Editing Hyades (star cluster) (section)

== Morphology and evolution ==
All stars form in clusters, but most clusters break up less than 50 million years after star formation concludes.<ref name="LadaLada">{{cite journal | last1 = Lada | first1 = CJ | last2 = Lada | first2 = EA | year = 2003 | title = Embedded clusters in molecular clouds | journal = Annual Review of Astronomy & Astrophysics | volume = 41 | issue = 1 | pages = 57–115 |bibcode= 2003ARA&A..41...57L | doi=10.1146/annurev.astro.41.011802.094844|arxiv = astro-ph/0301540 | s2cid = 16752089 }}</ref> The astronomical term for this process is "[[Mass_segregation_(astronomy)#Evaporation|evaporation]]." Only extremely massive clusters, orbiting far from the [[Galactic Center]], can avoid evaporation over extended timescales.<ref name="PavaniBica 2007">{{cite journal | last1 = Pavani | first1 = DB | last2 = Bica | first2 = E | year = 2007 | title = Characterization of open cluster remnants | journal = Astronomy & Astrophysics | volume = 468 | issue = 1| pages = 139–150 | doi=10.1051/0004-6361:20066240|arxiv = 0704.1159 |bibcode = 2007A&A...468..139P | s2cid = 11609818 }}</ref> As one such survivor, the Hyades Cluster probably contained a much larger star population in its infancy. Estimates of its original mass range from 800 to 1,600 times the [[Solar mass|mass of the Sun]] ({{Solar mass|link=y}}), implying still larger numbers of individual stars.<ref name="Weidemann1992">Weideman V, Jordan S, Iben I, Casertano S.  (1992)  White dwarfs in the halo of the Hyades Cluster – The case of the missing white dwarfs. ''Astronomical Journal,'' 104: 1876-1891.  1992AJ....104.1876W.</ref><ref name="Kroupa">{{cite journal | last1 = Kroupa | first1 = P | last2 = Boily | first2 = CM | year = 2002 | title = On the mass function of star clusters | journal = Monthly Notices of the Royal Astronomical Society | volume = 336 | issue = 4| pages = 1188–1194 |bibcode=2002MNRAS.336.1188K | doi=10.1046/j.1365-8711.2002.05848.x| doi-access = free |arxiv = astro-ph/0207514 | s2cid = 15225436 }}</ref>

===Star populations===
Theory predicts that a young cluster of this size should give birth to stars and substellar objects of all spectral types, from huge, hot O stars down to dim [[brown dwarf]]s.<ref name="Kroupa" /> However, studies of the Hyades show that it is deficient in stars at both extremes of mass.<ref name="Bouvier" /><ref name="Bohm-Vitense2007">{{cite journal | last1 = Böhm-Vitense | first1 = E | year = 2007 | title = Hyades morphology and star formation | journal = Astronomical Journal | volume = 133 | issue = 5| pages = 1903–1910 | bibcode = 2007AJ....133.1903B | doi=10.1086/512124| doi-access = free }}</ref> At an age of 625 million years, the cluster's main sequence turn-off is about {{Solar mass|2.3|link=y}}, meaning that all heavier stars have evolved into subgiants, giants, or [[white dwarf]]s, while less massive stars continue fusing hydrogen on the main sequence.<ref name="Weidemann1992" /> Extensive surveys have revealed a total of 8 white dwarfs in the cluster core,<ref name="Bohm-Vitense1995">{{cite journal |bibcode=1995AJ....110..228B |title=White Dwarf Companions to Hyades F Stars |last1=Bohm-Vitense |first1=Erika |journal=The Astronomical Journal |date=1995 |volume=110 |page=228 |doi=10.1086/117511 }}</ref> corresponding to the final evolutionary stage of its original population of B-type stars (each about {{Solar mass|3}}).<ref name="Weidemann1992" /> The preceding evolutionary stage is currently represented by the cluster's four red clump giants. Their present spectral type is K0 III, but all are "retired A stars" of around {{Solar mass|2.5}}.<ref name="Sato" /><ref name="Torres1997">{{cite journal | last1 = Torres | first1 = G | last2 = Stefanik | first2 = RP | last3 = Latham | first3 = DW | year = 1997 | title = The Hyades binaries Theta1 Tauri and Theta2 Tauri: The distance to the cluster and the mass-luminosity relation | journal = Astrophysical Journal | volume = 485 | issue = 1| pages = 167–181 | doi=10.1086/304422|bibcode = 1997ApJ...485..167T | doi-access = free }}</ref><ref name="Johnson">{{cite journal |bibcode=2007ApJ...665..785J |title=Retired a Stars and Their Companions: Exoplanets Orbiting Three Intermediate-Mass Subgiants |last1=Johnson |first1=John Asher |last2=Fischer |first2=Debra A. |last3=Marcy |first3=Geoffrey W. |last4=Wright |first4=Jason T. |last5=Driscoll |first5=Peter |last6=Butler |first6=R. Paul |last7=Hekker |first7=Saskia |last8=Reffert |first8=Sabine |last9=Vogt |first9=Steven S. |journal=The Astrophysical Journal |date=2007 |volume=665 |issue=1 |page=785 |doi=10.1086/519677 |arxiv=0704.2455 }}</ref> An additional "white giant" of type A7 III is the primary of [[Theta Tauri|θ<sup>2</sup> Tauri]], a [[binary star|binary]] system that includes a less massive companion of spectral type A; this pair is visually associated with [[Theta Tauri|θ<sup>1</sup> Tauri]], one of the four red giants, which also has an A-type binary companion.<ref name="Torres1997" /><ref name="Armstrong2006">{{cite journal | last1 = Armstrong | first1 = JT | last2 = Mozurkewich | first2 = D | last3 = Hajian | first3 = AR | display-authors = etal | year = 2006 | title = The Hyades binary Theta2 Tauri: Confronting evolutionary models with optical interferometry | journal = Astronomical Journal | volume = 131 | issue = 5| pages = 2643–2651 | doi=10.1086/501429|bibcode = 2006AJ....131.2643A | citeseerx = 10.1.1.1000.4076 | s2cid = 6268214 }}</ref>

