View source for Pleione (star)

{{Short description|Binary star in the Pleiades star cluster}}
{{Starbox begin
 |name=Pleione
}}
{{Starbox image
 | image =
  {{Location mark
  | image = Pleiades large.jpg | width = 280 
  | alt = Image of the Pleiades star cluster | float = center
  | mark = Red circle.svg | mark_width = 20
  | x = 110 | y = 367
  }}
 | caption = Pleione (circled) is located just above [[Atlas (star)|Atlas]] in the East of the [[Pleiades]] cluster.
}}
{{Starbox observe
 |epoch=J2000
 |ra={{RA|03|49|11.2166}}<ref name=edr3/>
 |dec={{DEC|24|08|12.157}}<ref name=edr3/>
 |appmag_v=4.77 - 5.50<ref name=gcvs/>
 |constell=[[Taurus (constellation)|Taurus]]
}}
{{Starbox character
 |type=[[Main sequence]]
 |class=B8Vne<ref name=taranova2007>{{cite journal |bibcode=2008PZP.....8....6T |title=Pleione (BU Tau): IR Fading of the Star in 1999 - 2007 |last1=Taranova |first1=O. |last2=Shenavrin |first2=V. |last3=Nadjip |first3=A. D. |journal=Peremennye Zvezdy Prilozhenie |year=2008 |volume=8 |issue=6 |page=6 }}</ref>
 |b-v=−0.08<ref name="AAA34">
{{cite journal
  |last=Nicolet |first=B.
  |date=1978
  |title=Catalogue of homogeneous data in the UBV photoelectric photometric system.
  |journal=[[Astronomy and Astrophysics Supplement Series]]
  |volume=34  |pages=1–49
  |bibcode=1978A&AS...34....1N
}}</ref>
 |variable=[[Variable star#Gamma Cassiopeiae variables|γ Cas]]<ref name=gcvs>{{cite journal|bibcode=2009yCat....102025S|title=VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)|journal=VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S|volume=1|pages=B/gcvs|display-authors=etal|last1=Samus|first1=N. N.|last2=Durlevich|first2=O. V.|year=2009}}</ref>
}}
{{Starbox astrometry
 |radial_v=5.10<ref name=pulkovo>{{cite journal |bibcode=2006AstL...32..759G |title=Pulkovo Compilation of Radial Velocities for 35 495 Hipparcos stars in a common system |last1=Gontcharov |first1=G. A. |journal=Astronomy Letters |year=2006 |volume=32 |issue=11 |pages=759–771 |doi=10.1134/S1063773706110065 |arxiv=1606.08053 |s2cid=119231169 }}</ref>
 |prop_mo_ra=19.496<ref name=edr3/>
 |prop_mo_dec=−47.650<ref name=edr3/>
 |parallax=7.2414
 |p_error=0.1255
 |parallax_footnote=<ref name=edr3/>
 |absmag_v=−0.347<ref name=katz2011>{{cite journal |bibcode=2011A&A...525A..90K |title=Probing the Galactic thick disc vertical properties and interfaces |last1=Katz |first1=D. |last2=Soubiran |first2=C. |last3=Cayrel |first3=R. |last4=Barbuy |first4=B. |last5=Friel |first5=E. |last6=Bienaymé |first6=O. |last7=Perrin |first7=M. -N. |journal=Astronomy and Astrophysics |year=2011 |volume=525 |pages=A90 |doi=10.1051/0004-6361/201014840 |doi-access=free }}</ref>
}}
{{Starbox detail
 |mass=2.888<ref name=tic>{{cite journal |bibcode=2019AJ....158..138S |title=The Revised TESS Input Catalog and Candidate Target List |last1=Stassun |first1=Keivan G. |last2=Oelkers |first2=Ryan J. |last3=Paegert |first3=Martin |last4=Torres |first4=Guillermo |last5=Pepper |first5=Joshua |last6=De Lee |first6=Nathan |last7=Collins |first7=Kevin |last8=Latham |first8=David W. |last9=Muirhead |first9=Philip S. |last10=Chittidi |first10=Jay |last11=Rojas-Ayala |first11=Bárbara |last12=Fleming |first12=Scott W. |last13=Rose |first13=Mark E. |last14=Tenenbaum |first14=Peter |last15=Ting |first15=Eric B. |last16=Kane |first16=Stephen R. |last17=Barclay |first17=Thomas |last18=Bean |first18=Jacob L. |last19=Brassuer |first19=C. E. |last20=Charbonneau |first20=David |last21=Ge |first21=Jian |last22=Lissauer |first22=Jack J. |last23=Mann |first23=Andrew W. |last24=McLean |first24=Brian |last25=Mullally |first25=Susan |last26=Narita |first26=Norio |last27=Plavchan |first27=Peter |last28=Ricker |first28=George R. |last29=Sasselov |first29=Dimitar |last30=Seager |first30=S. |journal=The Astronomical Journal |year=2019 |volume=158 |issue=4 |page=138 |doi=10.3847/1538-3881/ab3467 |arxiv=1905.10694 |s2cid=166227927 |display-authors=29 |doi-access=free }}</ref>
 |radius=3.701<ref name=tic/>
 |luminosity=184<ref name=tic/>
 |temperature=11,058<ref name=tic/>
 |metal_fe=+0.02<ref name=katz2011/>
 |gravity=3.087<ref name=mcdonald2017>{{cite journal |bibcode=2017MNRAS.471..770M |title=Fundamental parameters and infrared excesses of Tycho-Gaia stars |last1=McDonald |first1=I. |last2=Zijlstra |first2=A. A. |last3=Watson |first3=R. A. |journal=Monthly Notices of the Royal Astronomical Society |year=2017 |volume=471 |issue=1 |page=770 |doi=10.1093/mnras/stx1433 |doi-access=free |arxiv=1706.02208 |s2cid=73594365 }}</ref>
 |rotational_velocity=290<ref name=zorec2016>{{cite journal |bibcode=2016A&A...595A.132Z |title=Critical study of the distribution of rotational velocities of be stars. I. Deconvolution methods, effects due to gravity darkening, macroturbulence, and binarity |last1=Zorec |first1=J. |last2=Frémat |first2=Y. |last3=Domiciano De Souza |first3=A. |last4=Royer |first4=F. |last5=Cidale |first5=L. |last6=Hubert |first6=A. -M. |last7=Semaan |first7=T. |last8=Martayan |first8=C. |last9=Cochetti |first9=Y. R. |last10=Arias |first10=M. L. |last11=Aidelman |first11=Y. |last12=Stee |first12=P. |journal=Astronomy and Astrophysics |year=2016 |volume=595 |pages=A132 |doi=10.1051/0004-6361/201628760 |doi-access=free |hdl=11336/37946 |hdl-access=free }}</ref>
 |age_myr=125<ref name=su2006>{{cite journal |bibcode=2006ApJ...653..675S |title=Debris Disk Evolution around a Stars |last1=Su |first1=K. Y. L. |last2=Rieke |first2=G. H. |last3=Stansberry |first3=J. A. |last4=Bryden |first4=G. |last5=Stapelfeldt |first5=K. R. |last6=Trilling |first6=D. E. |last7=Muzerolle |first7=J. |last8=Beichman |first8=C. A. |last9=Moro-Martin |first9=A. |last10=Hines |first10=D. C. |last11=Werner |first11=M. W. |journal=The Astrophysical Journal |year=2006 |volume=653 |issue=1 |pages=675–689 |doi=10.1086/508649 |arxiv=astro-ph/0608563 |s2cid=14116473 }}</ref>
}}
{{Starbox catalog
 |names={{odlist | name=Pleione | V=BU&nbsp;Tau | F=28 Tau | BD=+23°558 | HD=23862 | HIP=17851 | HR=1180 | SAO=76229 | GC=4587 }}
}}
{{Starbox reference
 |Simbad=HD+23862
}}
{{Starbox end}}
{{Sky|03|49|11.2161|+|24|08|12.163|100000}}
'''Pleione'''<ref group=note>Pronounced {{IPAc-en|'|p|l|ai|@|n|iː}} {{respell|PLY|anee}} or {{IPAc-en|'|p|l|iː|@|n|iː}} {{respell|PLEE|ənee}}</ref> is a [[binary star]] and the [[Pleiades#Brightest stars|seventh-brightest]] star in the [[Pleiades]] [[open cluster|star cluster]] ([[Messier objects|Messier 45]]). It has the [[variable star designation]] '''BU Tauri''' (BU Tau) and the [[Flamsteed]] designation '''28 Tauri''' (28 Tau). The star is located approximately {{convert|138|pc|ly|abbr=off|lk=on}} from the [[Sun]], appearing in the [[constellation]] of [[Taurus (constellation)|Taurus]]. Pleione is located close on the sky to the brighter star [[Atlas (star)|Atlas]], so is difficult for stargazers to distinguish with the naked eye despite being a [[fifth magnitude star]].

