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| {{Star formation}}
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| '''Star formation''' is the process by which dense regions within [[molecular cloud]]s in [[<!--Interstellar medium|The "medium" is present further soon.-->interstellar space]], sometimes referred to as "stellar nurseries" or "star-forming regions", collapse to form [[star]]s. As a branch of [[astronomy]], star formation includes the study of the [[interstellar medium]] and [[giant molecular cloud]]s (GMC) as precursors to the star formation process, and the study of [[protostar]]s and [[young stellar object]]s as its immediate products. It is closely related to [[planet formation]], another branch of [[astronomy]]. Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of [[binary star]]s and the [[initial mass function]].
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| ==Stellar nurseries==
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| [[Image:Eagle nebula pillars.jpg|thumb|left|[[Hubble telescope]] image known as ''[[Pillars of Creation]],'' where stars are forming in the [[Eagle Nebula]].]]
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| ===Interstellar clouds===
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| A [[spiral galaxy]] like the [[Milky Way]] contains [[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]] by mass, with most of the remaining gas consisting of [[helium]]. This medium has been chemically enriched by trace amounts of [[Metallicity|heavier elements]] that were ejected from stars as they passed beyond the end of their [[main sequence]] lifetime. Higher density regions of the interstellar medium form clouds, or ''[[nebula|diffuse nebulae]]'',<ref>{{cite web
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| | first=C. R. | last=O'Dell
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| | title=Nebula | work=World Book at NASA | url=http://www.nasa.gov/worldbook/nebula_worldbook.html | publisher=World Book, Inc. | |
| | accessdate=2009-05-18 }}{{dead link|date=May 2012}}</ref> where star formation takes place.<ref name=prialnik>{{cite book
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| | first=Dina | last=Prialnik | title=An Introduction to the Theory of Stellar Structure and Evolution
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| | pages=195–212 | year=2000
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| | publisher=Cambridge University Press | isbn=0-521-65065-8
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| | nopp=true }}</ref> In contrast to spirals, an [[elliptical galaxy]] loses the cold component 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.; Casoli, F. | |
| | title=The Fate of the Molecular Gas from Mergers to Ellipticals | booktitle=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
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| | publisher=Kluwer Academic Publishers | |
| | bibcode=1991IAUS..146..373D
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| }}</ref>
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| 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 /> Observations indicate that the coldest clouds tend to form low-mass stars, observed first in the infrared inside the clouds, then in visible light at their surface when the clouds dissipate, while 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 | year=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 million [[solar mass]]es,<ref>{{cite conference
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| | author =Williams, J. P.; Blitz, L.; McKee, C. F. | |
| | title=The Structure and Evolution of Molecular Clouds: from Clumps to Cores to the IMF | page=97 | |
| | booktitle=Protostars and Planets IV | year=2000
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| | bibcode=2000prpl.conf...97W
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| | arxiv=astro-ph/9902246 }}</ref> and an average interior temperature of 10 K. About half the total mass of the galactic ISM is found in molecular clouds<ref>{{cite book | |
| | author=Alves, J.; Lada, C.; Lada, E. | |
| | title=Tracing H<sub>2</sub> Via Infrared Dust Extinction | |
| | booktitle=Molecular hydrogen in space | year=2001 | |
| | publisher=Cambridge University Press | page=217 | |
| | isbn=0-521-78224-4 }}</ref> and in the [[Milky Way]] there are an estimated 6,000 molecular clouds, each with more than 100,000 solar masses.<ref>{{cite journal | |
| | author=Sanders, D. B.; Scoville, N. Z.; Solomon, P. M. | |
| | title=Giant molecular clouds in the Galaxy. II – Characteristics of discrete features | journal=Astrophysical Journal, Part 1
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| | volume=289 | date=1985-02-01 | pages=373–387 | |
| | doi=10.1086/162897 | bibcode=1985ApJ...289..373S}}</ref> The nearest nebula to the [[Sun]] where massive stars are being formed is the [[Orion nebula]], {{convert|1300|ly|km|abbr=on}} away.<ref>{{cite journal | doi=10.1086/520922
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| | title=A Parallactic Distance of <math>389^{+24}_{-21}</math> Parsecs to the Orion Nebula Cluster from Very Long Baseline Array Observations | |
