Editing Molecular cloud (section)

== Cloud formation and destruction ==
Due to their short lifespan, it follows that molecular clouds are constantly being assembled and destroyed. By calculating the rate at which stars are forming in our galaxy, astronomers are able to suggest the amount of interstellar gas being collected into star-forming molecular clouds in our galaxy. The rate of mass being assembled into stars is approximately 3 [[Solar mass|''M''<sub>☉</sub>]] per year. Only 2% of the mass of a molecular cloud is assembled into stars, giving the number of 150 ''M''<sub>☉</sub> of gas being assembled in molecular clouds in the [[Milky Way]] per year.<ref name=":Physics439" /><ref name=":lada91">{{Cite book |title=The physics of star formation and early stellar evolution: proceedings of the NATO Advanced Study Institute on the Physics of Star Formation and Early Stellar Evolution, Agia Pelagia, Crete, Greece, May 27 - June 8, 1990 |date=1991 |publisher=Kluwer |isbn=978-0-7923-1349-6 |editor-last=Lada |editor-first=Charles J. |series=NATO ASI series Series C, Mathematical and physical sciences |location=Dordrecht}}</ref>

[[File:54345main ic1396 highres.jpg|thumb|left|The Elephant's Trunk Nebula is an elongated dark globule. The globule is a condensation of dense gas that is barely surviving the strong ionizing radiation from a nearby massive star.]]Two possible mechanisms for molecular cloud formation have been suggested by astronomers. Cloud growth by collision and gravitational instability in the gas layer spread throughout the galaxy. Models for the [[collision theory]] have shown it cannot be the main mechanism for cloud formation due to the very long timescale it would take to form a molecular cloud, beyond the average lifespan of such structures.<ref name=":lada91" /><ref name=":Physics439" />

Gravitational instability is likely to be the main mechanism. Those regions with more gas will exert a greater gravitational force on their neighboring regions, and draw surrounding material. This extra material increases the density, increasing their gravitational attraction. [[Mathematical model]]s of gravitational instability in the gas layer predict a formation time within the timescale for the estimated cloud formation time.<ref name=":lada91" /><ref name=":Physics439" />

Once a molecular cloud assembles enough mass, the densest regions of the structure will start to [[Gravitational collapse|collapse]] under gravity, creating [[Star formation|star-forming]] clusters. This process is highly destructive to the cloud itself. Once stars are formed, they begin to [[Ionization|ionize]] portions of the cloud around it due to their heat. The ionized gas then evaporates and is dispersed in formations called ‘[[Champagne flow model|champagne flows]]’.<ref>{{Cite journal |last=Tenorio-Tagle |first=G. |date=1979-01-01 |title=The gas dynamics of H II regions. I. The champagne model. |url=https://ui.adsabs.harvard.edu/abs/1979A&A....71...59T |journal=Astronomy and Astrophysics |volume=71 |pages=59–65 |bibcode=1979A&A....71...59T |issn=0004-6361}}</ref> This process begins when approximately 2% of the mass of the cloud has been converted into stars. [[Stellar wind]]s are also known to contribute to cloud dispersal. The cycle of cloud formation and destruction is closed when the gas dispersed by stars cools again and is pulled into new clouds by gravitational instability.<ref name=":Physics439" />