The remaining population of confirmed cluster members includes numerous bright stars of spectral types A (at least 21), F (about 60), and G (about 50).<ref name="Perryman" /><ref name="Bohm-Vitense2007" /> All these star types are concentrated much more densely within the tidal radius of the Hyades than within an equivalent 10-parsec radius of the Earth. By comparison, our local 10-parsec sphere contains only 4 A stars, 6 F stars, and 21 G stars.<ref name="RECONS">Research Consortium on Nearby Stars (RECONS). Ten-parsec census at http://joy.chara.gsu.edu/RECONS/census.posted.htm.</ref>

The Hyades' cohort of lower-mass stars – spectral types K and M – remains poorly understood, despite proximity and long observation. At least 48 K dwarfs are confirmed members, along with about a dozen [[M dwarf]]s of spectral types M0-M2.<ref name="Perryman" /><ref name="Bohm-Vitense2007" /><ref name="Endl">{{cite journal | last1 = Endl | first1 = M | last2 = Cochran | first2 = WD | last3 = Kurster | first3 = M | last4 = Paulson | first4 = DB | last5 = Wittenmyer | first5 = RA | last6 = MacQueen | first6 = PJ | last7 = Tull | first7 = RG | year = 2006 | title = Exploring the frequency of close-in Jovian planets around M dwarfs | journal = Astrophysical Journal | volume = 649 | issue = 1| pages = 436–443 | doi=10.1086/506465|arxiv = astro-ph/0606121 |bibcode = 2006ApJ...649..436E | s2cid = 14461746 }}</ref> Additional M dwarfs have been proposed in the past.<ref name="Stauffer">{{cite journal |last1=Stauffer |first1=JR |last2=Balachandran |first2=SC |last3=Krishnamurthi |first3=A |last4=Pinsonneault |first4=M |last5=Terndrup |first5=DM |last6=Stern |first6=RA |year=1997 |title=Rotational velocities and chromospheric activity of M dwarfs in the Hyades |journal=Astrophysical Journal |volume=475 |issue=2 |pages=604–622 |bibcode=1997ApJ...475..604S |doi=10.1086/303567 |doi-access=free}}</ref> This deficiency at the bottom of the mass range contrasts strongly with the distribution of stars within 10 parsecs of the Solar System, where at least 239 M dwarfs are known, comprising about 76% of all neighborhood stars.<ref name="RECONS" /> In more recent studies more low-mass members were discovered. This is due targeted searches<ref name="Bouvier" /><ref name="Hogan" /> and an improvement in [[proper motion]] searches.<ref name=":0">{{Cite journal |last1=Pérez-Garrido |first1=A. |last2=Lodieu |first2=N. |last3=Rebolo |first3=R. |date=2017-03-01 |title=A new L5 brown dwarf member of the Hyades cluster with chromospheric activity |journal=Astronomy and Astrophysics |volume=599 |pages=A78 |arxiv=1701.03398 |bibcode=2017A&A...599A..78P |doi=10.1051/0004-6361/201628778 |issn=0004-6361}}</ref><ref name=":1">{{Cite journal |last1=Schneider |first1=Adam C. |last2=Vrba |first2=Frederick J. |last3=Munn |first3=Jeffrey A. |last4=Dahm |first4=Scott E. |last5=Bruursema |first5=Justice |last6=Williams |first6=Stephen J. |last7=Dorland |first7=Bryan N. |last8=Faherty |first8=Jacqueline K. |last9=Rothermich |first9=Austin |last10=Calamari |first10=Emily |last11=Cushing |first11=Michael C. |last12=Caselden |first12=Dan |last13=Kabatnik |first13=Martin |last14=Pendrill |first14=William |last15=Sainio |first15=Arttu |date=2022-05-01 |title=Substellar Hyades Candidates from the UKIRT Hemisphere Survey |journal=The Astronomical Journal |volume=163 |issue=5 |pages=242 |arxiv=2203.11090 |bibcode=2022AJ....163..242S |doi=10.3847/1538-3881/ac5f50 |doi-access=free |issn=0004-6256}}</ref><ref name=":2">{{Cite journal |last1=Gagné |first1=Jonathan |last2=Faherty |first2=Jacqueline K. |date=2018-08-01 |title=BANYAN. XIII. A First Look at Nearby Young Associations with Gaia Data Release 2 |journal=The Astrophysical Journal |volume=862 |issue=2 |pages=138 |arxiv=1805.11715 |bibcode=2018ApJ...862..138G |doi=10.3847/1538-4357/aaca2e |doi-access=free |issn=0004-637X}}</ref> About 35 L-type (7<ref name="Hogan">{{cite journal |bibcode=2008MNRAS.388..495H |title=L dwarfs in the Hyades |last1=Hogan |first1=E. |last2=Jameson |first2=R. F. |last3=Casewell |first3=S. L. |last4=Osbourne |first4=S. L. |last5=Hambly |first5=N. C. |journal=Monthly Notices of the Royal Astronomical Society |date=2008 |volume=388 |issue=2 |page=495 |doi=10.1111/j.1365-2966.2008.13437.x |doi-access=free |arxiv=0805.1189 }}</ref><ref>{{Cite journal |last1=Lodieu |first1=N. |last2=Boudreault |first2=S. |last3=Béjar |first3=V. J. S. |date=2014-12-01 |title=Spectroscopy of Hyades L dwarf candidates★ |journal=Monthly Notices of the Royal Astronomical Society |volume=445 |issue=4 |pages=3908–3918 |arxiv=1410.0192 |bibcode=2014MNRAS.445.3908L |doi=10.1093/mnras/stu2059 |doi-access=free |issn=0035-8711}}</ref>+1<ref name=":0" />+8<ref>{{Cite journal |last1=Pérez-Garrido |first1=A. |last2=Lodieu |first2=N. |last3=Rebolo |first3=R. |last4=Chinchilla |first4=P. |date=2018-12-01 |title=Exploring the substellar population in the Hyades open cluster |journal=Astronomy and Astrophysics |volume=620 |pages=A130 |arxiv=1810.12867 |bibcode=2018A&A...620A.130P |doi=10.1051/0004-6361/201833672 |issn=0004-6361}}</ref>+6<ref name=":1" />+3<ref name=":3">{{Cite journal |last1=Schneider |first1=Adam C. |last2=Munn |first2=Jeffrey A. |last3=Vrba |first3=Frederick J. |last4=Bruursema |first4=Justice |last5=Dahm |first5=Scott E. |last6=Williams |first6=Stephen J. |last7=Liu |first7=Michael C. |last8=Dorland |first8=Bryan N. |date=2023-09-01 |title=Astrometry and Photometry for ≈1000 L, T, and Y Dwarfs from the UKIRT Hemisphere Survey |journal=The Astronomical Journal |volume=166 |issue=3 |pages=103 |arxiv=2307.11882 |bibcode=2023AJ....166..103S |doi=10.3847/1538-3881/ace9bf |doi-access=free |issn=0004-6256}}</ref>+4<ref name=":4">{{cite journal |bibcode=2024AJ....168..165S |title=Eight New Substellar Hyades Candidates from the UKIRT Hemisphere Survey |last1=Schneider |first1=Adam C. |last2=Cushing |first2=Michael C. |last3=Stiller |first3=Robert A. |last4=Munn |first4=Jeffrey A. |last5=Vrba |first5=Frederick J. |last6=Bruursema |first6=Justice |last7=Williams |first7=Stephen J. |last8=Liu |first8=Michael C. |last9=Bravo |first9=Alexia |last10=Faherty |first10=Jacqueline K. |last11=Rothermich |first11=Austin |last12=Calamari |first12=Emily |last13=Caselden |first13=Dan |last14=Kabatnik |first14=Martin |last15=Sainio |first15=Arttu |last16=Bickle |first16=Thomas P. |last17=Pendrill |first17=William |last18=Stevnbak Andersen |first18=Nikolaj |last19=Thévenot |first19=Melina |journal=The Astronomical Journal |date=2024 |volume=168 |issue=4 |page=165 |doi=10.3847/1538-3881/ad71d0 |doi-access=free |arxiv=2408.10112 }}</ref>+3<ref name=":3" /><ref>{{Cite journal |last1=Gaia Collaboration |last2=Smart |first2=R. L. |last3=Sarro |first3=L. M. |last4=Rybizki |first4=J. |last5=Reylé |first5=C. |last6=Robin |first6=A. C. |last7=Hambly |first7=N. C. |last8=Abbas |first8=U. |last9=Barstow |first9=M. A. |last10=de Bruijne |first10=J. H. J. |last11=Bucciarelli |first11=B. |last12=Carrasco |first12=J. M. |last13=Cooper |first13=W. J. |last14=Hodgkin |first14=S. T. |last15=Masana |first15=E. |date=2021-05-01 |title=Gaia Early Data Release 3. The Gaia Catalogue of Nearby Stars |journal=Astronomy and Astrophysics |volume=649 |pages=A6 |arxiv=2012.02061 |bibcode=2021A&A...649A...6G |doi=10.1051/0004-6361/202039498 |issn=0004-6361}}</ref>+3<ref name=":2" />) and 15 T-type (2<ref name="Bouvier" />+1<ref>{{Cite journal |last1=Kuzuhara |first1=Masayuki |last2=Currie |first2=Thayne |last3=Takarada |first3=Takuya |last4=Brandt |first4=Timothy D. |last5=Sato |first5=Bun'ei |last6=Uyama |first6=Taichi |last7=Janson |first7=Markus |last8=Chilcote |first8=Jeffrey |last9=Tobin |first9=Taylor |last10=Lawson |first10=Kellen |last11=Hori |first11=Yasunori |last12=Guyon |first12=Olivier |last13=Groff |first13=Tyler D. |last14=Lozi |first14=Julien |last15=Vievard |first15=Sebastien |date=2022-08-01 |title=Direct-imaging Discovery and Dynamical Mass of a Substellar Companion Orbiting an Accelerating Hyades Sun-like Star with SCExAO/CHARIS |journal=The Astrophysical Journal |volume=934 |issue=2 |pages=L18 |arxiv=2205.02729 |bibcode=2022ApJ...934L..18K |doi=10.3847/2041-8213/ac772f |doi-access=free |issn=0004-637X}}</ref>+3<ref name=":1" />+1<ref name=":3" />+4<ref name=":4" />+4<ref>{{Cite journal |last1=Zhang |first1=Zhoujian |last2=Liu |first2=Michael C. |last3=Best |first3=William M. J. |last4=Dupuy |first4=Trent J. |last5=Siverd |first5=Robert J. |date=2021-04-01 |title=The Hawaii Infrared Parallax Program. V. New T-dwarf Members and Candidate Members of Nearby Young Moving Groups |journal=The Astrophysical Journal |volume=911 |issue=1 |pages=7 |arxiv=2102.05045 |bibcode=2021ApJ...911....7Z |doi=10.3847/1538-4357/abe3fa |doi-access=free |issn=0004-637X}}</ref>) [[Brown dwarf|brown dwarfs]] are currently reported as Hyades members or candidate members. Meanwhile [[Gaia (spacecraft)|Gaia]] DR2 allowed the identification of 710 cluster members within 30 parsec, including 23 candidates with estimated masses between 60 and 80 {{Jupiter mass|link=true}}.<ref>{{Cite journal |last1=Lodieu |first1=N. |last2=Smart |first2=R. L. |last3=Pérez-Garrido |first3=A. |last4=Silvotti |first4=R. |date=2019-03-01 |title=A 3D view of the Hyades stellar and sub-stellar population |journal=Astronomy and Astrophysics |volume=623 |pages=A35 |arxiv=1901.07534 |bibcode=2019A&A...623A..35L |doi=10.1051/0004-6361/201834045 |issn=0004-6361}}</ref>