The brighter star of the Pleione binary pair, component A, is a hot [[B-type star|type B]] star 184 times more luminous than the Sun. It is classified as [[Be star]] with certain distinguishing traits: periodic phase changes and a complex circumstellar environment composed of two gaseous disks at different angles to each other. The primary star rotates rapidly, close to its [[Stellar rotation#Equatorial bulge|breakup velocity]], even faster than [[Achernar]]. Although some research on the companion star has been performed, stellar characteristics of the orbiting B component are not well known.

== Nomenclature ==
''28 Tauri'' is the star's [[Flamsteed designation]] and ''BU Tauri'' its [[variable star designation]]. The name ''Pleione'' originates with [[Greek mythology]]; she is the mother of seven daughters known as [[Pleiades (Greek mythology)|the Pleiades]]. In 2016, the [[International Astronomical Union]] organized a [[IAU Working Group on Star Names|Working Group on Star Names]] (WGSN)<ref name="WGSN">{{cite web | url=https://www.iau.org/science/scientific_bodies/working_groups/280/ | title=IAU Working Group on Star Names (WGSN)|access-date=22 May 2016}}</ref> to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016<ref name="WGSN1">{{cite web | url=http://www.pas.rochester.edu/~emamajek/WGSN/WGSN_bulletin1.pdf | title=Bulletin of the IAU Working Group on Star Names, No. 1 |access-date=28 July 2016}}</ref> included a table of the first two batches of names approved by the WGSN; which included ''Pleione'' for this star. It is now so entered in the IAU Catalog of Star Names.<ref name="IAU-CSN">{{cite web | url=http://www.pas.rochester.edu/~emamajek/WGSN/IAU-CSN.txt | title=IAU Catalog of Star Names |access-date=28 July 2016}}</ref>

