Editing Star formation (section)

==Stellar nurseries==

===Interstellar clouds===

[[Spiral galaxy|Spiral galaxies]] like the Milky Way contain [[star]]s, [[stellar remnant]]s, and a diffuse [[interstellar medium]] (ISM) of gas and dust. The interstellar medium consists of 10<sup>4</sup> to 10<sup>6</sup> particles per cm<sup>3</sup>, and is typically composed of roughly 70% [[hydrogen]], 28% [[helium]], and 1.5% [[Metallicity|heavier elements]] by mass. The trace amounts of heavier elements were and are produced within stars via [[stellar nucleosynthesis]] and ejected as the stars pass beyond the end of their [[main sequence]] lifetime. Higher density regions of the interstellar medium form clouds, or ''[[diffuse nebulae]]'',<ref>{{cite web | first=C. R. | last=O'Dell | title=Nebula | work=World Book at NASA | url=http://www.nasa.gov/worldbook/nebula_worldbook.html | archive-url=https://web.archive.org/web/20050429002503/http://www.nasa.gov/worldbook/nebula_worldbook.html |  url-status=dead | archive-date=2005-04-29 | publisher=World Book, Inc. | access-date=2009-05-18 }}</ref> where star formation takes place.<ref name=prialnik>{{cite book
 | first=Dina | last=Prialnik | title=An Introduction to the Theory of Stellar Structure and Evolution
 | pages=195–212 | date=2000
 | publisher=Cambridge University Press | isbn=0-521-65065-8
 | no-pp=true }}</ref> In contrast to spiral galaxies, [[elliptical galaxy|elliptical galaxies]] lose the cold component{{definition needed|date=August 2023}} of its interstellar medium within roughly a billion years, which hinders the galaxy from forming diffuse nebulae except through  mergers with other galaxies.<ref>{{cite conference
 | author=Dupraz, C. | author2=Casoli, F. | author2-link = Fabienne Casoli
 | chapter=The Fate of the Molecular Gas from Mergers to Ellipticals | title=Dynamics of Galaxies and Their Molecular Cloud Distributions: Proceedings of the 146th Symposium of the International Astronomical Union
 | date=June 4–9, 1990 | location=Paris, France
 | publisher=Kluwer Academic Publishers
 | bibcode=1991IAUS..146..373D
}}</ref>

[[Image:Eagle nebula pillars.jpg|thumb|left|[[Hubble Space Telescope]] image known as ''[[Pillars of Creation]],'' where stars are forming in the [[Eagle Nebula]]]]
In the dense nebulae where stars are produced, much of the hydrogen is in the molecular (H<sub>2</sub>) form, so these nebulae are called [[molecular cloud]]s.<ref name=prialnik /> The [[Herschel Space Observatory]] has revealed that filaments, or elongated dense gas structures, are truly ubiquitous in molecular clouds and central to the star formation process. They fragment into gravitationally bound cores, most of which will evolve into stars. Continuous accretion of gas, geometrical bending{{Definition needed|date=August 2023}}<!-- Is this pure bending, for which there's an article? -->, and magnetic fields may control the detailed manner in which the filaments are fragmented. Observations of supercritical filaments have revealed quasi-periodic chains of dense cores with spacing comparable to the filament inner width, and embedded protostars with outflows.{{technical inline|date=August 2023}}<ref>{{cite journal |last1=Zhang |first1=Guo-Yin |last2=André |first2=Ph |last3=Men'shchikov |first3=A. |last4=Wang |first4=Ke |title=Fragmentation of star-forming filaments in the X-shaped nebula of the California molecular cloud |journal=Astronomy and Astrophysics |date=October 2020 |volume=642 |pages=A76 |doi=10.1051/0004-6361/202037721 |url=https://ui.adsabs.harvard.edu/abs/2020A%26A...642A..76Z/abstract |language=en |issn=0004-6361|arxiv=2002.05984 |bibcode=2020A&A...642A..76Z |s2cid=211126855 }}</ref>

Observations indicate that the coldest clouds tend to form low-mass stars, which are first observed via the infrared light they emit inside the clouds, and then as visible light when the clouds dissipate. Giant molecular clouds, which are generally warmer, produce stars of all masses.<ref>{{cite book | first=James | last=Lequeux | title=Birth, Evolution and Death of Stars | publisher=World Scientific | date=2013 | isbn=978-981-4508-77-3}}</ref> These giant molecular clouds have typical densities of 100 particles per cm<sup>3</sup>, diameters of {{convert|100|ly|km|lk=on}}, masses of up to 6&nbsp;million [[solar mass|solar masses ({{Solar mass}})]], or six million times the mass of Earth's sun.<ref>{{cite conference
 | author =Williams, J. P. | author2 =Blitz, L. | author3 =McKee, C. F.
 | chapter=The Structure and Evolution of Molecular Clouds: from Clumps to Cores to the IMF | page=97
 | title=Protostars and Planets IV | date=2000
 | bibcode=2000prpl.conf...97W
 | arxiv=astro-ph/9902246 }}</ref> The average interior temperature is {{convert|10|K|F|lk=on}}.

