View source for Astronomy:Star formation

{{short description|Process by which dense regions of molecular clouds in interstellar space collapse to form stars}}
[[File:PIA23865-W51Nebula-StarFactory-20200825.jpg|thumb|right|The W51 nebula in Aquila - one of the largest star factories in the Milky Way (August 25, 2020)]]
'''Star formation''' is the process by which dense regions within [[Astronomy:Molecular cloud|molecular cloud]]s in The "medium" is present further soon.-->interstellar space, sometimes referred to as "stellar nurseries" or "[[Astronomy:Star|star]]-forming regions", [[Astronomy:Jeans instability|collapse]] and form [[Astronomy:Star|star]]s.<ref>{{Cite book|title = The Formation of Stars|author1=Stahler, S. W.  |author2=Palla, F. |name-list-style=amp |publisher = Wiley-VCH|year = 2004|isbn = 3-527-40559-3|location = Weinheim}}</ref> As a branch of [[Astronomy:Astronomy|astronomy]], star formation includes the study of the [[Astronomy:Interstellar medium|interstellar medium]] (ISM) and giant molecular clouds (GMC) as precursors to the star formation process, and the study of [[Astronomy:Protostar|protostar]]s and [[Astronomy:Young stellar object|young stellar object]]s as its immediate products.  It is closely related to planet formation, another branch of [[Astronomy:Astronomy|astronomy]].  Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of [[Astronomy:Binary star|binary star]]s and the [[Astronomy:Initial mass function|initial mass function]]. Most stars do not form in isolation but as part of a group of stars referred  as [[Astronomy:Star cluster|star cluster]]s or [[Astronomy:Stellar association|stellar association]]s.<ref>{{Cite journal|last1=Lada|first1=Charles J.|last2=Lada|first2=Elizabeth A.|date=2003-09-01|title=Embedded Clusters in Molecular Clouds|journal=Annual Review of Astronomy and Astrophysics|volume=41|issue=1|pages=57–115|doi=10.1146/annurev.astro.41.011802.094844|issn=0066-4146|arxiv=astro-ph/0301540|bibcode=2003ARA&A..41...57L|s2cid=16752089}}</ref>

==Stellar nurseries==

===Interstellar clouds===

[[Spiral galaxy|Spiral galaxies]] like the Milky Way contain [[Astronomy:Star|star]]s, stellar remnants, and a diffuse [[Astronomy:Interstellar medium|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% [[Chemistry:Hydrogen|hydrogen]], 28% [[Chemistry:Helium|helium]], and 1.5% [[Astronomy:Metallicity|heavier elements]] by mass. The trace amounts of heavier elements were and are produced within stars via [[Physics:Stellar nucleosynthesis|stellar nucleosynthesis]] and ejected as the stars pass beyond the end of their [[Astronomy:Main sequence|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, [[Astronomy: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
 | 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|[[Astronomy:Hubble Space Telescope|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 [[Astronomy:Molecular cloud|molecular cloud]]s.<ref name=prialnik /> The [[Astronomy:Herschel Space Observatory|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 [[Astronomy: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 [[Astronomy:Milky Way|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 [[Astronomy:Sun|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 [[Astronomy:Rho Ophiuchi cloud complex|ρ Ophiuchi cloud complex]].<ref>{{cite book
 | author=Wilking, B. A. | author2=Gagné, M. | author3=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
 | 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 [[Astronomy:Bok globule|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 [[Astronomy: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 [[Astronomy:Emission nebula|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 [[Astronomy:Hydrostatic equilibrium|hydrostatic equilibrium]] as long as the [[Physics:Kinetic energy|kinetic energy]] of the gas [[Physics:Pressure|pressure]] is in balance with the [[Physics:Potential energy|potential energy]] of the internal gravitational force. Mathematically this is expressed using the [[Physics:Virial theorem|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 [[Astronomy:Gravitational collapse|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 [[Astronomy:Embedded cluster|embedded clusters]]. The end product of a core collapse is an  [[Astronomy:Open cluster|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|[[Astronomy: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 [[Astronomy:Gravitational collapse|gravitational collapse]]. Molecular clouds may collide with each other, or a nearby [[Astronomy:Supernova|supernova]] explosion can be a trigger, sending [[Physics:Shock wave|shocked]] matter into the cloud at very high speeds.<ref name=prialnik /> (The resulting new stars may themselves soon produce supernovae, producing [[Astronomy:SSPSF model|self-propagating star formation]].)  Alternatively, [[Astronomy:Interacting galaxy|galactic collisions]] can trigger massive starbursts of star formation as the gas clouds in each galaxy are compressed and agitated by [[Astronomy: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 [[Astronomy:Globular cluster|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 [[Astronomy:Supermassive black hole|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=0
  | 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 
|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=0
   | 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 [[Physics:Potential energy|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 [[Physics:Turbulence|turbulence]], macroscopic flows, [[Physics:Rotation|rotation]], [[Physics:Magnetic fields|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>

