Pre-main-sequence star

{{Short description|Star in the stage when it has not yet reached the main sequence}} {{Refimprove|date=August 2014}} {{Star formation}}

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.<ref name=Larson_2003/><ref name=Comins_Kaufmann_2011/><ref name="Ward-Thompson_Whitworth_2011"/><ref name=Stahler_Palla_2004/>

An observed PMS object can either be a T Tauri star, if it has fewer than 2 solar masses ({{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 may have no observable pre-main-sequence stage because they contract too quickly as protostars and are surrounded by circumstellar gas. By the time they become visible, the hydrogen in their centers is already fusing and they are main-sequence objects.<ref name=Perryman_2009/> The creation large refracting telescopes has made it possible to study massive YSOs, which include protostars and PMS stars.<ref>{{Cite journal |last1=Kumar |first1=M. S. N. |last2=Peña |first2=C. Contreras |last3=Lucas |first3=P. W. |last4=Thompson |first4=M. A. |date=2016-12-05 |title=Highly Variable Young Massive Stars in Atlasgal Clumps |journal=The Astrophysical Journal |volume=833 |issue=1 |pages=24 |doi=10.3847/0004-637X/833/1/24 |doi-access=free |issn=1538-4357}}</ref><ref>{{Cite journal |last1=Ramírez-Tannus |first1=M. C. |last2=Kaper |first2=L. |last3=Koter |first3=A. de |last4=Tramper |first4=F. |last5=Bik |first5=A. |last6=Ellerbroek |first6=L. E. |last7=Ochsendorf |first7=B. B. |last8=Ramírez-Agudelo |first8=O. H. |last9=Sana |first9=H. |date=2017-08-01 |title=Massive pre-main-sequence stars in M17 |journal=Astronomy & Astrophysics |language=en |volume=604 |pages=A78 |doi=10.1051/0004-6361/201629503 |issn=0004-6361}}</ref><ref>{{Cite journal |last1=Derkink |first1=A. R. |last2=Ramírez-Tannus |first2=M. C. |last3=Kaper |first3=L. |last4=Koter |first4=A. de |last5=Backs |first5=F. |last6=Poorta |first6=J. |last7=Gelder |first7=M. L. van |date=2024-01-01 |title=Spectroscopic variability of massive pre-main-sequence stars in M17 |journal=Astronomy & Astrophysics |language=en |volume=681 |pages=A112 |doi=10.1051/0004-6361/202347369 |issn=0004-6361}}</ref>

The energy source of PMS objects is gravitational contraction and deuterium fusion,<ref name=Zwintz_Steindl_2022/> as opposed to 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 tracks as protostars, then leftward and horizontally along Henyey tracks until they finally halt at the main sequence. <ref>{{Cite journal |last=Henyey |first=L. G. |last2=Lelevier |first2=Robert |last3=Levée |first3=R. D. |date=1955 |title=The Early Phases of Stellar Evolution |url=https://iopscience.iop.org/article/10.1086/126791 |journal=Publications of the Astronomical Society of the Pacific |volume=67 |pages=154 |doi=10.1086/126791 |issn=0004-6280}}</ref><ref>{{Cite journal |last=Hayashi |first=Chushiro |last2=Hōshi |first2=Rěun |last3=Sugimoto |first3=Daiichiro |date=1962 |title=Evolution of the Stars |url=https://academic.oup.com/ptps/article-lookup/doi/10.1143/PTPS.22.1 |journal=Progress of Theoretical Physics Supplement |language=en |volume=22 |pages=165–171 |doi=10.1143/PTPS.22.1 |issn=0375-9687}}</ref> 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 lifetime of hydrogen fusion. During the early portion of the PMS stage, most stars have circumstellar disks, which are the sites of planet formation.

==References== <references> <ref name=Larson_2003>{{cite journal | title=The physics of star formation | first=Richard B. | last=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 name=Comins_Kaufmann_2011>{{cite book | title=Discovering the Universe | first1=Neil F. | last1=Comins | first2=William J. | last2=Kaufmann III | date=2011 | isbn=978-1-4292-5520-2 | page=350 | publisher=Macmillan }}</ref>

<ref name="Ward-Thompson_Whitworth_2011">{{cite book | title=An Introduction to Star Formation | first1=Derek | last1=Ward-Thompson | first2=Anthony P. | last2=Whitworth | isbn=978-1-107-62746-8 | page=119 | date=2011 | publisher=Cambridge University Press }}</ref>

<ref name=Stahler_Palla_2004>{{Cite book | title=The Formation of Stars | last1=Stahler | first1=S. W. | last2=Palla | first2=F. | publisher=Wiley-VCH | year=2004 | isbn=978-3-527-40559-6 | location=Weinheim }}</ref>

<ref name=Perryman_2009>{{cite book | title=Astronomical Applications of Astrometry: Ten Years of Exploitation of the Hipparcos Satellite Data | first=M. A. C. | last=Perryman | author-link=Michael Perryman | publisher=Cambridge University Press | year=2009 | isbn=978-0-521-51489-7 | page=414 | url=https://books.google.com/books?id=kvbLd6yCNikC&pg=PA414 }}</ref>

<ref name=Zwintz_Steindl_2022>{{cite journal | title=The Pre-main Sequence: Challenges and Prospects for Asteroseismology | last1=Zwintz | first1=Konstanze | last2=Steindl | first2=Thomas | journal=Frontiers in Astronomy and Space Sciences | volume=9 | at=id. 914738 | date=June 2022 | article-number=914738 | doi=10.3389/fspas.2022.914738 | doi-access=free | arxiv=2206.09171 | bibcode=2022FrASS...9.4738Z }}</ref>

</references>

Category:Star types Category:Star formation Category:Pre-main-sequence stars