Editing Molecular cloud (section)

==Occurrence==
[[Image:Barnard 68.jpg|thumb|left|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 it. The bulk of the molecular gas is contained in a ring between {{convert|3.5|and|7.5|kpc|ly|lk=on|abbr = off}} from the center of the Milky Way (the Sun is about 8.5 kiloparsecs from the center).<ref name="ferriere2001">{{cite journal |author=Ferriere, D. |date=2001 |title=The Interstellar Environment of our Galaxy |journal=Reviews of Modern Physics |volume=73 |issue=4 |pages=1031–1066 |arxiv=astro-ph/0106359 |bibcode=2001RvMP...73.1031F |doi=10.1103/RevModPhys.73.1031 |s2cid=16232084}}</ref> Large scale CO maps of the galaxy show that the position of this gas correlates with the spiral arms of the galaxy.<ref>{{cite journal |author=Dame |last2=Ungerechts |first2=H. |last3=Cohen |first3=R. S. |last4=De Geus |first4=E. J. |last5=Grenier |first5=I. A. |last6=May |first6=J. |last7=Murphy |first7=D. C. |last8=Nyman |first8=L.-A. |last9=Thaddeus |first9=P. |display-authors=1 |date=1987 |title=A composite CO survey of the entire Milky Way |url=https://openaccess.leidenuniv.nl/bitstream/handle/1887/6534/ApJ_322_706_720.pdf?sequence=1 |journal=Astrophysical Journal |volume=322 |pages=706–720 |bibcode=1987ApJ...322..706D |doi=10.1086/165766 |hdl-access=free |hdl=1887/6534}}</ref> 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.<ref name="williamsandblitz">{{cite conference |last1=Williams |first1=J. P. |last2=Blitz |first2=L. |last3=McKee |first3=C. F. |date=2000 |title=The Structure and Evolution of Molecular Clouds: from Clumps to Cores to the IMF |publisher=Tucson: University of Arizona Press |pages=97 |arxiv=astro-ph/9902246 |bibcode=2000prpl.conf...97W |book-title=Protostars and Planets IV}}</ref>

[[File:Violent birth announcement from an infant star.jpg|thumb|Circinus molecular cloud has a mass around 250,000 times that of the Sun.<ref>{{cite news |title=Violent birth announcement from an infant star |newspaper=ESA/Hubble Picture of the Week |url=http://www.spacetelescope.org/images/potw1421a/ |access-date=27 May 2014}}</ref>]]
Perpendicularly 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 [[atom]]ic (''Z ''from 130 to 400&nbsp;parsecs) and warm [[ion]]ized (''Z ''around 1000&nbsp;parsecs) gaseous [[Interstellar medium#Interstellar matter|components of the ISM]].<ref>{{cite journal |author=Cox, D. |date=2005 |title=The Three-Phase Interstellar Medium Revisited |journal=Annual Review of Astronomy and Astrophysics |volume=43 |issue=1 |pages=337–385 |bibcode=2005ARA&A..43..337C |doi=10.1146/annurev.astro.43.072103.150615}}</ref> The exceptions to the ionized-gas distribution are [[H II region]]s, which are bubbles of hot ionized gas created in molecular clouds by the intense radiation given off by [[OB star|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.<ref name="ferriere2001" />