==Protostar==
{{main|Astronomy:Protostar}}
[[Image:LH 95.jpg|thumb|right|[[Astronomy:LH 95|LH 95]] stellar nursery in Large Magellanic Cloud.]]
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
 | first=M. S. | last=Longair | date=2008 | page=478
 | title=Galaxy Formation  | edition=2nd
 | publisher=Springer | isbn=978-3-540-73477-2 }}</ref>

During the collapse, the density of the cloud increases towards 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/>
[[Image:Cepheus B.jpg|thumb|left|Composite image showing young stars in and around molecular cloud Cepheus B.]]
When the core temperature reaches about {{nowrap|2000 K}}, the thermal energy dissociates the H<sub>2</sub> molecules.<ref name=larson>{{cite journal
 | last=Larson | first=Richard B. | date=1969
 | title=Numerical calculations of the dynamics of collapsing proto-star | journal=[[Organization:Monthly Notices of the Royal Astronomical Society|Monthly Notices of the Royal Astronomical Society]]
 | volume=145 | issue=3 | pages=271–295 | bibcode=1969MNRAS.145..271L | doi = 10.1093/mnras/145.3.271 | doi-access=free }}</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
 | first=Maurizio | last=Salaris
 | editor=Cassisi, Santi | title=Evolution of stars and stellar populations
 | url=https://archive.org/details/evolutionofstars0000sala | url-access=registration | date=2005 | publisher=John Wiley and Sons
 | pages=[https://archive.org/details/evolutionofstars0000sala/page/108 108–109] | isbn=0-470-09220-3 }}</ref> After the density of infalling material has reached 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 [[Physics:Pressure|pressure]] to support the protostar against further gravitational collapse—a state called [[Astronomy:Hydrostatic equilibrium|hydrostatic equilibrium]]. When this accretion phase is nearly complete, the resulting object is known as a [[Astronomy:Protostar|protostar]].<ref name=prialnik />
[[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.]]
Accretion of material onto the protostar continues partially from the newly formed [[Astronomy:Circumstellar disc|circumstellar disc]]. When the density and temperature are high enough, [[Astronomy:Deuterium fusion|deuterium fusion]] begins, and the outward [[Astronomy:Radiation pressure|pressure]] of the resultant radiation slows (but does not stop) the collapse. Material comprising the cloud continues to "rain" onto the [[Astronomy:Protostar|protostar]]. In this stage bipolar jets are produced called [[Astronomy:Herbig–Haro object|Herbig–Haro object]]s. This is probably the means by which excess [[Physics:Angular momentum|angular momentum]] of the infalling material is expelled, allowing the star to continue to form.
[[File:Star formation region Lupus 3.jpg|left|thumb|Star formation region Lupus 3.<ref>{{cite web|title=Glory From Gloom|url=https://www.eso.org/public/news/eso1804/|website=www.eso.org|access-date=2 February 2018}}</ref>]]
When the surrounding gas and dust envelope disperses and accretion process stops, the star is considered a [[Astronomy:Pre-main-sequence star|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 [[Astronomy:Hayashi track|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 | date=1961 | volume=13 | pages=450–452 | bibcode=1961PASJ...13..450H }}</ref> The contraction will proceed until the [[Astronomy:Hayashi limit|Hayashi limit]] is reached, and thereafter contraction will continue on a [[Astronomy:Kelvin–Helmholtz mechanism|Kelvin–Helmholtz timescale]] with the temperature remaining stable. Stars with less than {{Solar mass|0.5}} 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 [[Astronomy:Henyey track|Henyey track]].<ref>{{cite journal | author = L. G. Henyey | author2 = R. Lelevier | author3 = R. D. Levée | title=The Early Phases of Stellar Evolution | journal=Publications of the Astronomical Society of the Pacific | date=1955 | volume=67 | issue=396 | pages=154 | bibcode=1955PASP...67..154H  | doi = 10.1086/126791 | doi-access=free }}</ref>