==Visibility==
{{Location mark
| image = Taurus constellation map.svg | type = thumb | float = left
| mark = Red circle.svg | mark_width = 20 | x = 810 | y = 323
| alt = Chart of the constellation Taurus 
| caption = The location of the Pleiades (circled) in the constellation [[Taurus (constellation)|Taurus]].
}}
With an [[apparent magnitude]] of +5.05 in [[UBV photometric system|V]], the star is rather difficult to make out with the naked eye, especially since its close neighbour [[Atlas (star)|Atlas]] is 3.7 times brighter and located less than 5 [[arcminute]]s away.<ref name="NOTEATLAS" group="note">The brightness ratio of Atlas versus Pleione is derived from the [[Apparent magnitude#Calculations|formula for apparent magnitude]] and is based on their respective [[UBV photometric system|visual]] [[apparent magnitude|magnitude]]s: Atlas '''(<math>{m_A}</math>)''' at 3.62 and Pleione '''(<math>{m_P}</math>)''' at 5.05. Therefore:
:<math>\frac{m_A}{m_P}=2.512^{(5.05-3.62)}=3.73</math></ref> Beginning in October of each year, Pleione along with the rest of the cluster can be seen rising in the east in the early morning before [[dawn]].<ref name="ESOPLEIADES">{{cite web
 |last1=Getsova
 |first1=I.
 |display-authors=etal
 |date=2002
 |title=All About The Pleiades
 |url=http://www.eso.org/public/outreach/eduoff/cas/cas2002/cas-projects/bulgaria_pleiades_1/
 |work=Catch a Star 2002
 |publisher=[[European Southern Observatory]]
 |access-date=2010-09-15
 |archive-date=2020-10-27
 |archive-url=https://web.archive.org/web/20201027033628/http://www.eso.org/public/outreach/eduoff/cas/cas2002/cas-projects/bulgaria_pleiades_1/
 |url-status=dead
 }}</ref> To see it after sunset, one will need to wait until December. By mid-February, the star is visible to virtually every inhabited region of the globe, with only those south of 66° unable to see it. Even in cities like [[Cape Town]], [[South Africa]], at the tip of the [[Africa]]n [[continent]], the star rises almost 32° above the [[horizon]]. Due to its declination of roughly +24°, Pleione is [[Circumpolar star|circumpolar]] in the [[northern hemisphere]] at latitudes greater than 66° North. Once late April arrives, the cluster can be spotted briefly in the deepening [[twilight]] of the western horizon, soon to disappear with the other setting stars.<ref name="ASTRONOMYMAGAZINE">
{{cite web
 |last=Bakich |first=M. E.
 |date=22 April 2009
 |title=See Mercury, the Moon, and the Pleiades together in the night sky
 |work=[[Astronomy (magazine)|Astronomy]]
 |url=http://www.astronomy.com/asy/default.aspx?c=a&id=8172
 |access-date=2010-09-14
 |quote=Don't miss a stunning sight around 9 P.M. local daylight time April 26 when a crescent Moon joins Mercury and the Pleiades in the deepening twilight.
}}</ref>

Pleione is classified as a [[Gamma Cassiopeiae variable|Gamma Cassiopeiae type]] [[variable star]], with brightness fluctuations that range between a 4.8 and 5.5 [[UBV photometric system|visual]] [[apparent magnitude|magnitude]].<ref name=gcvs/>  It has a spectral classification of B8Vne, a hot [[main sequence]] star with "nebulous" [[absorption line]]s due to its rapid rotation and [[emission line]]s from the surrounding [[circumstellar disk]]s formed of material being ejected from the star.

There has been significant debate as to the star's actual distance from [[Earth]]. The debate revolves around the different methodologies to measure distance—[[parallax]] being the most central, but [[Photometry (astronomy)|photometric]] and [[spectroscopic]] observations yielding valuable insights as well.<ref name="VANLEEUWEN">
For an in-depth discussion of [[Pleiades]] parallax measurements, see section 6.3 of {{cite journal
  |last=van Leeuwen |first=F.
  |date=2009
  |title=Parallaxes and proper motions for 20 open clusters as based on the new Hipparcos catalogue
  |journal=[[Astronomy and Astrophysics]]
  |volume=497 |issue=1 |pages=209–242
  |arxiv=0902.1039
  |bibcode=2009A&A...497..209V
  |doi=10.1051/0004-6361/200811382
|s2cid=16420237
 }}</ref><ref name="ASKASTRO">
{{cite web
 |last1=Allen |first1=J.
 |last2=Boyd |first2=P.
 |date=15 April 1997
 |title=Finding Stellar Distances
 |url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970415c.html
 |work=Ask an Astrophysicist
 |publisher=[[NASA]]
 |access-date=2010-09-14
 |quote=A straightforward summary of the different methods used by astronomers to measure stellar distances.
}}</ref> Before the [[Hipparcos]] mission, the estimated distance for the Pleiades star cluster was around 135 [[parsecs]] or 440 [[light year]]s. When the [[Hipparcos Catalogue]] was published in 1997, the new parallax measurement indicated a much closer distance of about {{val|119|1.0|ul=pc}} ({{val|388|3.2|ul=ly}}), triggering substantial controversy among astronomers.<ref name="VANLEEUWEN"/><ref name="PERRYMAN">
{{cite journal
 |last1=Perryman |first1=M. A. C.
 |display-authors=etal
 |date=1997
 |title=The Hipparcos Catalogue
 |journal=[[Astronomy and Astrophysics Letters]]
 |volume=323 |pages=L49–L52
 |bibcode=1997A&A...323L..49P
 |quote=The original parallax figure from the [[Hipparcos Catalogue]] as shown in the [[SIMBAD]] astronomical database was 8.42 ± 0.86[[milliarcsecond|mas]] yielding a distance of about 119 ± 1.0pc or 388 ± 3.2ly
}}</ref><ref name="HUBBLENEWS">
{{cite web
 |last1=Weaver |first1=D.
 |last2=Soderblom |first2=D.
 |title=Hubble Refines Distance to Pleiades Star Cluster
 |url=http://hubblesite.org/newscenter/archive/releases/2004/20/text/
 |publisher=Hubblesite Newscenter
 |date=1 June 2004
 |access-date=2010-09-13
}}</ref> The [[Hipparcos new reduction]] produced a broadly similar distance of {{val|120|2|u=pc}}.  If the Hipparcos estimate were accurate, some astronomers contend, then stars in the cluster would have to be fainter than Sun-like stars—a notion that would challenge some of the fundamental precepts of stellar structure. [[Interferometric]] measurements taken in 2004 by the [[Hubble Space Telescope|Hubble Telescope]]'s [[Fine Guidance Sensor]]s and corroborated by studies from [[Caltech]] and [[NASA]]'s [[Jet Propulsion Laboratory]] showed the original estimate of 135 [[parsec|pc]] or 440 [[light year|ly]] to be the correct figure.<ref name="HUBBLENEWS"/>  The [[Gaia EDR3]] parallax is {{val|7.24|0.1255|ul=mas}}, indicating a distance around {{val|138|u=pc}}.  This is relatively imprecise for a Gaia result due to the brightness of the star, but still with a statistical margin of error similar to the Hipparcos results.<ref name=edr3>{{Cite Gaia EDR3|66529975427235712}}</ref>