| | year=2007 | author=Sandstrom, Karin M. | |
| | journal=The Astrophysical Journal | volume=667 | |
| | pages=1161 | bibcode=2007ApJ...667.1161S | arxiv=0706.2361}}</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.; Gagné, M.; Allen, L. E. | |
| | 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
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| | arxiv=0811.0005
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| | bibcode=2008hsf2.book..351W | |
| }}</ref>
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| 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
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| | author=Khanzadyan, T.; Smith, M. D.; Gredel, R.; Stanke, T.; Davis, C. J. | doi=10.1051/0004-6361:20011531
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| | title=Active star formation in the large Bok globule CB 34
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| | journal=Astronomy and Astrophysics | volume=383
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| | issue=2
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| | pages=502–518 |date=February 2002 | bibcode=2002A&A...383..502K}}</ref> The Bok globules are typically up to a light year across and contain a few [[solar mass]]es.<ref>{{cite book
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| | first=Lee | last=Hartmann | year=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 | year=2004 | |
| | pages=43–44 | title=The Origin of Stars | |
| | publisher=Imperial College Press | isbn=1-86094-501-5 }}</ref> | |
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| ===Empty space===
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| [[File:The moment the lights went out.jpg|thumb|2MASX J09442693+0429569 marks a transitional phase in this process as young, star-forming galaxies settle to become massive, red and dead galaxies.<ref>{{cite news|title=The moment the lights went out|url=http://www.spacetelescope.org/images/potw1304a/|accessdate=12 February 2013|newspaper=ESA/Hubble Picture of the Week}}</ref>]]
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| A discovery by the infrared telescope [[Herschel Space Observatory|Herschel]] in conjunction with other ground based telescopes, determined that black patches of space in certain areas encompassing a star formation were not [[dark nebulae]] but actually vast holes of empty space. Such is the case of the area [[NGC 1999]] and its star [[V380 Orionis]]. The exact cause of this phenomenon is still being investigated, although it has been hypothesized that narrow jets of gas from some of the young stars in the region punctured the sheet of dust and gas, as well as powerful radiation from a nearby mature star may have helped to create the hole. This was a previously unknown and unexpected step in the star-forming process.<ref>[http://www.msnbc.msn.com/id/37088640/ns/technology_and_science-space/ Telescope discovers surprising hole in space], MSNBC, by [[Space.com]], 11-05-2010</ref>
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| ===Cloud collapse===
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| [[Image:Star-forming region.jpg|thumb|right|Stellar cluster and [[Stellar nursery|star-forming region]] [[Messier 17|M 17]].]]
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| 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
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| | first=Sun | last=Kwok | year=2006
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| | title=Physics and chemistry of the interstellar medium
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| | publisher=University Science Books
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| | isbn=1-891389-46-7 | pages=435–437
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| }}</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 /> This coincides with the typical mass of an [[open cluster]] of stars, which is the end product of a collapsing cloud.<ref>{{cite book | first=E. | last=Battaner
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| | title=Astrophysical Fluid Dynamics | |
| | publisher=Cambridge University Press | year=1996 | |
| | isbn=0-521-43747-4 | pages=166–167 }}</ref> | |
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| 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 /> 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
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| | last=Jog | first=C. J. | date=August 26–30, 1997 | |
| | editor=Barnes, J. E.; Sanders, D. B.
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| | title=Starbursts Triggered by Cloud Compression in Interacting Galaxies | booktitle=Proceedings of IAU Symposium #186, Galaxy Interactions at Low and High Redshift
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| | location=Kyoto, Japan
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| | bibcode=1999IAUS..186..235J }}</ref> The latter mechanism may be responsible for the formation of [[globular cluster]]s.<ref>{{cite journal | |
| | author=Keto, Eric; Ho, Luis C.; Lo, K.-Y.