Finally, [[Chemistry:Hydrogen|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 [[Astronomy:Main sequence|main sequence]] phase on the H–R diagram.

The stages of the process are well defined in stars with masses around {{Solar mass|1}} 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 is studied in [[Astronomy:Stellar evolution|stellar evolution]].
{| class="wikitable" style="margin:0.5em auto; width:600px;"
! [[Astronomy:Protostar|Protostar]]
|-
| style="font-size:88%" | [[File:PIA18928-Protostar-HOPS383-20150323.jpg|600px]]
{{center|Protostar outburst - [[Astronomy:HOPS 383|HOPS 383]] (2015).}}
|}

==Observations==
[[Image:Orion Nebula - Hubble 2006 mosaic 18000.jpg|thumb|left|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.]]
Key elements of star formation are only available by observing in [[Wavelength|wavelength]]s other than the [[Astronomy:Visible-light 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 GMC. Often, these star-forming cocoons known as [[Astronomy:Bok globule|Bok globule]]s, can be seen in [[Silhouette|silhouette]] against bright emission from surrounding gas.<ref>{{cite journal | bibcode=1947ApJ...105..255B | author=B. J. Bok | author2=E. F. Reilly | name-list-style=amp | title=Small Dark Nebulae | journal=Astrophysical Journal | date = 1947 | volume = 105 | pages=255 | doi=10.1086/144901 }}<br />{{cite journal | doi=10.1086/185891 | title=Star formation in small globules – Bart BOK was correct | date=1990 | author=Yun, Joao Lin | journal=The Astrophysical Journal | volume=365 | pages=L73 | last2=Clemens | first2=Dan P. | bibcode=1990ApJ...365L..73Y| doi-access=free }}</ref> Early stages of a star's life can be seen in [[Astronomy:Infrared astronomy|infrared]] light, which penetrates the dust more easily than [[Astronomy: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 | date= 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. | last10=Kobulnicky | first10=Henry A. | last11=Lazarian | first11=Alex | last12=Marston | first12=A. P. | last13=Mathis | first13=John S. | last14=Meade | first14=Marilyn R. | last15=Seager | first15=Sara | last16=Stolovy | first16=S. R. | last17=Watson | first17=C. | last18=Whitney | first18=Barbara A. | last19=Wolff | first19=Michael J. | last20=Wolfire | first20=Mark G. | bibcode=2003PASP..115..953B| s2cid=119510724 | display-authors=8 }}</ref>  
Observations from the [[Astronomy:Wide-field Infrared Survey Explorer|Wide-field Infrared Survey Explorer]] (WISE) have thus been especially important for unveiling numerous galactic protostars and their parent [[Astronomy:Star cluster|star cluster]]s.<ref name=wright>{{cite web|url=http://wise.ssl.berkeley.edu/ |title=Wide-field Infrared Survey Explorer Mission |publisher=NASA}}</ref><ref name=ma2013>Majaess, D. (2013). [http://adsabs.harvard.edu/abs/2013Ap&SS.344..175M ''Discovering protostars and their host clusters via WISE''], ApSS, 344, 1 ([http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=J%2Fother%2FApSS%2F344%2E175 ''VizieR catalog''])</ref>  Examples of such embedded star clusters are FSR 1184, FSR 1190, Camargo 14, Camargo 74, Majaess 64, and Majaess 98.<ref name=ca2015>Camargo et al. (2015). [http://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:1406.3099 ''New Galactic embedded clusters and candidates from a WISE Survey''], New Astronomy, 34</ref>
[[File:Star-forming region S106 (captured by the Hubble Space Telescope).jpg|thumb|Star-forming region S106.]]
The structure of the molecular cloud and the effects of the protostar can be observed in near-IR [[Astronomy:Extinction|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 [[Chemistry:Rotational transition|rotational transition]]s of [[Chemistry:Carbon monoxide|CO]] and other molecules; these last two are observed in the millimeter and [[Astronomy:Radio astronomy|submillimeter]] range. The radiation from the protostar and early star has to be observed in [[Physics:Infrared|infrared astronomy]] wavelengths, as the [[Astronomy:Extinction|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.