==Properties==
In 1942 [[Otto Struve]], one of the early researchers of Be Stars, stated that Pleione is "the most interesting member of the [[Pleiades]] cluster".<ref name="STRUVE">
{{cite journal
 |last=Struve |first=O.
 |date=1943
 |title=The Story of Pleione
 |journal=[[Popular Astronomy (US magazine)|Popular Astronomy]]
 |volume=51 |page=233
 |bibcode=1943PA.....51..233S
}}</ref> Like many of the stars in the [[open cluster|cluster]], Pleione is a blue-white [[stellar classification|B-type]] [[main sequence]] [[dwarf star]] with a temperature of about {{val|11,000|fmt=commas|ul=K}}. It has a [[bolometric luminosity]] of {{Solar luminosity|184|link=y}} assuming a distance of roughly 130 [[parsec|pc]].<ref name=tic/> With a radius of {{Solar radius|3.7|link=y}} and mass that is {{Solar mass|2.9|link=y}}, Pleione is considerably smaller than the brightest stars in the Pleiades. [[Alcyone (star)|Alcyone]] for instance has a radius that is {{Solar radius|10|link=y}} with a luminosity {{Solar luminosity|2,400|link=y}}, making it roughly 30 times more voluminous than Pleione and about 13 times brighter.<ref name="NOTEALCYONE" group="note">The relative size of Alcyone (''V''<sub>A</sub>) compared to Pleione (''V''<sub>P</sub>) is determined by comparing their [[Volume of a sphere|volumes]]. It is assumed that the volume of each star is reasonably approximated by the formula for a [[sphere]]:
:''V''<sub>A</sub> ≈ {{frac|4|3}}π × 10<sup>3</sup> ≈ 4,188.79 ''V''<sub>Sun</sub>
:''V''<sub>P</sub> ≈ {{frac|4|3}}π × 3.2<sup>3</sup> ≈ 137.26 ''V''<sub>Sun</sub>
Therefore, the volume of Alcyone compared to Pleione ≈ 4,188.79 ÷ 137.26 ≈ 30.52

Alcyone's luminosity compared to Pleione is:
:2,400{{Solar luminosity|link=y}} ÷ 190{{Solar luminosity|link=y}} = 12.63 (or 13 rounded)<br/></ref>

===Be star===
[[File:Classical Be star.jpg|thumb|right|Artist's impression of the [[Phi Persei]] system. Classical [[Be star]]s like Pleione and Phi Persei exhibit gaseous equatorial disks, likely caused by [[Stellar rotation|rapid rotation]].<ref name="LEHIGH">
{{cite web
 |title=Classical Be Stars
 |url=http://www.lehigh.edu/~mvm207/research.html
 |work=Research in Astronomy & Astrophysics at Lehigh
 |publisher=[[Lehigh University]]
 |access-date=2010-09-16
}}</ref>]]
Pleione is a classical [[Be star]], often referred to as an "active hot star".<ref name="STEE">
{{cite web
 |last=Stee |first=P.
 |title=What is a Be star?
 |work=Hot and Active Stars Research
 |url=http://www.oca.eu/stee/page1/page3/page3.html
 |access-date=2010-06-20
}}</ref> Classical Be stars are B-type stars close to the main sequence with the "e" in the [[stellar classification|spectral type]] signifying that Pleione exhibits [[spectral line|'''e'''mission lines]] in its [[stellar spectrum|spectrum]], rather than the [[spectroscopy|absorption lines]] typical of B-type stars.<ref name="BADASTRONOMY">{{cite web
 |last=Plait
 |first=P.
 |date=5 August 2009
 |url=http://blogs.discovermagazine.com/badastronomy/2009/08/05/to-be-or-not-to-be/
 |title=<nowiki>To B[e] or not to B[e]</nowiki>
 |work=[[Bad Astronomy]]
 |access-date=2010-09-20
 |archive-date=2010-07-11
 |archive-url=https://web.archive.org/web/20100711084034/http://blogs.discovermagazine.com/badastronomy/2009/08/05/to-be-or-not-to-be/
 |url-status=dead
 }}</ref> Emission lines usually indicate that a star is surrounded by gas. In the case of a Be star, the gas is typically in the form of an [[circumstellar disk|equatorial disk]], resulting in [[electromagnetic radiation]] that emanates not just from the [[photosphere]], but from the disk as well. The geometry and [[kinematics]] of this gaseous circumstellar environment are best explained by a [[Keplerian orbit|Keplerian]] disk – one that is supported against gravity by rotation, rather than gas or radiation pressure.<ref name="PORTER">
{{cite journal
 |last1=Porter |first1=J. M.
 |last2=Rivinius |first2=T.
 |date=2003
 |title=Classical Be Stars
 |journal=[[Publications of the Astronomical Society of the Pacific]]
 |volume=115 |issue=812 |pages=1153–1170
 |bibcode=2003PASP..115.1153P
 |doi=10.1086/378307
|doi-access=free
 }}</ref><ref name="SPACEREF"/> Circumstellar disks like this are sometimes referred to as ''"[[decretion disk]]s"'', because they consist of material being thrown off the star (as opposed to [[accretion disk]]s which comprise material falling toward the star).<ref name="THIZY">{{cite web
 |last         = Thizy
 |first        = O.
 |title        = Be Stars
 |url          = http://www.shelyak.com/en/be_stars.html
 |publisher    = [[Shelyak Instruments]]
 |access-date   = 2010-06-20
 |archive-url  = https://web.archive.org/web/20110927005933/http://shelyak.com/dossier.php?id_dossier=24
 |archive-date = 2011-09-27
 |url-status     = dead
}}</ref>

