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Pre-main-sequence star

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Star in the stage when it has not yet reached the main sequence

Star formation

Object classes
Interstellar medium Molecular cloud Bok globule Dark nebula Young stellar object Protostar Pre-main-sequence star T Tauri star Herbig Ae/Be star Herbig–Haro object
Theoretical concepts
Accretion Initial mass function Jeans instability Kelvin–Helmholtz mechanism Nebular hypothesis Planetary migration
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Apre-main-sequence star (also known as aPMS star andPMS object) is astar in the stage when it has not yet reached themain sequence. Earlier in its life, the object is aprotostar that grows by acquiring mass from its surrounding envelope of interstellar dust and gas. After theprotostar blows away this envelope, it is optically visible, and appears on thestellar birthline in theHertzsprung-Russell diagram. At this point, the star has acquired nearly all of its mass but has not yet startedhydrogen burning (i.e.nuclear fusion of hydrogen). The star continues to contract, its internal temperature rising until it begins hydrogen burning on thezero age main sequence. This period of contraction is the pre-main sequence stage.^[1]^[2]^[3]^[4] An observed PMS object can either be aT Tauri star, if it has fewer than 2solar masses (M_☉), or else aHerbig Ae/Be star, if it has 2 to 8M_☉. 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 aremain-sequence objects.

The energy source of PMS objects isgravitational contraction, as opposed tohydrogen burning in main-sequence stars. In theHertzsprung–Russell diagram, pre-main-sequence stars with more than 0.5M_☉ first move vertically downward alongHayashi tracks, then leftward and horizontally alongHenyey tracks, until they finally halt at themain sequence. Pre-main-sequence stars with less than 0.5M_☉ contract vertically along theHayashi 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 samestellar mass and thus has a lower surface gravity. Although they are optically visible, PMS objects are rare relative to those on themain sequence, because their contraction lasts for only 1 percent of the time required forhydrogen fusion. During the early portion of the PMS stage, most stars havecircumstellar disks, which are the sites ofplanet formation.

References

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^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.

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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
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