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PleionePronounced or is a binary star and the seventh-brightest star in the Pleiades star cluster (
Messier 45 Messier may refer to: People with the surname * Charles Messier, French astronomer * Éric Messier, former NHL defenseman * George Messier, French inventor * Jean-Marie Messier, former CEO of Vivendi Universal * Marc Messier, Canadian actor from Qu ...
). It has the variable star designation BU Tauri (BU Tau) and the
Flamsteed John Flamsteed (19 August 1646 – 31 December 1719) was an English astronomer and the first Astronomer Royal. His main achievements were the preparation of a 3,000-star catalogue, ''Catalogus Britannicus'', and a star atlas called '' Atlas ...
designation 28 Tauri (28 Tau). The star is located approximately from the Sun, appearing in the constellation of Taurus. Pleione is located close on the sky to the brighter star
Atlas An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth. Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geograp ...
, so is difficult for stargazers to distinguish with the naked eye despite being a
fifth magnitude star Apparent magnitude () is a measure of the Irradiance, brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction (astron ...
. The brighter star of the Pleione binary pair, component A, is a hot 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
breakup velocity A relationship breakup, breakup, or break-up is the termination of a relationship. The act is commonly termed "dumping omeone in slang when it is initiated by one partner. The term is less likely to be applied to a married couple, where a break ...
, 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 A major branch of classical mythology, Greek mythology is the body of myths originally told by the ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the origin and nature of the world, the lives and activities of ...
; she is the mother of seven daughters known as
the Pleiades The Pleiades (), also known as The Seven Sisters, Messier 45 and other names by different cultures, is an asterism and an open star cluster containing middle-aged, hot B-type stars in the north-west of the constellation Taurus. At a distance of ...
. In 2016, the
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach ...
organized a
Working Group on Star Names The International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in May 2016 to catalog and standardize List of proper names of stars, proper names for stars for the international astronomical community. It operates under ...
(WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 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.


Visibility

With an apparent magnitude of +5.05 in V, the star is rather difficult to make out with the naked eye, especially since its close neighbour
Atlas An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth. Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geograp ...
is 3.7 times brighter and located less than 5 arcminutes away.The brightness ratio of Atlas versus Pleione is derived from the formula for apparent magnitude and is based on their respective
visual The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight ...
magnitudes: Atlas () at 3.62 and Pleione () at 5.05. Therefore: :\frac=2.512^=3.73
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. 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
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,
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, at the tip of the
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n
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, the star rises almost 32° above the horizon. Due to its declination of roughly +24°, Pleione is 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. Pleione is classified as a Gamma Cassiopeiae type variable star, with brightness fluctuations that range between a 4.8 and 5.5
visual The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight ...
magnitude. It has a spectral classification of B8Vne, a hot main sequence star with "nebulous" absorption lines due to its rapid rotation and emission lines from the surrounding circumstellar disks formed of material being ejected from the star. There has been significant debate as to the star's actual distance from
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...
. The debate revolves around the different methodologies to measure distance— parallax being the most central, but photometric and spectroscopic observations yielding valuable insights as well. For an in-depth discussion of Pleiades parallax measurements, see section 6.3 of Before the Hipparcos mission, the estimated distance for the Pleiades star cluster was around 135
parsecs The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, a ...
or 440 light years. When the Hipparcos Catalogue was published in 1997, the new parallax measurement indicated a much closer distance of about (), triggering substantial controversy among astronomers. The
Hipparcos new reduction ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial obj ...
produced a broadly similar distance of . 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 Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber op ...
measurements taken in 2004 by the
Hubble Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...
's Fine Guidance Sensors and corroborated by studies from Caltech and
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedi ...
's
Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, United States. Founded in the 1930s by Caltech researchers, JPL is owned by NASA ...
showed the original estimate of 135 pc or 440 ly to be the correct figure. The Gaia EDR3 parallax is , indicating a distance around . 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.