Be Stars are [[Stellar rotation|fast rotators]] (>200&nbsp;km/s), causing them to be highly [[Flattening|oblate]], with a substantial [[stellar wind]] and high [[Stellar mass loss|mass loss rate]].<ref name="STEE"/> Pleione's rotational velocity of {{val|329|u=km/s}} is considerably faster than the {{val|251|u=km/s}} of [[Achernar]], a prototypical Be star.<ref name=zorec2016/><ref name="HR2">
{{cite web
 |title=HR 20472
 |url=http://vizier.u-strasbg.fr/viz-bin/VizieR-S?HR%20472
 |work=Bright Star Catalogue, 5th Revised Ed. (Hoffleit+, 1991)
 |publisher=[[VizieR]], [[Centre de Données astronomiques de Strasbourg]]
 |access-date=2010-06-20
}}</ref> Pleione revolves on its axis once every 11.8 hours, compared to 48.4 hours for Achernar.<ref name="NOTEROTATION" group="note">The time it takes for Achernar (''T''<sub>A</sub>) and Pleione (''T''<sub>P</sub>) to rotate on its own axis is determined by taking the star's radius in solar units, multiplying by the Sun's radius in kilometers, then calculating the star's circumference at the equator and dividing by its speed of rotation ''per hour''. Therefore:
:''T''<sub>A</sub> = 10 {{Solar radius}} × 696,000 km × 2 × π ÷ 251 km/s ÷ 3,600 ≈ 48.4 hrs
:''T''<sub>P</sub> = 3.2 {{Solar radius}} × 696,000 km × 2 × π ÷ 329 km/s ÷ 3,600 ≈ 11.8 hrs</ref> For comparison, the [[Sun]] takes 25.3 days to rotate. Pleione is spinning so fast that it is close to the estimated [[breakup velocity]] for a B8V star of about 370–390&nbsp;km/s, which is why it is losing so much mass.<ref name="HIRATA1"/>

[[File:Alpha Arae.jpg|thumb|left|Artist's impression of [[Alpha Arae]] rotating with a velocity close to [[Stellar rotation#Equatorial bulge|breakup]] and losing mass through a [[stellar wind]] emerging from the poles.<ref name="ESO0635">
{{cite press release
 |display-authors=etal
 |last1=Getsova |first1=I.
 |title=To Be or Not to Be: Is It All About Spinning?
 |date=20 September 2006
 |publisher=[[European Southern Observatory]]
 |url=http://www.eso.org/public/news/eso0635/
 |access-date=2010-09-16
}}</ref>]]
Pleione is unusual because it alternates between three different phases: 1) normal [[B star]], 2) [[Be star]] and 3) [[Shell star|Be shell star]]. The cause is changes in the decretion disc, which appears, disappears, and reforms. Material in the disc is pulled back towards the star by gravity, but if it has enough energy it can escape into space, contributing to the [[stellar wind]].<ref name="SPACEREF">{{cite web
 |date=1 November 2002
 |title=Glasgow astronomers explain hot star disks
 |publisher=[[SpaceRef]]
 |url=http://www.spaceref.com/news/viewpr.html?pid=9717
 |archive-url=https://archive.today/20130202200315/http://www.spaceref.com/news/viewpr.html?pid=9717
 |url-status=dead
 |archive-date=February 2, 2013
 |access-date=2010-06-20
 }}</ref> Sometimes, Be stars form multiple decretion discs simultaneously, producing complex circumstellar dynamics.<ref name="TANAKA"/>

