Editing Star formation (section)

===Cloud collapse===
An interstellar cloud of gas will remain in [[hydrostatic equilibrium]] as long as the [[kinetic energy]] of the gas [[pressure]] is in balance with the [[potential energy]] of the internal [[gravitational force]]. Mathematically this is expressed using the [[virial theorem]], which states that,  to maintain equilibrium, the gravitational potential energy must equal twice the internal thermal energy.<ref>{{cite book
 | first=Sun | last=Kwok | date=2006
 | title=Physics and chemistry of the interstellar medium
 | url=https://archive.org/details/physicschemistry0000kwok | url-access=registration | publisher=University Science Books 
 | isbn=1-891389-46-7 | pages=[https://archive.org/details/physicschemistry0000kwok/page/435 435–437]
}}</ref> If a cloud is massive enough that the gas pressure is insufficient to support it, the cloud will undergo [[gravitational collapse]]. The mass above which a cloud will undergo such collapse is called the [[Jeans mass]]. The Jeans mass depends on the temperature and density of the cloud, but is typically thousands to tens of thousands of solar masses.<ref name=prialnik /> During cloud collapse dozens to tens of thousands of stars form more or less simultaneously which is observable in so-called [[Embedded cluster|embedded clusters]]. The end product of a core collapse is an  [[open cluster]] of stars.<ref>{{cite book | first=E. | last=Battaner
 | title=Astrophysical Fluid Dynamics
 | publisher=Cambridge University Press | date=1996
 | isbn=0-521-43747-4 | pages=166–167 }}</ref>

[[File:ALMA views a stellar explosion in Orion.jpg|left|thumb|[[Atacama Large Millimeter Array|ALMA]] observations of the Orion Nebula complex provide insights into explosions at star birth.<ref>{{cite web|title=ALMA Captures Dramatic Stellar Fireworks|url=https://www.eso.org/public/news/eso1711/|website=www.eso.org|access-date=10 April 2017}}</ref>]]

In ''triggered star formation'', one of several events might occur to compress a molecular cloud and initiate its [[gravitational collapse]]. Molecular clouds may collide with each other, or a nearby [[supernova]] explosion can be a trigger, sending [[Shock wave|shocked]] matter into the cloud at very high speeds.<ref name=prialnik /> (The resulting new stars may themselves soon produce supernovae, producing [[SSPSF model|self-propagating star formation]].)  Alternatively, [[Interacting galaxy|galactic collisions]] can trigger massive [[starburst (astronomy)|starburst]]s of star formation as the gas clouds in each galaxy are compressed and agitated by [[galactic tide|tidal forces]].<ref>{{cite conference
 | last=Jog | first=C. J. | date=August 26–30, 1997
 | editor=Barnes, J. E. 
 | editor2=Sanders, D. B.
 | title=Starbursts Triggered by Cloud Compression in Interacting Galaxies | book-title=Proceedings of IAU Symposium #186, Galaxy Interactions at Low and High Redshift
 | location=Kyoto, Japan
 | bibcode=1999IAUS..186..235J }}</ref> The latter mechanism may be responsible for the formation of [[globular cluster]]s.<ref>{{cite journal
 | author=Keto, Eric
 | author2=Ho, Luis C.
 | author3=Lo, K.-Y.
 | title=M82, Starbursts, Star Clusters, and the Formation of Globular Clusters
 | journal=The Astrophysical Journal | volume=635
 | issue=2 | pages=1062–1076 |date=December 2005 | doi=10.1086/497575
 | bibcode=2005ApJ...635.1062K |arxiv = astro-ph/0508519 | s2cid=119359557
 }}</ref>