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". Like many of the stars in the
cluster may refer to: Science and technology Astronomy * Cluster (spacecraft), constellation of four European Space Agency spacecraft * Asteroid cluster, a small asteroid family * Cluster II (spacecraft), a European Space Agency mission to study th ...
, Pleione is a blue-white B-type main sequence dwarf star with a temperature of about . It has a bolometric luminosity of assuming a distance of roughly 130 pc. With a radius of and mass that is , Pleione is considerably smaller than the brightest stars in the Pleiades. Alcyone for instance has a radius that is with a luminosity , making it roughly 30 times more voluminous than Pleione and about 13 times brighter.The relative size of Alcyone (''V''A) compared to Pleione (''V''P) is determined by comparing their volumes. It is assumed that the volume of each star is reasonably approximated by the formula for a
sphere A sphere () is a Geometry, geometrical object that is a solid geometry, three-dimensional analogue to a two-dimensional circle. A sphere is the Locus (mathematics), set of points that are all at the same distance from a given point in three ...
: :''V''A ≈ π × 103 ≈ 4,188.79 ''V''Sun :''V''P ≈ π × 3.23 ≈ 137.26 ''V''Sun Therefore, the volume of Alcyone compared to Pleione ≈ 4,188.79 ÷ 137.26 ≈ 30.52 Alcyone's luminosity compared to Pleione is: :2,400 ÷ 190 = 12.63 (or 13 rounded)


Be star

Pleione is a classical Be star, often referred to as an "active hot star". Classical Be stars are B-type stars close to the main sequence with the "e" in the spectral type signifying that Pleione exhibits emission lines in its
spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of color ...
, rather than the absorption lines typical of B-type stars. 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 equatorial disk, resulting in
electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible ...
that emanates not just from the
photosphere The photosphere is a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...
, but from the disk as well. The geometry and kinematics of this gaseous circumstellar environment are best explained by a Keplerian disk – one that is supported against gravity by rotation, rather than gas or radiation pressure. Circumstellar disks like this are sometimes referred to as ''"
decretion disk Be stars are a heterogeneous set of stars with B spectral types and emission lines. A narrower definition, sometimes referred to as ''classical Be stars'', is a non-supergiant B star whose spectrum has, or had at some time, one or more Balmer e ...
s"'', because they consist of material being thrown off the star (as opposed to accretion disks which comprise material falling toward the star). Be Stars are fast rotators (>200 km/s), causing them to be highly oblate, with a substantial stellar wind and high mass loss rate. Pleione's rotational velocity of is considerably faster than the of Achernar, a prototypical Be star. Pleione revolves on its axis once every 11.8 hours, compared to 48.4 hours for Achernar.The time it takes for Achernar (''T''A) and Pleione (''T''P) 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''A = 10 × 696,000 km × 2 × π ÷ 251 km/s ÷ 3,600 ≈ 48.4 hrs :''T''P = 3.2 × 696,000 km × 2 × π ÷ 329 km/s ÷ 3,600 ≈ 11.8 hrs For comparison, the Sun takes 25.3 days to rotate. Pleione is spinning so fast that it is close to the estimated
breakup velocity A relationship breakup, breakup, or break-up is the termination of a relationship. The act is commonly termed "dumping omeone in slang when it is initiated by one partner. The term is less likely to be applied to a married couple, where a break ...
for a B8V star of about 370–390 km/s, which is why it is losing so much mass. Pleione is unusual because it alternates between three different phases: 1) normal B star, 2) Be star and 3) 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. Sometimes, Be stars form multiple decretion discs simultaneously, producing complex circumstellar dynamics. As a result of such dynamics, Pleione exhibits prominent long-term photometric and spectroscopic variations encompassing a period of about 35 years. 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). 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. These phase changes are ascribed to the evolution of a decretion disc that formed in 1972. Polarimetric observations show the intrinsic polarization angle has changed, indicating a change in orientation of the disc axis. 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 Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In o ...
(wobble) of the disk, with a precession period of roughly 81 years. 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°). Such a misaligned double-disc structure had not been observed around other Be stars.


Star system

Pleione is known to be a speckle binary, although its orbital parameters have yet to be fully established. 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 eccentricity of 0.6. The
Washington Double Star Catalogue The Washington Double Star Catalog, or WDS, is a catalog of double stars, maintained at the United States Naval Observatory. The catalog contains positions, magnitudes, proper motions and spectral types and has entries for (as of June 2017) 141,7 ...
lists an angular separation between the two components of 0.2 arcseconds—an angle which equates to a distance of about 24 AU, assuming a distance of 120 parsecs.


