A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit the formation of molecules, most commonly molecular hydrogen (H2). This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas.
Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most often used to determine the presence of H2 is carbon monoxide (CO). The ratio between CO luminosity and H2 mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies.[1]
Within molecular clouds are regions with higher density, where much dust and many gas cores reside, called clumps. These clumps are the beginning of star formation if gravitational forces are sufficient to cause the dust and gas to collapse.[2]
Occurrence
Molecular cloud Barnard 68, about 500 ly distant and 0.5 ly in diameter.
Within the Milky Way, molecular gas clouds account for less than one percent of the volume of the interstellar medium (ISM), yet it is also the densest part of the medium, comprising roughly half of the total gas mass interior to the Sun's galactic orbit. The bulk of the molecular gas is contained in a ring between 3.5 and 7.5 kiloparsecs (11,000 and 24,000 light-years) from the center of the Milky Way (the Sun is about 8.5 kiloparsecs from the center).[3] Large scale CO maps of the galaxy show that the position of this gas correlates with the spiral arms of the galaxy.[4] That molecular gas occurs predominantly in the spiral arms suggests that molecular clouds must form and dissociate on a timescale shorter than 10 million years—the time it takes for material to pass through the arm region.[5]
Circinus molecular cloud has a mass around 250,000 times that of the Sun.[6]
Vertically to the plane of the galaxy, the molecular gas inhabits the narrow midplane of the galactic disc with a characteristic scale height, Z, of approximately 50 to 75 parsecs, much thinner than the warm atomic (Z from 130 to 400 parsecs) and warm ionized (Z around 1000 parsecs) gaseous components of the ISM.[7] The exception to the ionized-gas distribution are H II regions, which are bubbles of hot ionized gas created in molecular clouds by the intense radiation given off by young massive stars and as such they have approximately the same vertical distribution as the molecular gas.
This distribution of molecular gas is averaged out over large distances; however, the small scale distribution of the gas is highly irregular with most of it concentrated in discrete clouds and cloud complexes.[3]
Types of molecular cloud
Giant molecular clouds
Part of the Taurus Molecular Cloud.[8]
A vast assemblage of molecular gas that has more than 10 thousand times the mass of the Sun[9] is called a giant molecular cloud (GMC). GMCs are around 15 to 600 light-years in diameter (5 to 200 parsecs) and typical masses of 10 thousand to 10 million solar masses.[10] Whereas the average density in the solar vicinity is one particle per cubic centimetre, the average density of a GMC is a hundred to a thousand times as great. Although the Sun is much more dense than a GMC, the volume of a GMC is so great that it contains much more mass than the Sun. The substructure of a GMC is a complex pattern of filaments, sheets, bubbles, and irregular clumps.[5]
The densest parts of the filaments and clumps are called "molecular cores", while the densest molecular cores are called "dense molecular cores" and have densities in excess of 104 to 106 particles per cubic centimeter. Observationally, typical molecular cores are traced with CO and dense molecular cores are traced with ammonia. The concentration of dust within molecular cores is normally sufficient to block light from background stars so that they appear in silhouette as dark nebulae.[11]
GMCs are so large that "local" ones can cover a significant fraction of a constellation; thus they are often referred to by the name of that constellation, e.g. the Orion Molecular Cloud (OMC) or the Taurus Molecular Cloud (TMC). These local GMCs are arrayed in a ring in the neighborhood of the Sun coinciding with the Gould Belt.[12] The most massive collection of molecular clouds in the galaxy forms an asymmetrical ring about the galactic center at a radius of 120 parsecs; the largest component of this ring is the Sagittarius B2 complex. The Sagittarius region is chemically rich and is often used as an exemplar by astronomers searching for new molecules in interstellar space.[13]
Distribution of molecular gas in 30 merging galaxies.[14]
Small molecular clouds
Main article: Bok globule
Isolated gravitationally-bound small molecular clouds with masses less than a few hundred times that of the Sun are called Bok globules. The densest parts of small molecular clouds are equivalent to the molecular cores found in GMCs and are often included in the same studies.