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| | title=M82, Starbursts, Star Clusters, and the Formation of Globular Clusters | |
| | journal=The Astrophysical Journal | volume=635
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| | issue=2 | pages=1062–1076 |date=December 2005 | doi=10.1086/497575
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| | bibcode=2005ApJ...635.1062K |arxiv = astro-ph/0508519 }}</ref>
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| 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. 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
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| | author=van Breugel, Wil | coauthors=''et al''. | |
| | editors=T. Storchi-Bergmann, L.C. Ho, and Henrique R. Schmitt | title=The Interplay among Black Holes, Stars and ISM in Galactic Nuclei | |
| | booktitle=Proceedings of IAU Symposium, No. 222 | |
| | publisher=Cambridge University Press |date=November 2004 | pages=485–488 | |
| | doi=10.1017/S1743921304002996 | |
| | month=2004IAUS..222..485V }}</ref> | |
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| 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
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| | first=Dina | last=Prialnik
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| | title=An Introduction to the Theory of Stellar Structure and Evolution | |
| | publisher=Cambridge University Press | year=2000 | |
| | isbn=0-521-65937-X | pages=198–199 | |
| }}</ref>
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| 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
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| | first=Lee | last=Hartmann | year=2000 | |
| | title=Accretion Processes in Star Formation | |
| | publisher=Cambridge University Press | |
| | isbn=0-521-78520-0 | page=22 | |
| }}</ref><ref>{{cite arXiv
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| | author=Li, Hua-bai; Dowell, C. Darren; Goodman, Alyssa; Hildebrand, Roger; Novak, Giles | |
| | title=Anchoring Magnetic Field in Turbulent Molecular Clouds | |
| | date=2009-08-11
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| | eprint=0908.1549
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| | class=astro-ph.GA }}</ref> Turbulence is instrumental in causing fragmentation of the cloud, and on the smallest scales it promotes collapse.<ref>{{cite book
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| | author=Ballesteros-Paredes, J.; Klessen, R. S.; Mac Low, M.-M.; Vazquez-Semadeni, E. | |
| | editor=Reipurth, B.; Jewitt, D.; Keil, K.
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| | chapter=Molecular Cloud Turbulence and Star Formation
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| | title=Protostars and Planets V | pages=63–80
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| | isbn=0-8165-2654-0 }}</ref>
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| ==Protostar==
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| {{main|Protostar}}
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| [[Image:LH 95.jpg|thumb|right|[[LH 95]] stellar nursery in Large Magellanic Cloud.]]
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| [[Image:Cepheus B.jpg|thumb|[[Composite image]] showing young stars in and around molecular cloud [[Cepheus (constellation)|Cepheus]] B.]]
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| [[Image:N11 (Hubble).jpg|thumb|right| N11, part of a complex network of gas clouds and star clusters within our neighbouring galaxy, the Large Magellanic Cloud.]]
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| A protostellar cloud will continue to collapse as long as the gravitational binding energy can be eliminated. This excess energy is primarily lost through radiation. However, the collapsing cloud will eventually become opaque to its own radiation, and the energy must be removed through some other means. The dust within the cloud becomes heated to temperatures of {{nowrap|60–100 K}}, and these particles radiate at wavelengths in the far infrared where the cloud is transparent. Thus the dust mediates the further collapse of the cloud.<ref>{{cite book
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| | first=M. S. | last=Longair | year=2008 | page=478
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| | title=Galaxy Formation | edition=2nd
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| | publisher=Springer | isbn=3-540-73477-5 }}</ref>
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| During the collapse, the density of the cloud increases toward the center and thus the middle region becomes optically opaque first. This occurs when the density is about {{nowrap|10<sup>−13</sup> g cm<sup>−3</sup>}}. A core region, called the First Hydrostatic Core, forms where the collapse is essentially halted. It continues to increase in temperature as determined by the virial theorem. The gas falling toward this opaque region collides with it and creates shock waves that further heat the core.<ref name=larson/>