[[Image:NASA-FlameNebula-NGC2024-20140507.jpg|thumb|Young stars (purple) revealed by X-ray inside the [[Astronomy:Flame Nebula|NGC 2024]] star-forming region.<ref name= Getman14>{{Cite journal | last = Getman | first = K. | display-authors = etal | year= 2014| title = Core-Halo Age Gradients and Star Formation in the Orion Nebula and NGC 2024 Young Stellar Clusters | journal = Astrophysical Journal Supplement | volume = 787 | issue =2 | pages = 109 | doi = 10.1088/0004-637X/787/2/109| bibcode = 2014ApJ...787..109G|arxiv = 1403.2742 | s2cid = 118503957 }}</ref>]]
[[Astronomy:X-ray astronomy|X-ray]] observations have proven useful for studying young stars, since X-ray emission from these objects is about 100–100,000 times stronger than X-ray emission from main-sequence stars.<ref name=Preibisch05>{{Cite journal | last = Preibisch | first = T. | display-authors = etal | year= 2005| title = The Origin of T Tauri X-Ray Emission: New Insights from the Chandra Orion Ultradeep Project | journal = Astrophysical Journal Supplement | volume = 160 | issue =2 | pages = 401–422 | doi = 10.1086/432891| bibcode = 2005ApJS..160..401P|arxiv = astro-ph/0506526 | s2cid = 18155082 }}</ref> The earliest detections of X-rays from T Tauri stars were made by the [[Astronomy:Einstein Observatory|Einstein X-ray Observatory]].<ref name= Feigelson81>{{Cite journal | last1 = Feigelson | first1 = E. D. | last2 = Decampli| first2=W. M. | year= 1981| title = Observations of X-ray emission from T-Tauri stars | journal = Astrophysical Journal Letters | volume = 243 | pages = L89–L93 | doi = 10.1086/183449| bibcode = 1981ApJ...243L..89F}}</ref><ref name= Montmerle83>{{Cite journal | last = Montmerle | first = T. | display-authors = etal | year= 1983| title = Einstein observations of the Rho Ophiuchi dark cloud - an X-ray Christmas tree | journal = Astrophysical Journal, Part 1 | volume = 269 | pages = 182–201 | doi = 10.1086/161029| bibcode = 1983ApJ...269..182M}}</ref> For low-mass stars X-rays are generated by the heating of the stellar corona through [[Astronomy:Magnetic reconnection|magnetic reconnection]], while for high-mass [[Astronomy:O-type star|O]] and early B-type stars X-rays are generated through supersonic shocks in the stellar winds. Photons in the soft X-ray energy range covered by the [[Astronomy:Chandra X-ray Observatory|Chandra X-ray Observatory]] and [[Astronomy:XMM-Newton|XMM-Newton]] may penetrate the interstellar medium with only moderate absorption due to gas, making the X-ray a useful wavelength for seeing the stellar populations within molecular clouds. X-ray emission as evidence of stellar youth makes this band particularly useful for performing censuses of stars in star-forming regions, given that not all young stars have infrared excesses.<ref name=feigelson13>{{Cite journal | last = Feigelson | first = E. D. | display-authors = etal | year= 2013| title = Overview of the Massive Young Star-Forming Complex Study in Infrared and X-Ray (MYStIX) Project | journal = Astrophysical Journal Supplement | volume = 209 | issue =2 | pages = 26 | doi = 10.1088/0067-0049/209/2/26| bibcode = 2013ApJS..209...26F|arxiv = 1309.4483 | s2cid = 56189137 }}</ref> X-ray observations have provided near-complete censuses of all stellar-mass objects in the Orion Nebula Cluster and [[Astronomy:Taurus Molecular Cloud|Taurus Molecular Cloud]].<ref name=Getman05>{{Cite journal | last = Getman | first = K. V. | display-authors = etal | year= 2005| title = Chandra Orion Ultradeep Project: Observations and Source Lists | journal = Astrophysical Journal Supplement | volume = 160 | issue =2 | pages = 319–352 | doi = 10.1086/432092| bibcode = 2005ApJS..160..319G|arxiv = astro-ph/0410136 | s2cid = 19965900 }}</ref><ref name=Gudel07>{{Cite journal | last = Güdel | first = M. | display-authors = etal | year= 2007| title = The XMM-Newton extended survey of the Taurus molecular cloud (XEST) | journal = Astronomy and Astrophysics | volume = 468 | issue =2 | pages = 353–377 | doi = 10.1051/0004-6361:20065724| bibcode = 2007A&A...468..353G|arxiv = astro-ph/0609160 | s2cid = 8846597 }}</ref>