High-latitude diffuse molecular clouds
Main article: Infrared cirrus
In 1984 IRAS identified a new type of diffuse molecular cloud.[15] These were diffuse filamentary clouds that are visible at high galactic latitudes. These clouds have a typical density of 30 particles per cubic centimeter.[16]
Processes
Young stars in and around molecular cloud Cepheus B. Radiation from one bright, massive star is destroying the cloud (from top to bottom in this image) while simultaneously triggering the formation of new stars.[17]
Star formation
Main article: Star formation
The formation of stars occurs exclusively within molecular clouds. This is a natural consequence of their low temperatures and high densities, because the gravitational force acting to collapse the cloud must exceed the internal pressures that are acting "outward" to prevent a collapse. There is observed evidence that the large, star-forming clouds are confined to a large degree by their own gravity (like stars, planets, and galaxies) rather than by external pressure. The evidence comes from the fact that the "turbulent" velocities inferred from CO linewidth scale in the same manner as the orbital velocity (a virial relation).
Physics
The Serpens South star cluster is embedded in a filamentary molecular cloud, seen as a dark ribbon passing vertically through the cluster. This cloud has served as a testbed for studies of molecular cloud stability.[18]
The physics of molecular clouds is poorly understood and much debated. Their internal motions are governed by turbulence in a cold, magnetized gas, for which the turbulent motions are highly supersonic but comparable to the speeds of magnetic disturbances. This state is thought to lose energy rapidly, requiring either an overall collapse or a steady reinjection of energy. At the same time, the clouds are known to be disrupted by some process—most likely the effects of massive stars—before a significant fraction of their mass has become stars.
Molecular clouds, and especially GMCs, are often the home of astronomical masers.
See also
Accretion (astrophysics)
Astrochemistry
Atomic and molecular astrophysics
Cosmic dust
Cosmochemistry
Evaporating gaseous globule
Formation and evolution of the Solar System
Interstellar ice
List of interstellar and circumstellar molecules
Nebula
Orion Molecular Cloud Complex
Perseus molecular cloud
References
Craig Kulesa. "Overview: Molecular Astrophysics and Star Formation". Research Projects. Retrieved September 7, 2005.
Astronomy (PDF). Rice University. 2016. p. 761. ISBN 978-1938168284 – via Open Stax.
Ferriere, D. (2001). "The Interstellar Environment of our Galaxy". Reviews of Modern Physics. 73 (4): 1031–1066.arXiv:astro-ph/0106359. Bibcode:2001RvMP...73.1031F. doi:10.1103/RevModPhys.73.1031. S2CID 16232084.
Dame; et al. (1987). "A composite CO survey of the entire Milky Way" (PDF). Astrophysical Journal. 322: 706–720. Bibcode:1987ApJ...322..706D. doi:10.1086/165766. hdl:1887/6534.
Williams, J. P.; Blitz, L.; McKee, C. F. (2000). "The Structure and Evolution of Molecular Clouds: from Clumps to Cores to the IMF". Protostars and Planets IV. Tucson: University of Arizona Press. p. 97.arXiv:astro-ph/9902246. Bibcode:2000prpl.conf...97W.
"Violent birth announcement from an infant star". ESA/Hubble Picture of the Week. Retrieved 27 May 2014.
Cox, D. (2005). "The Three-Phase Interstellar Medium Revisited". Annual Review of Astronomy and Astrophysics. 43 (1): 337–385. Bibcode:2005ARA&A..43..337C. doi:10.1146/annurev.astro.43.072103.150615.
"APEX Turns its Eye to Dark Clouds in Taurus". ESO Press Release. Retrieved 17 February 2012.
See, e.g., Fukui, Y.; Kawamura, A. (2010). "Molecular Clouds in Nearby Galaxies". The Annual Review of Astronomy and Astrophysics. 48: 547–580. Bibcode:2010ARA&A..48..547F. doi:10.1146/annurev-astro-081309-130854.
Murray, N. (2011). "Star Formation Efficiencies and Lifetimes of Giant Molecular Clouds in the Milky Way". The Astrophysical Journal. 729 (2): 133.arXiv:1007.3270. Bibcode:2011ApJ...729..133M. doi:10.1088/0004-637X/729/2/133. S2CID 118627665.
Di Francesco, J.; et al. (2006). "An Observational Perspective of Low-Mass Dense Cores I: Internal Physical and Chemical Properties". Protostars and Planets V.arXiv:astro-ph/0602379. Bibcode:2007prpl.conf...17D.