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| When the core temperature reaches about {{nowrap|2000 K}}, the thermal energy dissociates the H<sub>2</sub> molecules.<ref name=larson>{{cite journal
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| | last=Larson | first=Richard B. | year=1969
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| | title=Numerical calculations of the dynamics of collapsing proto-star | journal=Monthly Notices of the Royal Astronomical Society
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| | volume=145 | pages=271 | bibcode=1969MNRAS.145..271L }}</ref> This is followed by the ionization of the hydrogen and helium atoms. These processes absorb the energy of the contraction, allowing it to continue on timescales comparable to the period of collapse at free fall velocities.<ref>{{cite book
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| | first=Maurizio | last=Salaris
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| | editor=Cassisi, Santi | title=Evolution of stars and stellar populations
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| | year=2005 | publisher=John Wiley and Sons
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| | pages=108–109 | isbn=0-470-09220-3 }}</ref> After the density of infalling material has dropped below about 10<sup>−8</sup> g cm<sup>−3</sup>, that material is sufficiently transparent to allow energy radiated by the protostar to escape. The combination of convection within the protostar and radiation from its exterior allow the star to contract further.<ref name=larson/> This continues until the gas is hot enough for the internal [[pressure]] to support the protostar against further gravitational collapse—a state called [[hydrostatic equilibrium]]. When this accretion phase is nearly complete, the resulting object is known as a [[protostar]].<ref name=prialnik />
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| Accretion of material onto the protostar continues partially from the newly formed [[circumstellar disc]]. When the density and temperature are high enough, [[deuterium burning|deuterium fusion]] begins, and the outward [[radiation pressure|pressure]] of the resultant radiation slows (but does not stop) the collapse. Material comprising the cloud continues to "rain" onto the [[protostar]]. In this stage bipolar jets are produced called [[Herbig-Haro objects]]. This is probably the means by which excess [[angular momentum]] of the infalling material is expelled, allowing the star to continue to form.
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| When the surrounding gas and dust envelope disperses and accretion process stops, the star is considered a [[pre–main sequence star]] (PMS star). The energy source of these objects is gravitational contraction, as opposed to hydrogen burning in main sequence stars. The PMS star follows a [[Hayashi track]] on the [[Hertzsprung–Russell diagram|Hertzsprung–Russell (H–R) diagram]].<ref>{{cite journal | author = C. Hayashi | title=Stellar evolution in early phases of gravitational contraction | journal=Publications of the Astronomical Society of Japan | year=1961 | volume=13 | pages=450–452 | bibcode=1961PASJ...13..450H }}</ref> The contraction will proceed until the [[Hayashi limit]] is reached, and thereafter contraction will continue on a [[Kelvin–Helmholtz mechanism|Kelvin–Helmholtz timescale]] with the temperature remaining stable. Stars with less than 0.5 [[solar mass]]es thereafter join the main sequence. For more massive PMS stars, at the end of the Hayashi track they will slowly collapse in near hydrostatic equilibrium, following the [[Henyey track]].<ref>{{cite journal | author = L. G. Henyey, R. Lelevier, R. D. Levée | title=The Early Phases of Stellar Evolution | journal=Publications of the Astronomical Society of the Pacific | year=1955 | volume=67 | issue=396
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| | pages=154 | bibcode=1955PASP...67..154H | doi = 10.1086/126791
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| }}</ref>
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| Finally, [[hydrogen]] begins to fuse in the core of the star, and the rest of the enveloping material is cleared away. This ends the protostellar phase and begins the star's [[main sequence]] phase on the H–R diagram.
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| The stages of the process are well defined in stars with masses around one [[solar mass]] or less. In high mass stars, the length of the star formation process is comparable to the other timescales of their evolution, much shorter, and the process is not so well defined. The later evolution of stars are studied in [[stellar evolution]].
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| ==Observations==
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| [[Image:Orion Nebula - Hubble 2006 mosaic 18000.jpg|thumb|right|The [[Orion Nebula]] is an archetypical example of star formation, from the massive, young stars that are shaping the nebula to the pillars of dense gas that may be the homes of budding stars.]]