A [[supermassive black hole]] at the core of a galaxy may serve to regulate the rate of star formation in a galactic nucleus. A black hole that is accreting infalling matter can become [[Active galactic nucleus|active]], emitting a strong wind through a collimated [[relativistic jet]]. This can limit further star formation. Massive black holes ejecting radio-frequency-emitting particles at near-light speed can also block the formation of new stars in aging galaxies.<ref>{{cite journal
  | author=Gralla, Meg |display-authors=etal
  | title=A measurement of the millimetre emission and the Sunyaev–Zel'dovich effect associated with low-frequency radio sources
  | journal=Monthly Notices of the Royal Astronomical Society
  | issue=1
  | volume=445
  | publisher=Oxford University Press |date=September 29, 2014 | pages=460–478
  | doi=10.1093/mnras/stu1592 
|doi-access=free
 |arxiv = 1310.8281 |bibcode = 2014MNRAS.445..460G |s2cid=8171745
 }}</ref> However, the radio emissions around the jets may also trigger star formation. Likewise, a weaker jet may trigger star formation when it collides with a cloud.<ref>{{cite conference
  | author=van Breugel, Wil |display-authors=etal
   | editor=T. Storchi-Bergmann
   | editor2=L.C. Ho
   | editor3=Henrique R. Schmitt | title=The Interplay among Black Holes, Stars and ISM in Galactic Nuclei
  | publisher=Cambridge University Press |date=November 2004 | pages=485–488
  | doi=10.1017/S1743921304002996
  | bibcode=2004IAUS..222..485V |arxiv = astro-ph/0406668 }}</ref>

[[File:Size can be deceptive ESO 553-46.jpg|thumb|Dwarf galaxy [[ESO 553-46]] has one of the highest rates of star formation of the 1000 or so galaxies nearest to the Milky Way.<ref>{{cite web|title=Size can be deceptive|url=https://www.spacetelescope.org/images/potw1741a/|website=www.spacetelescope.org|access-date=9 October 2017}}</ref>]]

As it collapses, a molecular cloud breaks into smaller and smaller pieces in a hierarchical manner, until the fragments reach stellar mass. In each of these fragments, the collapsing gas radiates away the energy gained by the release of [[gravitational]] [[potential energy]].  As the density increases, the fragments become opaque and are thus less efficient at radiating away their energy. This raises the temperature of the cloud and inhibits further fragmentation. The fragments now condense into rotating spheres of gas that serve as stellar embryos.<ref>{{cite book
 | first=Dina | last=Prialnik
 | title=An Introduction to the Theory of Stellar Structure and Evolution
 | publisher=Cambridge University Press | date=2000
 | isbn=0-521-65937-X | pages=198–199
}}</ref>

Complicating this picture of a collapsing cloud are the effects of [[turbulence]], macroscopic flows, [[rotation]], [[magnetic fields]] and the cloud geometry. Both rotation and magnetic fields can hinder the collapse of a cloud.<ref>{{cite book
 | first=Lee | last=Hartmann | date=2000
 | title=Accretion Processes in Star Formation
 | publisher=Cambridge University Press
 | isbn=0-521-78520-0 | page=22
}}</ref><ref>{{Cite journal| author=Li, Hua-bai
 | author2=Dowell, C. Darren
 | author3=Goodman, Alyssa
 | author4=Hildebrand, Roger
 | author5=Novak, Giles
 | title=Anchoring Magnetic Field in Turbulent Molecular Clouds
 | journal=The Astrophysical Journal
 | volume=704
 | issue=2
 | pages=891
 | date=2009-08-11
 | arxiv=0908.1549
 | doi=10.1088/0004-637X/704/2/891
 |bibcode = 2009ApJ...704..891L | s2cid=118341372
 }}</ref> Turbulence is instrumental in causing fragmentation of the cloud, and on the smallest scales it promotes collapse.<ref>{{cite book
 | author=Ballesteros-Paredes, J.
 | author2=Klessen, R. S.
 | author3=Mac Low, M.-M.
 | author4=Vazquez-Semadeni, E.
 | editor=Reipurth, B.
 | editor2=Jewitt, D.
 | editor3=Keil, K.
 | chapter=Molecular Cloud Turbulence and Star Formation
 | title=Protostars and Planets V | pages=63–80
 | isbn=978-0-8165-2654-3 | year=2007
 | publisher=University of Arizona Press
 }}</ref>