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 cubic centimeter) 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 some known interstellar molecules

 

Diatomic

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

 

Triatomic

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

 

4-atomic

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

blue

5-atomic

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

 

6-atomic

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

 

7-atomic

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

 

8-atomic

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

 

9-atomic

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

 

10-atomic

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

 

11-atomic

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

 

12-atomic

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

 

13-atomic

Molecular species	Name	Discovery year
HC11N	cyanodecapentayne	1997