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| Key elements of star formation are only available by observing in [[wavelength]]s other than the [[optical astronomy|optical]]. The protostellar stage of stellar existence is almost invariably hidden away deep inside dense clouds of gas and dust left over from the [[giant molecular cloud|GMC]]. Often, these star-forming cocoons known as [[Bok globule]]s, can be seen in [[silhouette]] against bright emission from surrounding gas.<ref>{{cite journal | bibcode=1947ApJ...105..255B | author=B. J. Bok & E. F. Reilly | title=Small Dark Nebulae | journal=Astrophysical Journal | year = 1947 | volume = 105 | pages=255 | format=PDF | doi=10.1086/144901 }}<br />{{cite journal | doi=10.1086/185891 | title=Star formation in small globules – Bart BOK was correct | year=1990 | author=Yun, Joao Lin | journal=The Astrophysical Journal | volume=365 | pages=L73 | last2=Clemens | first2=Dan P. | bibcode=1990ApJ...365L..73Y}}</ref> Early stages of a star's life can be seen in [[infrared astronomy|infrared]] light, which penetrates the dust more easily than [[visible-light astronomy|visible]] light.<ref>{{cite journal | doi= 10.1086/376696 | arxiv=astro-ph/0306274 | title= GLIMPSE. I. An ''SIRTF'' Legacy Project to Map the Inner Galaxy | year= 2003 | author= Benjamin, Robert A. | journal= Publications of the Astronomical Society of the Pacific | volume= 115 | issue= 810 | pages= 953–964 | last2= Churchwell | first2= E. | last3= Babler | first3= Brian L. | last4= Bania | first4= T. M. | last5= Clemens | first5= Dan P. | last6= Cohen | first6= Martin | last7= Dickey | first7= John M. | last8= Indebetouw | first8= Rémy | last9= Jackson | first9= James M. | bibcode=2003PASP..115..953B}}</ref>
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| The structure of the molecular cloud and the effects of the protostar can be observed in near-IR [[extinction (astronomy)|extinction]] maps (where the number of stars are counted per unit area and compared to a nearby zero extinction area of sky), continuum dust emission and [[rotational transition]]s of [[Carbon monoxide|CO]] and other molecules; these last two are observed in the millimeter and [[radio astronomy|submillimeter]] range. The radiation from the protostar and early star has to be observed in [[infrared|infrared astronomy]] wavelengths, as the [[extinction (astronomy)|extinction]] caused by the rest of the cloud in which the star is forming is usually too big to allow us to observe it in the visual part of the spectrum. This presents considerable difficulties as the Earth's atmosphere is almost entirely opaque from 20μm to 850μm, with narrow windows at 200μm and 450μm. Even outside this range, atmospheric subtraction techniques must be used.
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| The formation of individual stars can only be directly observed in [[Milky Way|our Galaxy]], but in distant galaxies star formation has been detected through its unique [[Gas chromatography-Mass spectrometry|spectral signature]].
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| ===Notable pathfinder objects===
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| *[[MWC 349]] was first discovered in 1978, and is estimated to be only 1,000 years old.
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| *VLA 1623 – The first exemplar Class 0 protostar, a type of embedded protostar that has yet to accrete the majority of its mass. Found in 1993, is possibly younger than 10,000 years [http://www.newscientist.com/article/mg13718613.200-science-youngest-star.html].
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| *[[L1014]] – An incredibly faint embedded object representative of a new class of sources that are only now being detected with the newest telescopes. Their status is still undetermined, they could be the youngest low-mass Class 0 protostars yet seen or even very low-mass evolved objects (like a [[brown dwarf]] or even an [[interstellar planet]]). [http://www.sciencenews.org/articles/20041113/fob5.asp].
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| *[[IRS 8*]] – The youngest known [[main sequence]] star in the [[Galactic Center]] region, discovered in August 2006. It is estimated to be 3.5 million years old [http://www.newscientistspace.com/article.ns?id=dn9738&feedId=space_rss20].
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| ==Low mass and high mass star formation==
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| [[File:Star-forming region S106 (captured by the Hubble Space Telescope).jpg|thumb|Star-forming region S106.]]
