Editing Molecular cloud (section)

==Types of molecular cloud==

===Giant molecular clouds===
[[File:"Finger of God" Bok globule in the Carina Nebula.jpg|thumb|left|Within a few million years the light from bright stars will have boiled away this molecular cloud of gas and dust. [[Finger of God Globule|The Finger of God Globule]] has broken off from the [[Carina Nebula]]. Newly formed stars are visible nearby, their images reddened by blue light being preferentially scattered by the pervasive dust. This image spans about two light-years and was taken by the [[Hubble Space Telescope]] in 1999.]]

[[File:Part of the Taurus Molecular Cloud.jpg|thumb|Part of the Taurus molecular cloud<ref>{{cite news|title=APEX Turns its Eye to Dark Clouds in Taurus|url=http://www.eso.org/public/news/eso1209/|access-date=17 February 2012|newspaper=ESO Press Release}}</ref>]]

A vast assemblage of molecular gas that has more than 10 thousand times the mass of the Sun<ref name="Fukui_Kawamura_2010">See, e.g., {{Cite journal | last1 = Fukui | first1 = Y. | last2 = Kawamura | first2 = A. | title = Molecular Clouds in Nearby Galaxies | doi = 10.1146/annurev-astro-081309-130854 | journal = Annual Review of Astronomy and Astrophysics | volume = 48 | pages = 547–580 | year = 2010 | bibcode = 2010ARA&A..48..547F }}</ref> is called a '''giant molecular cloud''' ('''GMC'''). GMCs are around 15 to 600 light-years (5 to 200 parsecs) in diameter, with typical masses of 10 thousand to 10 million solar masses.<ref name="murray">{{Cite journal | last1 = Murray | first1 = N. | title = Star Formation Efficiencies and Lifetimes of Giant Molecular Clouds in the Milky Way | doi = 10.1088/0004-637X/729/2/133 | journal = The Astrophysical Journal | volume = 729 | issue = 2 | pages = 133 | year = 2011 |arxiv = 1007.3270 |bibcode = 2011ApJ...729..133M | s2cid = 118627665 }}</ref> Whereas the average density in the solar vicinity is one particle per cubic centimetre, the average volume density of a GMC is about ten to a thousand times higher. Although the Sun is much denser 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.<ref name="williamsandblitz"/>

Filaments are truly ubiquitous in the molecular cloud. Dense molecular filaments will fragment into gravitationally bound cores, most of which will evolve into stars. Continuous accretion of gas, geometrical bending, and magnetic fields may control the detailed fragmentation manner of the filaments. In supercritical filaments, observations have revealed quasi-periodic chains of dense cores with spacing of 0.15 parsec comparable to the filament inner width.<ref>{{cite journal |last1=Zhang |first1=Guo-Yin |last2=André |first2=Ph. |last3=Men'shchikov |first3=A. |last4=Wang |first4=Ke |title=Fragmentation of star-forming filaments in the X-shaped nebula of the California molecular cloud |journal=Astronomy and Astrophysics |date=1 October 2020 |volume=642 |pages=A76 |doi=10.1051/0004-6361/202037721 |url=https://ui.adsabs.harvard.edu/abs/2020A%26A...642A..76Z/abstract |issn=0004-6361|arxiv=2002.05984 |bibcode=2020A&A...642A..76Z |s2cid=211126855 }}</ref> A substantial fraction of filaments contained prestellar and protostellar cores, supporting the important role of filaments in gravitationally bound core formation.<ref>{{cite journal |last1=Li |first1=Xue-Mei |last2=Zhang |first2=Guo-Yin |last3=Men’shchikov |first3=Alexander |last4=Li |first4=Jin-Zeng |last5=Zhang |first5=Chang |last6=Wu |first6=Zhong-Zu |title=Properties of the dense cores and filamentary structures in the Vela C molecular cloud |journal=Astronomy & Astrophysics |date=June 2023 |volume=674 |pages=A225 |doi=10.1051/0004-6361/202345846 |arxiv=2304.10863 |bibcode=2023A&A...674A.225L |s2cid=258291496 |url=https://doi.org/10.1051/0004-6361/202345846}}</ref> Recent studies have suggested that filamentary structures in molecular clouds play a crucial role in the initial conditions of star formation and the origin of the stellar IMF.<ref name="arxiv">{{cite journal | last1 = Zhang | first1 = Guo-Yin | last2 = Andre | first2 = Philippe | last3 = Menshchikov | first3 = Alexander | last4 = Li | first4 = Jin-Zeng | title = Probing the filamentary nature of star formation in the California giant molecular cloud | journal = Astronomy & Astrophysics | volume = 689 | page = A3 | year = 2024 | doi = 10.1051/0004-6361/202449853 | url = https://ui.adsabs.harvard.edu/link_gateway/2024A&A...689A...3Z/doi:10.1051/0004-6361/202449853 | arxiv = 2406.08004 }}</ref>

