Editing Molecular cloud (section)

== General structure and chemistry of molecular clouds ==
Molecular clouds typically have [[interstellar medium]] densities of 10 to 30 {{math|cm<sup>−3</sup>}}, and constitute approximately 50% of the total interstellar gas in a [[galaxy]].<ref name=":Dominik">{{Cite web |last1=Dominik |first1=Carsten |last2=Inga |first2=Kamp |date=November 2023 |title=Star and Planet Formation |url=https://staff.fnwi.uva.nl/c.dominik/Teaching/SPF/syllabus.pdf |website=University of Amsterdam}}</ref> Most of the gas is found in a [[Molecule|molecular state]]. The visual boundaries of a molecular cloud is not where the cloud effectively ends, but where molecular gas changes to atomic gas in a fast transition, forming "envelopes" of mass, giving the impression of an edge to the cloud structure. The structure itself is generally irregular and filamentary.<ref name="williamsandblitz"/>

[[Cosmic dust]] and [[Ultraviolet|ultraviolet radiation]] emitted by stars are key factors that determine not only gas and column density, but also the molecular composition of a cloud. The dust provides shielding to the molecular gas inside, preventing [[Dissociation (chemistry)|dissociation]] by the ultraviolet radiation. The dissociation caused by [[Ultraviolet|UV photons]] is the main mechanism for transforming molecular material back to the atomic state inside the cloud.<ref name=":wardthompson">{{Cite book |last1=Ward-Thompson |first1=Derek |title=An introduction to star formation |last2=Whitworth |first2=Anthony P. |date=2015 |publisher=Cambridge University Press |isbn=978-0-521-63030-6 |edition=1 |location=Cambridge}}</ref>  Molecular content in a region of a molecular cloud can change rapidly due to variation in the radiation field and dust movement and disturbance.<ref name=":Physics439">{{Cite book |url=http://link.springer.com/10.1007/3-540-58621-0 |title=The Structure and Content of Molecular Clouds 25 Years of Molecular Radioastronomy: Proceedings of a Conference Held at Schloss Ringberg, Tegernsee, Germany 14–16 April 1993 |date=1994 |publisher=Springer Berlin Heidelberg |isbn=978-3-540-58621-0 |editor-last=Wilson |editor-first=Thomas L. |series=Lecture Notes in Physics |volume=439 |location=Berlin, Heidelberg |language=en |doi=10.1007/3-540-58621-0 |editor-last2=Johnston |editor-first2=Kenneth J. }}</ref> 
[[File:Ngc1555.jpg|thumb|left|The star T Tauri with NGC 1555 cloud nearby.]]
Most of the gas constituting a molecular cloud is [[Hydrogen|molecular hydrogen]], with [[carbon monoxide]] being the second most common compound.<ref name=":Dominik" /> Molecular clouds also usually contain other elements and compounds. Astronomers have observed the presence of long chain compounds such as [[methanol]], [[ethanol]] and [[Benzene|benzene rings]] and their several [[hydride]]s. Large molecules known as [[polycyclic aromatic hydrocarbon]]s have also been detected.<ref name=":wardthompson" />

The density across a molecular cloud is fragmented and its regions can be generally categorized in clumps and cores. Clumps form the larger substructure of the cloud, having the average size of 1 [[Parsec|pc]]. Clumps are the precursors of [[star cluster]]s, though not every clump will eventually form stars. Cores are much smaller (by a factor of 10) and have higher densities. Cores are [[Gravitational binding energy|gravitationally bound]] and go through a [[Gravitational collapse|collapse]] during [[star formation]].<ref name=":Dominik" />

In astronomical terms, molecular clouds are short-lived structures that are either destroyed or go through major structural and chemical changes approximately 10 million years into their existence. Their short life span can be inferred from the range in age of young stars associated with them, of 10 to 20 million years, matching molecular clouds’ internal timescales.<ref name=":Physics439" />

Direct observation of [[T Tauri star]]s inside dark clouds and [[OB star]]s in star-forming regions match this predicted age span. The fact OB stars older than 10 million years don’t have a significant amount of cloud material about them, seems to suggest most of the cloud is dispersed after this time. The lack of large amounts of frozen molecules inside the clouds also suggest a short-lived structure. Some astronomers propose the molecules never froze in very large quantities due to turbulence and the fast transition between atomic and molecular gas.<ref name=":Physics439" />