Grenier (2004). "The Gould Belt, star formation, and the local interstellar medium". The Young Universe.arXiv:astro-ph/0409096. Bibcode:2004astro.ph..9096G. Electronic preprint
Sagittarius B2 and its Line of Sight Archived 2007-03-12 at the Wayback Machine
"Violent Origins of Disc Galaxies Probed by ALMA". www.eso.org. European Southern Observatory. Retrieved 17 September 2014.
Low; et al. (1984). "Infrared cirrus – New components of the extended infrared emission". Astrophysical Journal. 278: L19. Bibcode:1984ApJ...278L..19L. doi:10.1086/184213.
Gillmon, K. & Shull, J.M. (2006). "Molecular Hydrogen in Infrared Cirrus". Astrophysical Journal. 636 (2): 908–915.arXiv:astro-ph/0507587. Bibcode:2006ApJ...636..908G. doi:10.1086/498055. S2CID 18995587.
"Chandra :: Photo Album :: Cepheus B :: August 12, 2009".
Friesen, R. K.; Bourke, T. L.; Francesco, J. Di; Gutermuth, R.; Myers, P. C. (2016). "The Fragmentation and Stability of Hierarchical Structure in Serpens South". The Astrophysical Journal. 833 (2): 204.arXiv:1610.10066. Bibcode:2016ApJ...833..204F. doi:10.3847/1538-4357/833/2/204. ISSN 1538-4357. S2CID 118594849.
External links
Molecular cloud at the Encyclopædia Britannica
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Accretion Molecular cloud Bok globule Young stellar object
Protostar Pre-main-sequence Herbig Ae/Be T Tauri FU Orionis Herbig–Haro object Hayashi track Henyey track
Main sequence Red-giant branch Horizontal branch
Red clump Asymptotic giant branch
super-AGB Blue loop Protoplanetary nebula Planetary nebula PG1159 Dredge-up OH/IR Instability strip Luminous blue variable Blue straggler Stellar population Supernova Superluminous supernova / Hypernova
Early Late Main sequence
O B A F G K M Brown dwarf WR OB Subdwarf
O B Subgiant Giant
Blue Red Yellow Bright giant Supergiant
Blue Red Yellow Hypergiant
Yellow Carbon
S CN CH White dwarf Chemically peculiar
Am Ap/Bp HgMn Helium-weak Barium Extreme helium Lambda Boötis Lead Technetium Be
Shell B[e]
White dwarf
Helium planet Black dwarf Neutron
Radio-quiet Pulsar
Binary X-ray Magnetar Stellar black hole X-ray binary
Burster
Hypothetical
Blue dwarf Green Black dwarf Exotic
Boson Electroweak Strange Preon Planck Dark Dark-energy Quark Q Black Gravastar Frozen Quasi-star Thorne–Żytkow object Iron Blitzar
Deuterium burning Lithium burning Proton–proton chain CNO cycle Helium flash Triple-alpha process Alpha process Carbon burning Neon burning Oxygen burning Silicon burning S-process R-process Fusor Nova
Symbiotic Remnant Luminous red nova
Structure
Core Convection zone
Microturbulence Oscillations Radiation zone Atmosphere
Photosphere Starspot Chromosphere Stellar corona Stellar wind
Bubble Bipolar outflow Accretion disk Asteroseismology
Helioseismology Eddington luminosity Kelvin–Helmholtz mechanism
Properties
Designation Dynamics Effective temperature Luminosity Kinematics Magnetic field Absolute magnitude Mass Metallicity Rotation Starlight Variable Photometric system Color index Hertzsprung–Russell diagram Color–color diagram
Star systems
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Sun
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Substellar object
Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Guest Gravity Intergalactic Planet-hosting stars Tidal disruption event
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Nebula
Visible nebula
Dark nebula Diffuse nebula
Emission nebula
Planetary nebula Supernova remnant Nova remnant H II region Reflection nebula
Variable nebula Preplanetary nebula
Pre-stellar nebula
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Clouds
