Editing Molecular cloud (section)

{{short description|Type of interstellar cloud}}

{{Star formation}}
A '''molecular cloud'''—sometimes called a '''stellar nursery''' if [[star formation]] is occurring within—is a type of [[interstellar cloud]] of which the density and size permit [[absorption nebula]]e, the formation of molecules (most commonly [[molecular hydrogen]], H<sub>2</sub>), and the formation of [[H II region]]s. 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 H<sub>2</sub> is [[carbon monoxide]] (CO). The ratio between CO [[luminosity]] and H<sub>2</sub> [[mass]] is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies.<ref name=":0">{{cite web | author=Craig Kulesa | title=Overview: Molecular Astrophysics and Star Formation | work=Research Projects | url=http://loke.as.arizona.edu/~ckulesa/research/overview.html | access-date=September 7, 2005 }}</ref>

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.<ref>{{Cite book|title=Astronomy|publisher=[[Rice University]]|year=2016|isbn=978-1938168284|url=https://d3bxy9euw4e147.cloudfront.net/oscms-prodcms/media/documents/Astronomy-Draft-20160817.pdf|pages=761|via=Open Stax}}</ref>