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 10<sup>4</sup> to 10<sup>6</sup> particles per cubic centimeter. Typical molecular cores are traced with CO and dense molecular cores are traced with [[ammonia]]. The concentration of [[Cosmic dust|dust]] within molecular cores is normally sufficient to block light from background stars so that they appear in silhouette as [[dark nebula]]e.<ref name="francesco2006">{{cite conference | author = Di Francesco, J. |display-authors=etal | title = An Observational Perspective of Low-Mass Dense Cores I: Internal Physical and Chemical Properties| book-title = Protostars and Planets V | date = 2006 | arxiv = astro-ph/0602379 |bibcode = 2007prpl.conf...17D }}</ref>

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]].<ref>{{cite conference | author = Grenier| title = The Gould Belt, star formation, and the local interstellar medium| book-title = The Young Universe | date = 2004 | arxiv = astro-ph/0409096|bibcode = 2004astro.ph..9096G }} [https://arxiv.org/abs/astro-ph/0409096 Electronic preprint]</ref> 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.<ref>[http://www.mpifr-bonn.mpg.de/staff/epolehampton/thesis/node23.html Sagittarius B2 and its Line of Sight] {{webarchive|url=https://web.archive.org/web/20070312062920/http://www.mpifr-bonn.mpg.de/staff/epolehampton/thesis/node23.html |date=2007-03-12 }}</ref>[[File:Distribution of molecular gas in 30 merging galaxies.jpg|thumb|Distribution of molecular gas in 30 merging galaxies.<ref>{{cite web|title=Violent Origins of Disc Galaxies Probed by ALMA|url=http://www.eso.org/public/news/eso1429/|website=www.eso.org|publisher=[[European Southern Observatory]]|access-date=17 September 2014}}</ref>]]

===Small molecular clouds===
{{main|Bok globule}}
Isolated gravitationally-bound small molecular clouds with masses less than a few hundred times that of the Sun are called [[Bok globule]]s. 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|Infrared cirrus}}

In 1984 [[IRAS]]{{clarify|which astronomers identified the "new type of diffuse molecular cloud" in circa 1984?  ...a science instrument, a telescope, cannot "identify" anything; it takes data, and comparisons, and decisions to identify something.|date=October 2021}} identified a new type of diffuse molecular cloud.<ref>{{cite journal | author=Low | title=Infrared cirrus – New components of the extended infrared emission | journal=Astrophysical Journal | date=1984| volume=278 | pages = L19 | doi = 10.1086/184213 | last2=Young | first2=E. | last3=Beintema | first3=D. A. | last4=Gautier | first4=T. N. | last5=Beichman | first5=C. A. | last6=Aumann | first6=H. H. | last7=Gillett | first7=F. C. | last8=Neugebauer | first8=G. | last9=Boggess | first9=N. | bibcode=1984ApJ...278L..19L | display-authors=1}}</ref> These were diffuse filamentary clouds that are visible at high [[Galactic coordinate system|galactic latitudes]]. These clouds have a typical density of 30 particles per cubic centimetre.<ref>{{cite journal | author=Gillmon, K. |author2=Shull, J.M. |name-list-style=amp | title= Molecular Hydrogen in Infrared Cirrus | journal=Astrophysical Journal | date=2006| volume=636 | pages= 908–915 | doi = 10.1086/498055 | bibcode=2006ApJ...636..908G|arxiv = astro-ph/0507587 | issue=2 |s2cid=18995587 }}</ref>[[File:Serpens_south.jpg|thumb|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.<ref name="FriesenBourke2016">{{cite journal|last1=Friesen|first1=R. K.|last2=Bourke|first2=T. L.|last3=Francesco|first3=J. Di|last4=Gutermuth|first4=R.|last5=Myers|first5=P. C.|title=The Fragmentation and Stability of Hierarchical Structure in Serpens South|journal=The Astrophysical Journal|volume=833|issue=2|year=2016|pages=204|issn=1538-4357|doi=10.3847/1538-4357/833/2/204|arxiv = 1610.10066 |bibcode = 2016ApJ...833..204F |s2cid=118594849 |doi-access=free }}</ref>]]