Ethnological influences


Mythology

Pleione was an Oceanid nymph of Mount Kyllene in Arkadia (southern Greece), one of the three thousand daughters of the Titans
Oceanus In Greek mythology, Oceanus (; grc-gre, , Ancient Greek pronunciation: , also Ὠγενός , Ὤγενος , or Ὠγήν ) was a Titans (mythology), Titan son of Uranus (mythology), Uranus and Gaia, the husband of his sister the Titan Tethy ...
and Tethys. The nymphs in
Greek mythology A major branch of classical mythology, Greek mythology is the body of myths originally told by the ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the origin and nature of the world, the lives and activities of ...
were the spirits of nature; oceanids, spirits of the sea. 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 An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth. Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geograp ...
, the
Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictiona ...
, and mother of the Hyas, Hyades and Pleiades.


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 mythological Pleiades (the 'Seven Sisters'). The term "Pleiades" was used by Valerius Flaccus to apply to the cluster as a whole, and Riccioli called the star ''Mater Pleione''. 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". This is particularly plausible given that ancient Greece was a seafaring culture and because of Pleione's
myth Myth is a folklore genre consisting of narratives that play a fundamental role in a society, such as foundational tales or origin myths. Since "myth" is widely used to imply that a story is not objectively true, the identification of a narrati ...
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. Another derivation of the name is the Greek word Πλειόνη ''(pr. plêionê)'', meaning "more", "plenty", or "full"—a lexeme with many English derivatives like pleiotropy, pleomorphism, pleonasm, pleonexia, plethora and
Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58 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". Finally, the last comes from Peleiades (
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
: , " doves"), a reference to the sisters' mythical transformation by
Zeus Zeus or , , ; grc, Δῐός, ''Diós'', label=genitive Boeotian Aeolic and Laconian grc-dor, Δεύς, Deús ; grc, Δέος, ''Déos'', label=genitive el, Δίας, ''Días'' () is the sky and thunder god in ancient Greek religion, ...
into a flock of doves following their pursuit by Orion, th
giant huntsman
across the heavens.


Modern legacy

In the best-selling 1955
nature Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans ar ...
book published by Time-Life called '' The World We Live In'', there is an artist's impression of Pleione entitled ''Purple Pleione''. 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 Sailing employs the wind—acting on sails, wingsails or kites—to propel a craft on the surface of the ''water'' ( sailing ship, sailboat, raft, windsurfer, or kitesurfer), on ''ice'' ( iceboat) or on ''land'' ( land yacht) over a chose ...
and orchids, the name Pleione is often associated with grace, speed and elegance. Some of the finest designs in racing
yacht A yacht is a sailing or power vessel used for pleasure, cruising, or racing. There is no standard definition, though the term generally applies to vessels with a cabin intended for overnight use. To be termed a , as opposed to a , such a pleasu ...
s have the name Pleione, and the recent Shanghai Oriental Art Center draws its inspiration from an orchid. Fat Jon in his new
album An album is a collection of audio recordings issued on compact disc (CD), Phonograph record, vinyl, audio tape, or another medium such as Digital distribution#Music, digital distribution. Albums of recorded sound were developed in the early ...
Hundred Eight Stars Jon Marshall (born September 8, 1975), better known by his stage name Fat Jon the Ample Soul Physician, is an American hip hop producer and rapper from Cincinnati, Ohio. He is a member of the American hip-hop group Five Deez. He is also one ha ...
has a prismatic track dedicated to 28 Tauri.


See also

* Lists of stars in the constellation Taurus * Class B Stars * Be stars * Shell star * Circumstellar disk


Notes


References


External links

*Jim Kaler's Stars, University of Illinois
PLEIONE (28 Tauri)
* Philippe Stee's in-depth information on

* Olivier Thizy's in-depth information on
Be Stars
*High-resolution LRGB image based on 4 hrs total exposure

* APOD Pictures:
Orion, the giant huntsman, in pursuit of the Pleiades






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