The formation of individual stars can only be directly observed in the [[Astronomy:Milky Way|Milky Way Galaxy]], but in distant galaxies star formation has been detected through its unique [[Gas chromatography–mass spectrometry|spectral signature]].

Initial research indicates star-forming clumps start as giant, dense areas in turbulent gas-rich matter in young galaxies, live about 500 million years, and may migrate to the center of a galaxy, creating the central bulge of a galaxy.<ref>{{Cite web|title = Young Star-Forming Clump in Deep Space Spotted for First Time|website = Space.com|date = 10 May 2015|url = http://www.space.com/29333-star-forming-clump-discovery.html|access-date = 2015-05-11}}</ref>

On February 21, 2014, [[Organization:NASA|NASA]] announced a [http://www.astrochem.org/pahdb/ greatly upgraded database] for tracking [[Astronomy:Polycyclic aromatic hydrocarbon|polycyclic aromatic hydrocarbon]]s (PAHs) in the [[Astronomy:Universe|universe]]. According to scientists, more than 20% of the [[Chemistry:Carbon|carbon]] in the universe may be associated with PAHs, possible [[Biology:PAH world hypothesis|starting materials]] for the [[Biology:Abiogenesis#PAH world hypothesis|formation]] of life. PAHs seem to have been formed shortly after the [[Astronomy:Big Bang|Big Bang]], are widespread throughout the universe, and are associated with new stars and [[Astronomy:Exoplanet|exoplanet]]s.<ref name="NASA-201409221">{{cite web |last=Hoover |first=Rachel |title=Need to Track Organic Nano-Particles Across the Universe? NASA's Got an App for That |url=http://www.nasa.gov/ames/need-to-track-organic-nano-particles-across-the-universe-nasas-got-an-app-for-that/ |date=February 21, 2014 |work=[[Organization:NASA|NASA]] |access-date=February 22, 2014 }}</ref>

