Editing Star (section)

===Star formation===
{{Main|Star formation}}
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 <!--image 1-->
 | image1            = Witness the Birth of a Star.jpg
 | alt1              =
 | caption1          = Artist's conception of the birth of a star within a dense [[molecular cloud]]
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 | image2            = W40_star-forming_region.jpg
 | alt2              =
 | caption2          = A cluster of approximately 500 young stars lies within the nearby [[Westerhout 40|W40]] stellar nursery.
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The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density—often triggered by compression of clouds by radiation from massive stars, expanding bubbles in the interstellar medium, the collision of different molecular clouds, or the [[Interacting galaxy|collision of galaxies]] (as in a [[starburst galaxy]]).<ref>
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</ref><ref>
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</ref> When a region reaches a sufficient density of matter to satisfy the criteria for [[Jeans instability]], it begins to collapse under its own gravitational force.<ref>
{{cite book
 | first=Michael David | last=Smith | date=2004
 | title=The Origin of Stars | url=https://archive.org/details/originstars00smit | url-access=limited | publisher=Imperial College Press
 | isbn=978-1-86094-501-4 | pages=[https://archive.org/details/originstars00smit/page/n70 57]–68}}
</ref>

As the cloud collapses, individual conglomerations of dense dust and gas form "[[Bok globule]]s". As a globule collapses and the density increases, the gravitational energy converts into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of [[hydrostatic equilibrium]], a [[protostar]] forms at the core.<ref>
{{cite web
 | last= Seligman | first= Courtney
 | url= http://courtneyseligman.com/text/stars/starevol2.htm
 | archive-url= https://web.archive.org/web/20080623190408/http://courtneyseligman.com/text/stars/starevol2.htm
 | archive-date= 2008-06-23
 | title= Slow Contraction of Protostellar Cloud | work= Self-published
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</ref> These [[pre-main-sequence star]]s are often surrounded by a [[protoplanetary disk]] and powered mainly by the conversion of gravitational energy. The period of gravitational contraction lasts about 10&nbsp;million years for a star like the sun, up to 100&nbsp;million years for a red dwarf.<ref name="Hanslmeier2010">{{cite book |author=Hanslmeier |first=Arnold |url=https://books.google.com/books?id=Mj5tSld5tjMC&pg=PA163 |title=Water in the Universe |date=2010 |publisher=Springer Science & Business Media |isbn=978-90-481-9984-6 |pages=163}}</ref>

Early stars of less than {{Solar mass|2}} are called [[T Tauri star]]s, while those with greater mass are [[Herbig Ae/Be star]]s. These newly formed stars emit jets of gas along their axis of rotation, which may reduce the [[angular momentum]] of the collapsing star and result in small patches of nebulosity known as [[Herbig–Haro object]]s.<ref>
{{cite conference |last1=Bally |first1=J. |last2=Morse |first2=J. |last3=Reipurth |first3=B. |date=1996 |editor1-last=Benvenuti |editor1-first=Piero |editor2-last=Macchetto |editor2-first=F. D. |editor3-last=Schreier |editor3-first=Ethan J. |title=The Birth of Stars: Herbig-Haro Jets, Accretion and Proto-Planetary Disks |conference=Space Science Institute Workshop, Paris, France, December 4–8, 1995 |publisher=Space Telescope Science Institute |page=491 |bibcode=1996swhs.conf..491B |book-title=Science with the Hubble Space Telescope – II. Proceedings of a workshop held in Paris, France, December 4–8, 1995}}
</ref><ref name=smith04>
{{cite book
 | first=Michael David | last=Smith
 | title=The origin of stars | url=https://archive.org/details/originstars00smit | url-access=limited | page=[https://archive.org/details/originstars00smit/page/n189 176] | date=2004
 | isbn=978-1-86094-501-4
 | publisher=Imperial College Press
}}
</ref>
These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud from which the star was formed.<ref>
{{cite news
 | first=Tom | last=Megeath | date=2010-05-11
 | title=Herschel finds a hole in space
 | url=http://www.esa.int/esaCP/SEMFEAKPO8G_index_0.html
 | publisher=ESA | access-date=2010-05-17}}
</ref>

Early in their development, T Tauri stars follow the [[Hayashi track]]—they contract and decrease in luminosity while remaining at roughly the same temperature. Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the [[Henyey track]].<ref name="Darling2004">{{cite book |author=Darling |first=David |url=https://books.google.com/books?id=L5zuAAAAMAAJ |title=The Universal Book of Astronomy: From the Andromeda Galaxy to the Zone of Avoidance |publisher=Wiley |year=2004 |isbn=978-0-471-26569-6 |page=229}}</ref>

Most stars are observed to be members of binary star systems, and the properties of those binaries are the result of the conditions in which they formed.<ref>
{{cite journal
 | last1=Duquennoy | first1=A.
 | last2=Mayor | first2= M.
 | title= Multiplicity among solar-type stars in the solar neighbourhood. II – Distribution of the orbital elements in an unbiased sample
 | journal= Astronomy & Astrophysics
 | date=1991 | volume= 248 | issue=2
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 | bibcode=1991A&A...248..485D
}}
</ref> A gas cloud must lose its angular momentum in order to collapse and form a star. The fragmentation of the cloud into multiple stars distributes some of that angular momentum. The primordial binaries transfer some angular momentum by gravitational interactions during close encounters with other stars in young stellar clusters. These interactions tend to split apart more widely separated (soft) binaries while causing hard binaries to become more tightly bound. This produces the separation of binaries into their two observed populations distributions.<ref name="Padmanabhan2000">{{cite book |author=Padmanabhan |first=T. |url=https://books.google.com/books?id=TOjwtYYb63cC&pg=PA557 |title=Theoretical Astrophysics: Volume 2, Stars and Stellar Systems |publisher=Cambridge University Press |year=2000 |isbn=978-0-521-56631-5 |pages=557}}</ref>