[[File:BUTauLightCurve.png|thumb|right|Broad-band optical [[light curve]] for Pleione<ref name="White2017"/>]]
As a result of such dynamics, Pleione exhibits prominent long-term [[Photometry (astronomy)|photometric]] and [[spectroscopic]] variations encompassing a period of about 35 years.<ref name="TANAKA">
{{cite journal
 |last1=Tanaka |first1=K.
 |display-authors=etal
 |date=2007
 |title=Dramatic Spectral and Photometric Changes of Pleione (28 Tau) between 2005 November and 2007 April
 |journal=[[Publications of the Astronomical Society of Japan]]
 |volume=59 |issue=4 |pages=L35–L39
 |url=http://pasj.asj.or.jp/v59/n4/590404/590404.pdf
 |access-date=2010-06-13
 |doi=10.1093/pasj/59.4.l35
 |bibcode=2007PASJ...59L..35T
|doi-access=free
 }}</ref> During the 20th century, Pleione went through several phase changes: it was in a Be phase until 1903, a B phase (1905–1936), a B-shell phase (1938–1954), followed by another Be phase (1955–1972).<ref name="HIRATA1">
{{cite journal
 |last=Hirata |first=Ryuko
 |date=1995
 |title=Interpretation of the Long-Term Variation in Late-Type Active Be Stars
 |journal=[[Publications of the Astronomical Society of Japan]]
 |volume=47 |pages=195–218
 |bibcode=1995PASJ...47..195H
}}</ref> It then returned to the Be-shell phase in 1972, developing numerous shell absorption lines in its spectrum. At the same time, the star showed a decrease in brightness, beginning at the end of 1971. After reaching a minimum brightness in late 1973, the star gradually re-brightened. In 1989, Pleione entered a Be phase which lasted until the summer of 2005.<ref name="TANAKA" />

These phase changes are ascribed to the evolution of a decretion disc that formed in 1972.<ref name="TANAKA"/> [[Polarimetry|Polarimetric]] observations show the intrinsic [[Brewster's angle|polarization angle]] has changed, indicating a change in orientation of the disc axis.<ref name="HIRATA2">
{{cite journal
 |last=Hirata |first=R.
 |date=2007
 |title=Disk Precession in Pleione
 |journal=[[ASP Conference Series]]
 |volume=361 |page=267
 |bibcode=2007ASPC..361..267H
}}</ref> Because Pleione has a stellar companion with a close orbit, the shift in the polarization angle has been attributed to the companion causing a [[precession]] (wobble) of the disk, with a precession period of roughly 81 years.<ref name="HIRATA2"/>

Photometric and spectroscopic observations from 2005 to 2007 indicated that a new disc had formed around the equator – producing a two discs at different inclination angles (60° and 30°).<ref name="TANAKA"/><ref name="HIRATA2"/> Such a misaligned double-disc structure had not been observed around other Be stars.<ref name="TANAKA"/><ref name="HIRATA2"/>

==Star system==
Pleione is known to be a [[Speckle imaging|speckle]] [[Binary star|binary]], although its orbital parameters have yet to be fully established.<ref name="MCALISTER">
{{cite journal
 |last1=McAlister |first1=H. A.
 |display-authors=etal
 |date=1989
 |title=ICCD speckle observations of binary stars. IV – Measurements during 1986–1988 from the Kitt Peak 4 M telescope
 |journal=[[Astronomical Journal]]
 |volume=97 |pages=510–531
 |bibcode=1989AJ.....97..510M
 |doi=10.1086/115001
}}</ref> In 1996 a group of Japanese and French astronomers discovered that Pleione is a single-lined [[spectroscopic binary]] with an [[orbital period]] of 218.0 days and a large [[Orbital elements|eccentricity]] of 0.6.<ref name="TANAKA" /><ref name="KATAHIRA">
{{cite journal
 |author1=Katahira, Jun-Ichi
 |display-authors=etal
 |date=1996
 |title=Period Analysis of the Radial Velocity in PLEIONE
 |journal=[[Publications of the Astronomical Society of Japan]]
 |volume=48 |issue=2
 |pages=317–334
 |bibcode=1996PASJ...48..317K
 |doi=10.1093/pasj/48.2.317
|doi-access=free
 }}</ref> The [[Washington Double Star Catalogue]] lists an [[angular separation]] between the two components of 0.2 [[arcseconds]]—an angle which equates to a distance of about 24 [[Astronomical Units|AU]], assuming a distance of 120 parsecs.<ref name="ALCYONE">
{{cite web
 |last1=Mason |first1=B. D.
 |display-authors=etal
 |date=1996
 |title=Pleione
 |url=http://www.alcyone.de/SIT/mainstars/SIT000781.htm
 |publisher=Alcyone (Star Information Tool)
 |access-date=2010-09-21
}}</ref>

==Ethnological influences==
[[File:Les Oceanides Les Naiades de la mer.jpg|thumb|right|260px|Depiction of Pleione as an [[Oceanid]] [[nymph]]. Painting from French artist [[Gustave Doré]].]]

===Mythology===
{{Main|Pleione (mythology)}}
Pleione was an [[Oceanid]] [[nymph]] of Mount Kyllene in Arkadia (southern Greece), one of the three thousand daughters of the [[Titan (mythology)|Titans]] [[Oceanus]] and [[Tethys (mythology)|Tethys]].<ref name="ANDREWS">
{{cite book
 |last=Andrews |first=M.
 |date=2004
 |title=The Seven Sisters of the Pleiades – Stories from around the world
 |url=https://books.google.com/books?id=3GbYg26S8pUC&pg=PA61
 |publisher=[[Spinifex Press]]
 |isbn=1-876756-45-4
 |access-date=2010-10-07
}}</ref><ref name="SMITH"/> The nymphs in [[Greek mythology]] were the spirits of nature; oceanids, spirits of the sea.<ref name="ATHENA">
{{cite news
 |last=Athena |first=A.
 |date=8 July 2010
 |title=Nymphs
 |url=http://www.paleothea.com/Nymphs.html
 |work=Women in Greek Myths
 |access-date=2010-10-07
}}</ref> Though considered lesser divinities, they were still very much venerated as the protectors of the natural world. Each oceanid was thence a patroness of a particular body of water — be it ocean, river, lake, spring or even cloud — and by extension activities related thereto. The sea-nymph, Pleione, was the consort of [[Atlas (mythology)|Atlas]], the [[Titan (mythology)|Titan]], and mother of the [[Hyas]], [[Hyades (mythology)|Hyades]] and [[Pleiades (Greek mythology)|Pleiades]].<ref name="FRAZER"/>