In February 2018, astronomers reported, for the first time, a signal of the [[Astronomy:Reionization|reionization]] epoch, an indirect detection of light from the earliest stars formed - about 180 million years after the [[Astronomy:Big Bang|Big Bang]].<ref name="NAT-20180228">{{cite journal |last=Gibney |first=Elizabeth |title=Astronomers detect light from the Universe's first stars - Surprises in signal from cosmic dawn also hint at presence of dark matter. |url=https://www.nature.com/articles/d41586-018-02616-8 |date=February 28, 2018 |journal=Nature |access-date=February 28, 2018 |doi=10.1038/d41586-018-02616-8 }}</ref>

An article published on October 22, 2019, reported on the detection of [[Astronomy:3MM-1|3MM-1]], a massive star-forming galaxy about 12.5 billion light-years away that is obscured by clouds of [[Astronomy:Cosmic dust|dust]].<ref name="WilliamsLabbe2019">{{cite journal|last1=Williams|first1=Christina C.|last2=Labbe|first2=Ivo|last3=Spilker|first3=Justin|last4=Stefanon|first4=Mauro|last5=Leja|first5=Joel|last6=Whitaker|first6=Katherine|last7=Bezanson|first7=Rachel|last8=Narayanan|first8=Desika|last9=Oesch|first9=Pascal|last10=Weiner|first10=Benjamin|title=Discovery of a Dark, Massive, ALMA-only Galaxy at z ∼ 5–6 in a Tiny 3 mm Survey|journal=The Astrophysical Journal|volume=884|issue=2|year=2019|pages=154|issn=1538-4357|doi=10.3847/1538-4357/ab44aa|arxiv=1905.11996|bibcode=2019ApJ...884..154W|s2cid=168169681 |doi-access=free }}</ref> At a mass of about 10<sup>10.8</sup> [[Astronomy:Solar mass|solar mass]]es, it showed a star formation rate about 100 times as high as in the [[Astronomy:Milky Way|Milky Way]].<ref name="UAnews">{{Cite web|url=https://uanews.arizona.edu/story/cosmic-yeti-dawn-universe-found-lurking-dust|title=Cosmic Yeti from the Dawn of the Universe Found Lurking in Dust|author=University of Arizona|website=UANews|date=22 October 2019|language=en|access-date=2019-10-22}}</ref>

===Notable pathfinder objects===
*[[Astronomy:MWC 349|MWC 349]] was first discovered in 1978, and is estimated to be only 1,000 years old.
*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.<ref name="AndreWard-Thompson1993">{{cite journal |last1=Andre |first1=Philippe |last2=Ward-Thompson |first2=Derek |last3=Barsony |first3=Mary |title=Submillimeter continuum observations of Rho Ophiuchi A - The candidate protostar VLA 1623 and prestellar clumps |journal=The Astrophysical Journal |volume=406 |year=1993 |pages=122–141 |issn=0004-637X |doi=10.1086/172425 |bibcode=1993ApJ...406..122A|doi-access=free }}</ref>
*[[Astronomy:L1014|L1014]] – An extremely 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 [[Astronomy:Brown dwarf|brown dwarf]]s or even [[Astronomy:Rogue planet|rogue planet]]s).<ref>{{cite journal |last1=Bourke |first1=Tyler L. |last2=Crapsi |first2=Antonio |last3=Myers |first3=Philip C. |display-authors=0 |title=Discovery of a Low-Mass Bipolar Molecular Outflow from L1014-IRS with the Submillimeter Array |journal=The Astrophysical Journal |volume=633 |issue=2 |pages=L129 |year=2005 |doi=10.1086/498449 |arxiv = astro-ph/0509865 |bibcode = 2005ApJ...633L.129B |s2cid=14721548 }}</ref>
*[[Astronomy:GCIRS 8*|GCIRS 8*]] – The youngest known [[Astronomy:Main sequence|main sequence]] star in the [[Galactic Center]] region, discovered in August 2006. It is estimated to be 3.5 million years old.<ref name="GeballeNajarro2006">{{cite journal |last1=Geballe |first1=T. R. |last2=Najarro |first2=F. |last3=Rigaut |first3=F. |last4=Roy |first4=J.-R. |title=The K-Band Spectrum of the Hot Star in IRS 8: An Outsider in the Galactic Center? |journal=The Astrophysical Journal |volume=652 |issue=1 |year=2006 |pages=370–375 |issn=0004-637X |doi=10.1086/507764 |bibcode=2006ApJ...652..370G |arxiv=astro-ph/0607550|s2cid=9998286 }}</ref>

