a light year), warm (a few hundred K, compared to 10 or 20K for the general interstellar gas), and dense (more than 106 hydrogen molecules per cm3) condensations with remarkably rich millimeter-wave emission-line spectra. They also show up as powerful infrared objects. The Orion Nebula contains two hot cores, which, being the nearest such objects, are also the most well-studied.


Interstellar molecules are formed through a complicated network of chemical reactions in situ, inside the interstellar or circumstellar clouds where they are found. Crucial to their synthesis is the presence of dust grains. The icy surface of these grains both shields molecules from stellar ultraviolet that would otherwise disrupt the chemical bonds and provides a surface on which atoms, radicals, and molecules can congregate and interact. Laboratory studies have shown that not only simple molecules can be made in this way but elaborate molecules (yet to be detected in space) that may be important in the origin of life itself. Among the molecules of biochemical interest already detected in the Sagittarius B2 cloud, near the center of the Galaxy, is glycolaldehyde (C2H4O2), a simple sugar that can combine with other molecules to form the more complex sugars ribose and glucose. Ribose is a building block of nucleic acids such, as RNA and DNA, which carry the genetic code of living organisms. Observations also suggest the presence of glycine, the simplest amino acid (a chemical unit of proteins), in Sag B2.


List of known interstellar molecules



molecular species name discovery year
CH methylidyne radical 1937
CN cyanogen radical 1940
CH+ methylidyne cation 1941
OH hydroxyl radical 1963
CO carbon monoxide 1970
H2 molecular hydrogen 1970
SiO silicon monoxide 1971
CS carbon monosulfide 1971
SO sulfur monoxide 1973
SiS silicon sulfide 1975
NS nitrogen sulfide 1975
C2 carbon dimer 1977
NO nitric oxide 1978
HCl hydrogen chloride 1985
NaCl sodium chloride 1987
AlCl aluminum (I) chloride 1987
KCl potassium chloride 1987
AlF aluminum (I) fluoride 1987
PN phosphorus nitride 1987
SiC silicon carbide 1989
CP   1990
NH   1991
SiN silicon nitride 1992
SO+ sulfur monoxide cation 1992
CO+ carbon monoxide cation 1993
HF hydrogen fluoride 1997
LiF lithium hydride 1998
SH   2000
FeO iron (II) oxide; 2002
N2 molecular nitrogen 2004
CF+   2006
O2 molecular oxygen 2007



Molecular species Name Discovery year
H2O water 1969
HCO+ formyl cation 1970
HCN hydrogen cyanide 1971
OCS carbonyl sulfide 1971
H2S hydrogen sulfide 1972
HNC hydrogen isocynaide 1973
N2H+ protated nitrogen ion 1974
C2H ethynyl radical 1974
SO2 sulfur dioxide 1975
HDO deuterated water 1975
HCO formyl radical 1976
HNO   1977
OCN-   1979
HCS+   1981
HOC+   1983
c-SiC2   1984
MgNC magnesium isocyanide 1986
C2S   1987
C3 carbon trimer 1987
CO2 carbon dioxide 1989
CH2   1989
C2O carbene 1991
NH2   1993
NaCN sodium cyanide 1994
N2O nitrous oxide 1994
MgCN magnesium cyanide 1995
H3+   1996
SiCN silicon cyanide 2000
AlNC aluminum isocynaide 2002
SiNC silicon isocyanide 2004



Molecular species Name Discovery year
NH3 ammonia 1968
H2CO formaldehyde 1969
HNCO isocyanic acid 1972
H2CS thioformaldehyde 1973
C3N cyanoethynyl 1977
HNCS isothiocyanic acid 1979
HOCO+ protonated carbon dioxide 1981
C3H propynlidyne radical 1985
C3O tricarbon monoxide 1985
HCNH+ protonated hydrogen cyanide 1986
H3O+ hydronium ion 1986
C3S tricarbon sulfide 1987
c-C3H cyclic propynlidyne radical 1987
C2H2 acetylene 1989
HC2N   1991
H2CN methylene amidogen 1994
SiC3 silicon tricarbide 1999
CH3   2000


Molecular species Name Discovery year
HC3N cyanoacetylene 1971
HCOOH formic acid 1971
CH2NH methylenimine 1973
NH2CN cyanamide 1975
H2C2O ketene 1977
C4H   1978
SiH4 silane 1984
c-C3H2 cyclopropenylidene 1985
CH2CN cyanomethyl radical 1988
C5   1989
SiC4 silicon tetracarbide 1989
l-C3H2 l-cyclopropenylidene 1991
CH4 methane 1991
HC2NC isocyanoacetylene 1992
HNC3   1992
H2COH+   1996
C4H-   2007



Molecular species Name Discovery year
CH3OH methanol 1970
CH3CN methyl cyanide 1971
NH2HCO formamide 1973
CH3CHO acetaldehyde 1973
C2H4 ethene 1983
C5H   1986
CH2CHO propynal 1988
HC3NH+   1994
C5N cyanobutadiynyl 1998
H2COH+   1996
C4H-   2007
C4H2 diacetylene 2001
HC4N   2004
c-C2H3O cyclopropenone 2006
CH2CNH ketenimine 2006



Molecular species Name Discovery year
CH3NH2 methylamine 1974
CH2CHCN acrylonitrile 1975
CH3C2H methlacetylene 1973
HC5N cyanodiacetylene 1978
C6H hexatriynyl 1986
c-C2H4O ethylene oxide 1997
CH2CHOH vinyl alcohol 2001
C6H-   2006



Molecular species Name Discovery year
CH3C3N methylcyanoacetylene 1984
C7H  methylamine 1997
CH3COOH acetic acid 1997
CH2OHCHO glycoaldehyde 2000
C6H2 triacetylene 2001
CH2CHCHO propenal 2004
CH2CCHCN cyanoallene 2006



Molecular species Name >Discovery year
CH3OCH3 dimethyl ether 1974
C2H5OH trans-ethanol 1975
CH3CH2CN propionitrile (ethyl cyanide) 1977
HC7N cyanohexatriyne (cyanotriacetylene) 1978
CH3C4H methyldiacetylene 1984
C8H   1996
CH3CONH2 acetamide 2006



Molecular species Name Discovery year
(CH3)2CO acetone 1987
NH2CH2COOH glycine 2003
CH3CH2CHO propanal 2004
CH3C5N methylcyanodiacetylene 2006



Molecular species Name Discovery year
HC9N cyanotetra-acetylene 1978
HOCHC2CH2OH ethylene glycol 2002
CH3C6H methyltriacetylene 2006



Molecular species Name Discovery year
C6H6 benzene 2001
CO(CH2OH)2 1,3 dihydroxyacetone 2005



Molecular species Name Discovery year
HC11N cyanodecapentayne 1997