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A pre-main-sequence star (also known as a PMS star and PMS object) is a star in the stage when it has not yet reached the main sequence. Earlier in its life, the object is a protostar that grows by acquiring mass from its surrounding envelope of interstellar dust and gas. After the protostar blows away this envelope, it is optically visible, and appears on the stellar birthline in the Hertzsprung-Russell diagram. At this point, the star has acquired nearly all of its mass but has not yet started hydrogen burning (i.e. nuclear fusion of hydrogen). The star continues to contract, its internal temperature rising until it begins hydrogen burning on the zero age main sequence. This period of contraction is the pre-main sequence stage.^[1]^[2]^[3]^[4] An observed PMS object can either be a T Tauri star, if it has fewer than 2 solar masses (M_☉), or else a Herbig Ae/Be star, if it has 2 to 8 M_☉. Yet more massive stars have no pre-main-sequence stage because they contract too quickly as protostars. By the time they become visible, the hydrogen in their centers is already fusing and they are main-sequence objects.

The energy source of PMS objects is gravitational contraction, as opposed to hydrogen burning in main-sequence stars. In the Hertzsprung–Russell diagram, pre-main-sequence stars with more than 0.5 M_☉ first move vertically downward along Hayashi tracks, then leftward and horizontally along Henyey tracks, until they finally halt at the main sequence. Pre-main-sequence stars with less than 0.5 M_☉ contract vertically along the Hayashi track for their entire evolution.

PMS stars can be differentiated empirically from main-sequence stars by using stellar spectra to measure their surface gravity. A PMS object has a larger radius than a main-sequence star with the same stellar mass and thus has a lower surface gravity. Although they are optically visible, PMS objects are rare relative to those on the main sequence, because their contraction lasts for only 1 percent of the time required for hydrogen fusion. During the early portion of the PMS stage, most stars have circumstellar disks, which are the sites of planet formation.

References

^ Richard B. Larson (10 September 2003). "The physics of star formation" (PDF). Reports on Progress in Physics. 66 (10): 1669–1673. arXiv:astro-ph/0306595. Bibcode:2003RPPh...66.1651L. doi:10.1088/0034-4885/66/10/r03. S2CID 18104309.
^ Neil F. Comins; William J. Kaufmann III (2011). Discovering the Universe. p. 350. ISBN 978-1-4292-5520-2.
^ Derek Ward-Thompson; Anthony P. Whitworth (2011). An Introduction to Star Formation. Cambridge University Press. p. 119. ISBN 978-1-107-62746-8.
^ Stahler, S. W.; Palla, F. (2004). The Formation of Stars. Weinheim: Wiley-VCH. ISBN 978-3-527-40559-6.

[1] Richard B. Larson (10 September 2003). "The physics of star formation" (PDF). Reports on Progress in Physics. 66 (10): 1669–1673. arXiv:astro-ph/0306595. Bibcode:2003RPPh...66.1651L. doi:10.1088/0034-4885/66/10/r03. S2CID 18104309.

[2] Neil F. Comins; William J. Kaufmann III (2011). Discovering the Universe. p. 350. ISBN 978-1-4292-5520-2.

[3] Derek Ward-Thompson; Anthony P. Whitworth (2011). An Introduction to Star Formation. Cambridge University Press. p. 119. ISBN 978-1-107-62746-8.

[4] Stahler, S. W.; Palla, F. (2004). The Formation of Stars. Weinheim: Wiley-VCH. ISBN 978-3-527-40559-6.