===Etymology===
When names were assigned to the stars in the Pleiades cluster, the bright pair of stars in the East of the cluster were named Atlas and Pleione, while the seven other bright stars were named after the [[mythology|mythological]] Pleiades (the 'Seven Sisters'). The term "Pleiades" was used by [[Gaius Valerius Flaccus|Valerius Flaccus]] to apply to the cluster as a whole, and [[Giovanni Battista Riccioli|Riccioli]] called the star ''Mater Pleione''.<ref name="ALLEN">
{{cite web
 |last=Allen |first=R. H.
 |date=1963
 |title=Star Names: Their Lore and Meaning
 |url=https://penelope.uchicago.edu/Thayer/E/Gazetteer/Topics/astronomy/_Texts/secondary/ALLSTA/Taurus*.html
 |page=408
 |publisher=[[Dover Publications]]
 |access-date=2010-06-11
}}</ref>

There is some diversity of opinion as to the origin of the names Pleione and Pleiades. There are three possible derivations of note. Foremost is that both names come from the Greek word πλεῖν, ''(pr. ple'-ō)'', meaning "to sail".<ref name="ALLEN"/><ref name="GIBSON">{{cite web
 |last=Gibson
 |first=S.
 |date=5 April 2007
 |title=Pleiades Mythology
 |url=http://www.naic.edu/~gibson/pleiades/pleiades_myth.html
 |publisher=[[National Astronomy and Ionosphere Center]]
 |access-date=2010-06-18
 |archive-date=2021-03-26
 |archive-url=https://web.archive.org/web/20210326235401/http://www.naic.edu/~gibson/pleiades/pleiades_myth.html
 |url-status=dead
 }}</ref> This is particularly plausible given that ancient Greece was a seafaring culture and because of Pleione's [[myth]]ical status as an Oceanid nymph. Pleione, as a result, is sometimes referred to as the "sailing queen" while her daughters the "sailing ones". Also, the appearance of these stars coincided with the sailing season in antiquity; sailors were well advised to set sail only when the Pleiades were visible at night, lest they meet with misfortune.<ref name="FRAZER">
{{cite web
 |author1=Apollodorus
 |others=Translated by Frazer, J. G
 |date=1921
 |title=Book 3, Chapter 10, Section 1
 |url=https://www.perseus.tufts.edu/hopper/text?doc=Apollod.%203.10.1&lang=original
 |work=[[Bibliotheca (Pseudo-Apollodorus)|The Library]]
 |access-date=2010-06-17
}}</ref>

Another derivation of the name is the Greek word Πλειόνη<ref name="SMITH">
{{cite web
 |last=Smith |first=W.
 |date=1873
 |title=Plei'one
 |work=A dictionary of Greek and Roman biography and mythology
 |publisher=[[John Murray (publishing house)|John Murray]]
 |url=https://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0104:entry=pleione-bio-1
 |access-date=2010-06-17
}}</ref> ''(pr. plêionê)'', meaning "more", "plenty", or "full"—a [[lexeme]] with many English derivatives like [[pleiotropy]], [[wikt:pleomorphism|pleomorphism]], [[pleonasm]], [[pleonexia]], [[wiktionary:plethora|plethora]] and [[Pliocene]]. This meaning also coincides with the biblical Kīmāh and the Arabic word for the Pleiades — Al Thurayya.<ref name="ALLEN"/> In fact, Pleione may have been numbered amongst the [[Epimelides]] (nymphs of meadows and pastures) and presided over the multiplication of the animals, as her name means "to increase in number".<ref name="THEOI">
{{cite web
 |last=Atsma |first=A. J.
 |date=8 March 2010
 |url=http://www.theoi.com/Nymphe/NymphePleione.html
 |title=Pleione
 |publisher=Theoi Greek Mythology
 |access-date=2010-06-14
}}</ref>

Finally, the last comes from [[Peleiades]] ({{langx|grc|Πελειάδες|lit=[[dove]]s}}), a reference to the sisters' mythical transformation by [[Zeus]] into a flock of doves following their pursuit by [[Orion (mythology)|Orion]], the [http://antwrp.gsfc.nasa.gov/apod/ap100329.html giant huntsman], across the heavens.<ref name="HESIOD">
{{cite book
 |author1=Hesiod
 |author1-link=Hesiod
 |others=Translated by Evelyn-White, H. G.
 |date=1914
 |chapter=ll. 618–640
 |title=Works and Days
 |url=http://www.sacred-texts.com/cla/hesiod/works.htm
 |publisher=The Internet Sacred Text Archive
 |access-date=2010-10-13
 |isbn=0-585-30250-2
}}</ref>

===Modern legacy===
In the best-selling 1955 [[nature]] book published by [[Time-Life]] called ''[[The World We Live In (Life magazine)|The World We Live In]]'', there is an artist's impression of Pleione entitled ''Purple Pleione''.<ref>
{{cite book
 |last=Barnett |first=L.
 |date=1955
 |title=[[The World We Live In (Life magazine)|The World We Live In]]
 |publisher=[[Simon & Schuster]]
 |page=284
}}</ref> The illustration is from the famed space artist [[Chesley Bonestell]] and carries the caption: "Purple Pleione, a star of the familiar Pleiades cluster, rotates so rapidly that it has flattened into a flying saucer and hurled forth a dark red ring of hydrogen. Where the excited gas crosses Pleione's equator, it obscures her violet light."