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Theoretical concepts
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Molecules detected in outer space
Molecules
Diatomic
Aluminium monochloride Aluminium monofluoride Aluminium monoxide Argonium Carbon monophosphide Carbon monosulfide Carbon monoxide Carborundum Cyanogen radical Diatomic carbon Fluoromethylidynium Helium hydride ion Hydrogen chloride Hydrogen fluoride Hydrogen (molecular) Hydroxyl radical Iron(II) oxide Magnesium monohydride cation Methylidyne radical Nitric oxide Nitrogen (molecular) Nitrogen monohydride Nitrogen sulfide Oxygen (molecular) Phosphorus monoxide Phosphorus mononitride Potassium chloride Silicon carbide Silicon mononitride Silicon monoxide Silicon monosulfide Sodium chloride Sodium iodide Sulfur monohydride Sulfur monoxide Titanium oxide
Nitrous-oxide-3D-balls.png
Ethanol-3D-balls.png
Buckminsterfullerene-perspective-3D-balls.png
Triatomic
Aluminium(I) hydroxide Aluminium isocyanide Amino radical Carbon dioxide Carbonyl sulfide CCP radical Chloronium Diazenylium Dicarbon monoxide Disilicon carbide Ethynyl radical Formyl radical Hydrogen cyanide (HCN) Hydrogen isocyanide (HNC) Hydrogen sulfide Hydroperoxyl Iron cyanide Isoformyl Magnesium cyanide Magnesium isocyanide Methylene radical N2H+ Nitrous oxide Nitroxyl Ozone Phosphaethyne Potassium cyanide Protonated molecular hydrogen Sodium cyanide Sodium hydroxide Silicon carbonitride c-Silicon dicarbide SiNC Sulfur dioxide Thioformyl Thioxoethenylidene Titanium dioxide Tricarbon Water
Four
atoms
Acetylene Ammonia Cyanic acid Cyanoethynyl Cyclopropynylidyne Formaldehyde Fulminic acid HCCN Hydrogen peroxide Hydromagnesium isocyanide Isocyanic acid Isothiocyanic acid Ketenyl Methylene amidogen Methyl radical Propynylidyne Protonated carbon dioxide Protonated hydrogen cyanide Silicon tricarbide Thioformaldehyde Tricarbon monoxide Tricarbon monosulfide Thiocyanic acid
Five
atoms
Ammonium ion Butadiynyl Carbodiimide Cyanamide Cyanoacetylene Cyanoformaldehyde Cyanomethyl Cyclopropenylidene Formic acid Isocyanoacetylene Ketene Methane Methoxy radical Methylenimine Propadienylidene Protonated formaldehyde Protonated formaldehyde Silane Silicon-carbide cluster
Six
atoms
Acetonitrile Cyanobutadiynyl radical E-Cyanomethanimine Cyclopropenone Diacetylene Ethylene Formamide HC4N Ketenimine Methanethiol Methanol Methyl isocyanide Pentynylidyne Propynal Protonated cyanoacetylene
Seven
atoms
Acetaldehyde Acrylonitrile
Vinyl cyanide Cyanodiacetylene Ethylene oxide Glycolonitrile Hexatriynyl radical Methylacetylene Methylamine Methyl isocyanate Vinyl alcohol
Eight
atoms
Acetic acid Aminoacetonitrile Cyanoallene Ethanimine Glycolaldehyde Heptatrienyl radical Hexapentaenylidene Methylcyanoacetylene Methyl formate Propenal
Nine
atoms
Acetamide Cyanohexatriyne Cyanotriacetylene Dimethyl ether Ethanol Methyldiacetylene Octatetraynyl radical Propene Propionitrile
Ten
atoms
or more
Acetone Benzene Benzonitrile Buckminsterfullerene (C60, C60+, fullerene, buckyball) C70 fullerene Cyanodecapentayne Cyanopentaacetylene Cyanotetra-acetylene Ethylene glycol Ethyl formate Methyl acetate Methyl-cyano-diacetylene Methyltriacetylene Propanal n-Propyl cyanide Pyrimidine
Deuterated
molecules
Ammonia Ammonium ion Formaldehyde Formyl radical Heavy water Hydrogen cyanide Hydrogen deuteride Hydrogen isocyanide Methylacetylene N2D+ Trihydrogen cation
Unconfirmed
Anthracene Dihydroxyacetone Ethyl methyl ether Glycine Graphene Hemolithin (possibly 1st extraterrestrial protein found) H2NCO+ Linear C5 Naphthalene cation Phosphine Pyrene Silylidine
Related
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