==Low mass and high mass star formation==
[[File:Infrared Image of Dark Cloud in Aquila.jpg|thumb|Star-forming region [[Astronomy:Westerhout 40|Westerhout 40]] and the [[Astronomy:Serpens-Aquila Rift|Serpens-Aquila Rift]]- cloud filaments containing new stars fill the region.<ref name=kuhn10>{{Cite journal| last = Kuhn | first = M. A. | display-authors = etal | year=2010 | title = A Chandra Observation of the Obscured Star-forming Complex W40 | journal = Astrophysical Journal | volume = 725 | issue=2 | pages = 2485–2506 | doi=10.1088/0004-637X/725/2/2485 | bibcode=2010ApJ...725.2485K|arxiv = 1010.5434 | s2cid = 119192761 }}</ref><ref name=andre10>{{Cite journal |last=André |first=Ph. |display-authors=0 |date=2010 |title= From filamentary clouds to prestellar cores to the stellar IMF: Initial highlights from the Herschel Gould Belt Survey |journal=Astronomy & Astrophysics |volume=518 |pages=L102 |doi=10.1051/0004-6361/201014666 |bibcode= 2010A&A...518L.102A|arxiv = 1005.2618 |s2cid=248768 }}</ref>
]]
Stars of different masses are thought to form by slightly different mechanisms.  The theory of low-mass star formation, which is well-supported by observation, 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 {{Solar mass|8}}, however, the mechanism of star formation is not well understood.

Massive stars emit copious quantities of radiation which pushes against infalling material.  In the past, it was thought that this [[Astronomy:Radiation pressure|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 | author2 = J. P. Cassinelli | title = Conditions for the formation of massive stars | journal = Astrophysical Journal | date = 1987 | volume = 319 | issue = 1 | pages = 850–867 | bibcode = 1987ApJ...319..850W  | doi = 10.1086/165503| doi-access = free }}</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 | author2 = J. C. Tan | title = Massive star formation in 100,000 years from turbulent and pressurized molecular clouds | journal = Nature | date = 2002 | volume = 416 | issue = 6876 | pages = 59–61 | bibcode = 2002Natur.416...59M  | doi = 10.1038/416059a | pmid = 11882889|arxiv = astro-ph/0203071 | s2cid = 4330710 }}</ref><ref>{{cite journal | author = R. Banerjee | author2 = R. E. Pudritz | title = Massive star formation via high accretion rates and early disk-driven outflows | journal = Astrophysical Journal | date = 2007 | volume = 660 | issue = 1 | pages = 479–488 | bibcode = 2007ApJ...660..479B  | doi = 10.1086/512010|arxiv = astro-ph/0612674 | s2cid = 9769562 }}</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.