Given its mythical connection with [[sailing]] and [[Pleione (plant)|orchid]]s, the name Pleione is often associated with grace, speed and elegance. Some of the finest designs in racing [[yacht]]s have the name Pleione,<ref name="DYSON">
{{cite web
 |title      = Team Pleione
 |publisher  = Marblehead International One Design Class
 |url        = http://ww.marbleheadiod.com/8mr/
 |access-date = 2010-10-07
}}{{dead link|date=January 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref name="TAYLOR">
{{cite web
 |last        = Taylor
 |first       = J.
 |date        = 19 March 2009
 |title       = Fast Boats in the 'Spirit of Tradition'
 |url         = http://www.tayloryachtdesigns.com/yacht_design.php#post-33
 |work        = Jim Taylor Yacht Designs
 |access-date  = 2010-10-07
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20100217064337/http://www.tayloryachtdesigns.com/yacht_design.php#post-33
 |archive-date = 17 February 2010
}}</ref> and the recent [[Shanghai Oriental Art Center]] draws its inspiration from an orchid.<ref name="CHAMI">{{cite web
 |last=Chami
 |first=C.
 |date=9 January 2008
 |title=Paul Andreu – The Oriental Arts Centre in Shanghai
 |url=http://www.archinnovations.com/featured-projects/public-buildings/paul-andreu-the-oriental-arts-centre-in-shanghai/
 |publisher=Archinnovations
 |access-date=2010-10-07
 |archive-date=2012-03-07
 |archive-url=https://web.archive.org/web/20120307080647/http://www.archinnovations.com/featured-projects/public-buildings/paul-andreu-the-oriental-arts-centre-in-shanghai/
 |url-status=usurped
 }}</ref> [[Fat Jon]] in his new [[album]] [[Hundred Eight Stars]] has a prismatic [[Track (CD)|track]] dedicated to 28 Tauri.<ref name="DISCOGS">
{{discogs release|id=998205|name=Hundred Eight Stars}}</ref>

==See also==
* [[Lists of stars]] in the constellation [[List of stars in Taurus|Taurus]]
* [[B star|Class B Stars]]
* [[Be stars]]
* [[Shell star]]
* [[Circumstellar disk]]

==Notes==
{{Reflist|group=note}}

==References==
{{Reflist|refs=

<ref name="White2017">{{cite journal |last1=White |first1=T. R. |last2=Pope |first2=B. J. S. |last3=Antoci |first3=V. |last4=Pápics |first4=P. I. |last5=Aerts |first5=C. |last6=Gies |first6=D. R. |last7=Gordon |first7=K. |last8=Huber |first8=D. |last9=Schaefer |first9=G. H. |last10=Aigrain |first10=S. |last11=Albrecht |first11=S. |last12=Barclay |first12=T. |last13=Barentsen |first13=G. |last14=Beck |first14=P. G. |last15=Bedding |first15=T. R. |last16=Andersen |first16=M. Fredslund |last17=Grundahl |first17=F. |last18=Howell |first18=S. B. |last19=Ireland |first19=M. J. |last20=Murphy |first20=S. J. |last21=Nielsen |first21=M. B. |last22=Aguirre |first22=V. Silva |last23=Tuthill |first23=P. G. |title=Beyond the Kepler/K2 bright limit: variability in the seven brightest members of the Pleiades |journal=Monthly Notices of the Royal Astronomical Society |date=November 2017 |volume=471 |issue=3 |pages=2882–2901 |doi=10.1093/mnras/stx1050 |doi-access=free |arxiv=1708.07462 |bibcode=2017MNRAS.471.2882W |url=https://academic.oup.com/mnras/article/471/3/2882/4081952 |access-date=27 March 2022}}</ref>

}}

==External links==
{{Commons category|Pleione (star)}}
*Jim Kaler's Stars, University of Illinois: [http://stars.astro.illinois.edu/sow/pleione.html PLEIONE (28 Tauri)]
* Philippe Stee's in-depth information on: [http://www.oca.eu/stee/page1/page3/page3.html Hot and Active Stars Research]
* Olivier Thizy's in-depth information on: [https://web.archive.org/web/20110927005933/http://shelyak.com/dossier.php?id_dossier=24 Be Stars]
*High-resolution LRGB image based on 4 hrs total exposure: [http://www.perseus.gr/Astro-DSO-M45.htm M45 – Pleiades Open Cluster]
*[[Astronomy Picture of the Day|APOD]] Pictures:
# [http://antwrp.gsfc.nasa.gov/apod/ap100329.html Orion, the giant huntsman, in pursuit of the Pleiades]
# [http://antwrp.gsfc.nasa.gov/apod/ap091205.html Himalayan Skyscape]
# [http://antwrp.gsfc.nasa.gov/apod/ap090212.html Pleiades and the Milky Way]
# [http://antwrp.gsfc.nasa.gov/apod/ap071122.html Pleiades and the Interstellar Medium]

{{Stars of Taurus}}
{{Authority control}}
{{Good article}}

{{DEFAULTSORT:Pleione (Star)}}
[[Category:Taurus (constellation)]]
[[Category:Pleiades]]
[[Category:B-type main-sequence stars]]
[[Category:Be stars]]
[[Category:Gamma Cassiopeiae variable stars]]
[[Category:Binary stars]]
[[Category:Flamsteed objects|Tauri, 028]]
[[Category:Objects with variable star designations|Tauri, BU]]
[[Category:Henry Draper Catalogue objects|023862]]
[[Category:Bright Star Catalogue objects|1180]]
[[Category:Hipparcos objects|017851]]
[[Category:Durchmusterung objects]]