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| Stars of different masses are thought to form by slightly different mechanisms. The theory of low-mass star formation, which is well-supported by a plethora of observations, suggests that low-mass stars form by the gravitational collapse of rotating density enhancements within molecular clouds. As described above, the collapse of a rotating cloud of gas and dust leads to the formation of an accretion disk through which matter is channeled onto a central protostar. For stars with masses higher than about 8 solar masses, however, the mechanism of star formation is not well understood.
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| Massive stars emit copious quantities of radiation which pushes against infalling material. In the past, it was thought that this [[radiation pressure]] might be substantial enough to halt accretion onto the massive protostar and prevent the formation of stars with masses more than a few tens of solar masses.<ref>{{cite journal | author = M. G. Wolfire, J. P. Cassinelli | title = Conditions for the formation of massive stars | journal = Astrophysical Journal | year = 1987 | volume = 319 | issue = 1 | pages = 850–867 | bibcode = 1987ApJ...319..850W | doi = 10.1086/165503}}</ref> Recent theoretical work has shown that the production of a jet and outflow clears a cavity through which much of the radiation from a massive protostar can escape without hindering accretion through the disk and onto the protostar.<ref>{{cite journal | author = C. F. McKee, J. C. Tan | title = Massive star formation in 100,000 years from turbulent and pressurized molecular clouds | journal = Nature | year = 2002 | volume = 416 | issue = 6876 | pages = 59–61 | bibcode = 2002Natur.416...59M | doi = 10.1038/416059a | pmid = 11882889|arxiv = astro-ph/0203071 }}</ref><ref>{{cite journal | author = R. Banerjee, R. E. Pudritz | title = Massive star formation via high accretion rates and early disk-driven outflows | journal = Astrophysical Journal | year = 2007 | volume = 660 | issue = 1 | pages = 479–488 | bibcode = 2007ApJ...660..479B | doi = 10.1086/512010|arxiv = astro-ph/0612674 }}</ref> Present thinking is that massive stars may therefore be able to form by a mechanism similar to that by which low mass stars form.
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| There is mounting evidence that at least some massive protostars are indeed surrounded by accretion disks. Several other theories of massive star formation remain to be tested observationally. Of these, perhaps the most prominent is the theory of competitive accretion, which suggests that massive protostars are "seeded" by low-mass protostars which compete with other protostars to draw in matter from the entire parent molecular cloud, instead of simply from a small local region.<ref>{{cite journal | author = I. A. Bonnell, M. R. Bate, C. J. Clarke, J. E. Pringle | title = Accretion and the stellar mass spectrum in small clusters | journal = Monthly Notices of the Royal Astronomical Society | year = 1997 | volume = 285 | issue = 1 | pages = 201–208 | bibcode = 1997MNRAS.285..201B | last2 = Bate | last3 = Clarke | last4 = Pringle }}</ref><ref>{{cite journal | author = I. A. Bonnell, M. R. Bate | title = Star formation through gravitational collapse and competitive accretion | journal = Monthly Notices of the Royal Astronomical Society | year = 2006 | volume = 370 | issue = 1 | pages = 488–494 | bibcode = 2006MNRAS.370..488B | doi = 10.1111/j.1365-2966.2006.10495.x |arxiv = astro-ph/0604615 }}</ref>
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| Another theory of massive star formation suggests that massive stars may form by the coalescence of two or more stars of lower mass.<ref>{{cite journal | author = I. A. Bonnell, M. R. Bate, H. Zinnecker | title = On the formation of massive stars | journal = Monthly Notices of the Royal Astronomical Society | year = 1998 | volume = 298 | issue = 1 | pages = 93–102 | bibcode = 1998MNRAS.298...93B | doi = 10.1046/j.1365-8711.1998.01590.x|arxiv = astro-ph/9802332 }}</ref>
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| ==See also==
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| * [[Galaxy formation and evolution]]
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| * [[Formation and evolution of the Solar System]]
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| * [[Structure formation]]
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| * [[Timeline of the Big Bang]]
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| * [[Chronology of the universe]]
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| * [[Big Bang]]
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| ==References==
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| {{Reflist|2}}
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| {{Star}}
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| [[Category:Star formation| ]]
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| [[Category:Stellar astronomy]]
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| {{Link FA|it}}
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