There is mounting evidence that at least some massive protostars are indeed surrounded by accretion disks.<ref name=rab>{{Cite journal |last1=Burns |first1=R. A. |last2=Uno |first2=Y. |last3=Sakai |first3=N. |last4=Blanchard |first4=J. |last5=Rosli |first5=Z. |last6=Orosz |first6=G. |last7=Yonekura |first7=Y. |last8=Tanabe |first8=Y. |last9=Sugiyama |first9=K. |last10=Hirota |first10=T. |last11=Kim |first11=Kee-Tae |last12=Aberfelds |first12=A. |last13=Volvach |first13=A. E. |last14=Bartkiewicz |first14=A. |last15=Caratti o Garatti |first15=A. |date=May 2023 |title=A Keplerian disk with a four-arm spiral birthing an episodically accreting high-mass protostar |url=https://www.nature.com/articles/s41550-023-01899-w |journal=Nature Astronomy |language=en |volume=7 |issue=5 |pages=557–568 |doi=10.1038/s41550-023-01899-w |s2cid=257252773 |issn=2397-3366|arxiv=2304.14740 }}</ref>  Disk accretion in high-mass protostars, similar to their low-mass counterparts, is expected to exhibit bursts of episodic accretion as a result of a gravitationally instability leading to clumpy and in-continuous accretion rates. Recent evidence of accretion bursts in high-mass protostars has indeed been confirmed observationally.<ref name=rab/><ref>{{Cite journal |last1=Caratti o Garatti |first1=A. |last2=Stecklum |first2=B. |last3=Garcia Lopez |first3=R. |last4=Eislöffel |first4=J. |last5=Ray |first5=T. P. |last6=Sanna |first6=A. |last7=Cesaroni |first7=R. |last8=Walmsley |first8=C. M. |last9=Oudmaijer |first9=R. D. |last10=de Wit |first10=W. J. |last11=Moscadelli |first11=L. |last12=Greiner |first12=J. |last13=Krabbe |first13=A. |last14=Fischer |first14=C. |last15=Klein |first15=R. |date=March 2017 |title=Disk-mediated accretion burst in a high-mass young stellar object |url=https://www.nature.com/articles/nphys3942 |journal=Nature Physics |language=en |volume=13 |issue=3 |pages=276–279 |doi=10.1038/nphys3942 |issn=1745-2481|arxiv=1704.02628 }}</ref><ref>{{Cite journal |last1=Hunter |first1=T. R. |last2=Brogan |first2=C. L. |last3=MacLeod |first3=G. |last4=Cyganowski |first4=C. J. |last5=Chandler |first5=C. J. |last6=Chibueze |first6=J. O. |last7=Friesen |first7=R. |last8=Indebetouw |first8=R. |last9=Thesner |first9=C. |last10=Young |first10=K. H. |date=2017-03-15 |title=An Extraordinary Outburst in the Massive Protostellar System NGC 6334I-MM1: Quadrupling of the Millimeter Continuum |journal=The Astrophysical Journal |volume=837 |issue=2 |pages=L29 |doi=10.3847/2041-8213/aa5d0e |doi-access=free |issn=2041-8213|arxiv=1701.08637 }}</ref> 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 | author2 = M. R. Bate | author3 = C. J. Clarke| author4 = J. E. Pringle | title = Accretion and the stellar mass spectrum in small clusters | journal = Monthly Notices of the Royal Astronomical Society | date = 1997 | volume = 285 | issue = 1 | pages = 201–208 | bibcode = 1997MNRAS.285..201B | doi=10.1093/mnras/285.1.201| doi-access = free }}</ref><ref>{{cite journal | author = I. A. Bonnell | author2 = M. R. Bate | title = Star formation through gravitational collapse and competitive accretion | journal = Monthly Notices of the Royal Astronomical Society | date = 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 | s2cid = 15652967 }}</ref>

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 | author2 = M. R. Bate | author3 = H. Zinnecker | title = On the formation of massive stars | journal = Monthly Notices of the Royal Astronomical Society | date = 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 | s2cid = 119346630 }}</ref>

==See also==
* {{annotated link|Physics:Accretion (astrophysics)|Accretion}}
* {{annotated link|Astronomy:Champagne flow model}}
* {{annotated link|Astronomy:Chronology of the universe}}
* {{annotated link|Astronomy:Formation and evolution of the Solar System}} 
* {{annotated link|Astronomy:Galaxy formation and evolution}}

==References==
{{Reflist|30em}}

{{Star}}
{{Star formation navbox}}

[[Category:Star formation| ]]
[[Category:Stellar astronomy]]

{{Sourceattribution|Star formation}}