About half the total mass of the [[Milky Way]]'s galactic ISM is found in molecular clouds<ref>{{cite book
 | author=Alves, J. | author2=Lada, C. | author3=Lada, E.
 | chapter=Tracing H<sub>2</sub> Via Infrared Dust Extinction
 | title=Molecular hydrogen in space | date=2001 | publisher=Cambridge University Press 
| page=217 | isbn=0-521-78224-4
 }}</ref> and the galaxy includes an estimated 6,000 molecular clouds, each with more than {{Solar mass|100,000}}.<ref>{{cite journal
 | author=Sanders, D. B. | author2=Scoville, N. Z. | author3=Solomon, P. M.
 | title=Giant molecular clouds in the Galaxy. II – Characteristics of discrete features 
 | journal=Astrophysical Journal, Part 1
 | volume=289 | date=1985-02-01 | pages=373–387
 | doi=10.1086/162897 | bibcode=1985ApJ...289..373S
}}</ref> The nebula nearest to the [[Sun]] where massive stars are being formed is the [[Orion Nebula]], {{convert|1300|ly|km}} away.<ref>{{cite journal | doi=10.1086/520922
 | title=A Parallactic Distance of <math>389^{+24}_{-21}</math> Parsecs to the Orion Nebula Cluster from Very Long Baseline Array Observations
 | date=2007 | author=Sandstrom, Karin M.
 | journal=The Astrophysical Journal | volume=667 | issue=2
 | pages=1161 | bibcode=2007ApJ...667.1161S | arxiv=0706.2361
 | last2=Peek | first2=J. E. G. 
 | last3=Bower | first3=Geoffrey C. 
 | last4=Bolatto | first4=Alberto D.
 | last5=Plambeck | first5=Richard L.
 | s2cid=18192326 }}</ref> However, lower mass star formation is occurring about 400–450 light-years distant in the [[Rho Ophiuchi cloud complex|ρ Ophiuchi cloud complex]].<ref>{{cite book
 | author=Wilking, B. A. | author2=Gagné, M. | author3=Allen, L. E.|author3-link=Lori Allen (astronomer)
 | chapter=Star Formation in the ρ Ophiuchi Molecular Cloud
 | editor=Bo Reipurth | title=Handbook of Star Forming Regions, Volume II: The Southern Sky ASP Monograph Publications
 | arxiv=0811.0005 | bibcode=2008hsf2.book..351W | year=2008
 }}</ref>

A more compact site of star formation is the opaque clouds of dense gas and dust known as [[Bok globule]]s, so named after the astronomer [[Bart Bok]]. These can form in association with collapsing molecular clouds or possibly independently.<ref>{{cite journal
 | author=Khanzadyan, T. | author2=Smith, M. D.
 | author3=Gredel, R. | author4=Stanke, T.
 | author5=Davis, C. J. | doi=10.1051/0004-6361:20011531
 | title=Active star formation in the large Bok globule CB 34
 | journal=Astronomy and Astrophysics | volume=383 | issue=2
 | pages=502–518 |date=February 2002 | bibcode=2002A&A...383..502K| doi-access=free
 }}</ref> The Bok globules are typically up to a light-year across and contain a few [[solar mass|solar masses]].<ref>{{cite book
 | first=Lee | last=Hartmann | date=2000
 | title=Accretion Processes in Star Formation | page=4
 | publisher=Cambridge University Press | isbn=0-521-78520-0 }}</ref> They can be observed as dark clouds silhouetted against bright [[emission nebula]]e or background stars. Over half the known Bok globules have been found to contain newly forming stars.<ref>{{cite book | first=Michael David | last=Smith | date=2004
 | pages=43–44 | title=The Origin of Stars
 | publisher=Imperial College Press | isbn=1-86094-501-5 }}</ref>
[[File:ALMA witnesses assembly of galaxy in early Universe (annotated).jpg|thumb|Assembly of galaxy in early Universe.<ref>{{cite web|title=ALMA Witnesses Assembly of Galaxies in the Early Universe for the First Time|url=http://www.eso.org/public/news/eso1530/|access-date=23 July 2015}}</ref>]]

===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>