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+{{Short description|Star in the stage when it has not yet reached the main sequence}}
 {{Star formation}}
 {{Refimprove|date=August 2014}}
-A '''pre-main-sequence star''' (also known as a '''PMS star''' and '''PMS object''') is a [[star]] in the stage when it has not yet reached the [[main sequence]]. Earlier in its life, the object is a [[protostar]] that grows by acquiring mass from its surrounding envelope of interstellar dust and gas. After the [[protostar]] blows away this envelope, it is optically visible, and appears on the [[stellar birthline]] in the [[Hertzsprung–Russell diagram|Hertzsprung-Russell diagram]]. At this point, the star has acquired nearly all of its mass but has not yet started [[hydrogen burning]] (i.e. [[nuclear fusion]] of hydrogen). The star then contracts, its internal temperature rising until it begins hydrogen burning on the [[zero age main sequence]]. This period of contraction is the pre-main sequence stage.<ref>{{cite journal|url=http://www.ifa.hawaii.edu/~reipurth/reviews/larson.pdf|title=The physics of star formation|author=Richard B. Larson|date=10 September 2003|journal=Reports on Progress in Physics|volume=66|number=10|pages= 1669–73|doi=10.1088/0034-4885/66/10/r03 |arxiv = astro-ph/0306595 |bibcode = 2003RPPh...66.1651L }}</ref><ref>{{cite book|title= Discovering the Universe|author=Neil F. Comins|author2= William J. Kaufmann III|date=2011| isbn=978-1429255202|page=350}}</ref><ref>{{cite book|title=An Introduction to Star Formation |date=2011|author=Derek Ward-Thompson |author2=Anthony P. Whitworth |isbn=978-1107627468|page=119|publisher=Cambridge University Press}}</ref><ref>{{Cite book|title = The Formation of Stars|last = Stahler, S. W. and Palla, F.|publisher = Wiley-VCH|year = 2004|isbn = 3-527-40559-3|location = Weinheim}}</ref>  An observed PMS object can either be a [[T Tauri star]], if it has fewer than 2 [[solar mass]]es ({{Solar mass|link=y}}), or else a [[Herbig Ae/Be star]], if it has 2 to 8 {{Solar mass|link=y}}. Yet more massive stars have no pre-main-sequence stage because they contract too quickly as protostars. By the time they become visible, the hydrogen in their centers is already fusing and they are [[Main sequence|main-sequence]] objects.
+A '''pre-main-sequence  star''' (also known as a '''PMS star''' and '''PMS object''') is a [[star]] in the stage when it has not yet reached the [[main sequence]]. Earlier in its life, the object is a [[protostar]] that grows by acquiring mass from its surrounding envelope of interstellar dust and gas. After the [[protostar]] blows away this envelope, it is optically visible, and appears on the [[stellar birthline]] in the [[Hertzsprung–Russell diagram|Hertzsprung-Russell diagram]]. At this point, the star has acquired nearly all of its mass but has not yet started [[hydrogen burning]] (i.e. [[nuclear fusion]] of hydrogen). The star continues to contract, its internal temperature rising until it begins hydrogen burning on the [[zero age main sequence]]. This period of contraction is the pre-main sequence stage.<ref>{{cite journal|url=http://www.ifa.hawaii.edu/~reipurth/reviews/larson.pdf|title=The physics of star formation|author=Richard B. Larson|date=10 September 2003|journal=Reports on Progress in Physics|volume=66|number=10|pages= 1669–1673|doi=10.1088/0034-4885/66/10/r03 |arxiv = astro-ph/0306595 |bibcode = 2003RPPh...66.1651L |s2cid=18104309}}</ref><ref>{{cite book|title= Discovering the Universe|author=Neil F. Comins|author2= William J. Kaufmann III|date=2011| isbn=978-1-4292-5520-2|page=350}}</ref><ref>{{cite book|title=An Introduction to Star Formation |date=2011|author=Derek Ward-Thompson |author2=Anthony P. Whitworth |isbn=978-1-107-62746-8|page=119|publisher=[[Cambridge University Press]]}}</ref><ref>{{Cite book|title = The Formation of Stars|author1=Stahler, S. W.  |author2=Palla, F. |publisher = [[Wiley-VCH]]|year = 2004|isbn = 978-3-527-40559-6|location = Weinheim}}</ref>  An observed PMS object can either be a [[T Tauri star]], if it has fewer than 2 [[solar mass]]es ({{Solar mass|link=y}}), or else a [[Herbig Ae/Be star]], if it has 2 to 8 {{Solar mass|link=y}}. Yet more massive stars have no pre-main-sequence stage because they contract too quickly as protostars. By the time they become visible, the hydrogen in their centers is already fusing and they are [[Main sequence|main-sequence]] objects.
-The energy source of PMS objects is [[Kelvin-Helmholtz mechanism|gravitational contraction]], as opposed to [[Proton-proton chain reaction|hydrogen burning]] in main-sequence stars. In the [[Hertzsprung–Russell diagram]], pre-main-sequence stars with more than 0.5 {{Solar mass}} first move vertically downward along [[Hayashi track]]s, then leftward and horizontally along [[Henyey track]]s, until they finally halt at the [[main sequence]]. Pre-main-sequence stars with less than 0.5 {{Solar mass}} contract vertically along the [[Hayashi track]] for their entire evolution.
+The energy source of PMS objects is [[Kelvin-Helmholtz mechanism|gravitational contraction]], as opposed to [[proton–proton chain|hydrogen burning]] in main-sequence stars. In the [[Hertzsprung–Russell diagram]], pre-main-sequence stars with more than 0.5 {{Solar mass}} first move vertically downward along [[Hayashi track]]s, then leftward and horizontally along [[Henyey track]]s, until they finally halt at the [[main sequence]]. Pre-main-sequence stars with less than 0.5 {{Solar mass}} contract vertically along the [[Hayashi track]] for their entire evolution.
-PMS stars can be differentiated empirically from main-sequence stars by using stellar spectra to measure their surface gravity.  A PMS object has a larger radius than a main-sequence star with the same [[stellar mass]] and thus has a lower surface gravity. Although they are optically visible, PMS objects are rare relative to those on the [[main sequence]], because their contraction lasts for only 1 percent of the time required for [[hydrogen fusion]]. During the early portion of the PMS stage, most stars have [[circumstellar disk]]s, which are probable sites of [[planet formation]].
+PMS stars can be differentiated empirically from main-sequence stars by using stellar spectra to measure their surface gravity.  A PMS object has a larger radius than a main-sequence star with the same [[stellar mass]] and thus has a lower surface gravity. Although they are optically visible, PMS objects are rare relative to those on the [[main sequence]], because their contraction lasts for only 1 percent of the time required for [[hydrogen fusion]]. During the early portion of the PMS stage, most stars have [[circumstellar disk]]s, which are the sites of [[planet formation]].
 ==See also==
-{{Portal|Star}}
 * [[Protoplanetary disk]]
 * [[Protostar]]
+* [[Stellar evolution]]
 * [[Young stellar object]]
-* [[stellar birthline]]
 ==References==
 {{Reflist}}
 {{Star}}
+{{Star formation navbox}}
 {{DEFAULTSORT:Pre-Main-Sequence Star}}
 [[Category:Star types]]
 [[Category:Star formation]]
-[[Category:Hertzsprung–Russell classifications]]
 [[Category:Pre-main-sequence stars| ]]
-{{Astronomy-stub}}
-{{star-stub}}

v t e Star formation
Object classes	Interstellar medium Molecular cloud Bok globule Dark nebula Young stellar object Protostar T Tauri star Pre-main-sequence star Herbig Ae/Be star Herbig–Haro object
Theoretical concepts	Initial mass function Jeans instability Kelvin–Helmholtz mechanism Nebular hypothesis Planetary migration
Category Stars portal Commons