===Mass segregation===

The observed distribution of stellar types in the Hyades Cluster demonstrates a history of [[Mass segregation (astronomy)|mass segregation]]. With the exception of its white dwarfs, the cluster's central {{Convert|2|pc|abbr=off|spell=in}} contain only star systems of at least {{Solar mass|1}}.<ref name="Perryman" /> This tight concentration of heavy stars gives the Hyades its overall structure, with a core defined by bright, closely packed systems and a halo consisting of more widely separated stars in which later spectral types are common. The core radius is 2.7 parsecs (8.8 light-years, a little more than the distance between the Sun and [[Sirius]]), while the half-mass radius, within which half the cluster's mass is contained, is {{Convert|5.7|pc|abbr=off}}. The tidal radius of {{Convert|10|pc|abbr=off|spell=in}} represents the Hyades' average outer limit, beyond which a star is unlikely to remain gravitationally bound to the cluster core.<ref name="Perryman" /><ref name="Weidemann1992" />

Stellar evaporation occurs in the cluster halo as smaller stars are scattered outward by more massive insiders. From the halo they may then be lost to tides exerted by the Galactic core or to shocks generated by collisions with drifting hydrogen clouds.<ref name="PavaniBica 2007" /> In this way the Hyades probably lost much of its original population of M dwarfs, along with substantial numbers of brighter stars.

===Stellar multiplicity===
Another result of mass segregation is the concentration of binary systems in the cluster core.<ref name="Perryman" /><ref name="Bohm-Vitense2007" /> More than half of the known F and G stars are binaries, and these are preferentially located within this central region. As in the immediate Solar neighborhood, [[binary star|binarity]] increases with increasing stellar mass. The fraction of binary systems in the Hyades increases from 26% among K-type stars to 87% among A-type stars.<ref name="Bohm-Vitense2007" /> Hyades binaries tend to have small separations, with most binary pairs in shared orbits whose [[semi-major axis|semimajor axes]] are smaller than 50 [[astronomical unit]]s.<ref name="Patience1998">{{cite journal |bibcode=1998AJ....115.1972P |title=The Multiplicity of the Hyades and Its Implications for Binary Star Formation and Evolution |last1=Patience |first1=J. |last2=Ghez |first2=A. M. |last3=Reid |first3=I. N. |last4=Weinberger |first4=A. J. |last5=Matthews |first5=K. |journal=The Astronomical Journal |date=1998 |volume=115 |issue=5 |page=1972 |doi=10.1086/300321 |arxiv=astro-ph/9801216 }}</ref> Although the exact ratio of single to multiple systems in the cluster remains uncertain, this ratio has considerable implications for our understanding of its population. For example, Perryman and colleagues list about 200 high-probability Hyades members.<ref name="Perryman" /> If the binary fraction is 50%, the total cluster population would be at least 300 individual stars.

===Future evolution===
Surveys indicate that 90% of open clusters dissolve less than 1 billion years after formation, while only a tiny fraction survive for the present age of the Solar System (about 4.6 billion years).<ref name="PavaniBica 2007" /> Over the next few hundred million years, the Hyades will continue to lose both mass and membership as its brightest stars evolve off the main sequence and its dimmest stars evaporate out of the cluster halo. It may eventually be reduced to a remnant containing about a dozen star systems, most of them binary or multiple, which will remain vulnerable to ongoing dissipative forces.<ref name="PavaniBica 2007" />