Pleione (star)
Encyclopedia
Pleione is a binary star
in the Pleiades
star cluster
(M45), located roughly 390 light years away in the constellation
of Taurus
. Pleione was not given a Bayer designation
, but did receive a Flamsteed number—hence its designation 28 Tauri. Since the star is located close to Atlas
, it's difficult for stargazers to distinguish with the naked eye, even though it's a hot type B star 190 times more luminous than the Sun
. Pleione rotates even faster than Achernar
on its axis, close to its breakup velocity.
The brighter star of the binary pair, component A, is a classical 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. Although some research on the system has been performed, stellar characteristics of the orbiting B component are not well known. Pleione is the seventh brightest star in the Pleiades, after Taygeta
.
of +5.05 in V
, the star is rather difficult to make out with the naked eye, especially since its close neighbour Atlas
is 3.7 times brighter and located less than 5 arcminutes away. 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 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
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
magnitude
. Its variable star designation
is BU Tauri. The SIMBAD
astronomical database lists its spectral class
as B8IVev, although the current classification recognized by many researchers is B8IVpe. The suffix "ev" stands for "Spectral emission that exhibits variability" whilst the suffix "pe" refers to "Emission lines with peculiarity". In the case of Pleione, the "peculiar" emissions come from gaseous 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 photometric
and spectroscopic observations yielding valuable insights as well. Before the Hipparcos
mission, the estimated distance for the Pleiades star cluster was around 135 parsecs or 440 light years. However, when the Hipparcos Catalogue was published in 1997, the new parallax measurement indicated a much closer distance of about 119 ± 1.0 pc
(388 ± 3.2 ly), triggering substantial controversy among astronomers. 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 Telescope
's Fine Guidance Sensor
s and corroborated by studies from Caltech and NASA
's Jet Propulsion Laboratory
showed the original estimate of 135 pc
or 440 ly to be the correct figure. However a recent study published in 2009 has argued otherwise, publishing a new parallax measurement of 8.32 mas with a very tight error factor of 0.13 mas yielding a distance of 120.2 ± 1.9 pc
or 392.0 ± 6.0 ly. Which distance estimate future astrometric calculations will corroborate remains to be seen, although the upcoming Gaia mission with its expected launch in late 2012 could well prove to be the ultimate arbiter in this debate.
, 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
, Pleione is a blue-white B-type
main sequence
dwarf star
with a temperature of about 12,000 Kelvin
. It has a bolometric luminosity of 190 L☉ assuming a distance of roughly 120 pc
. With a radius of 3.2 R☉ and mass that is 3.4 M☉
, Pleione is considerably smaller than the brightest stars in the Pleiades. Alcyone
for instance has a radius that is 10 R☉ with a luminosity 2,400 L☉, making it roughly 30 times more voluminous than Pleione and about 13 times brighter.
, often referred to as an "active hot star". Classical Be stars are B-type stars close to the main sequence with an "e" added on, signifying that Pleione exhibits emission lines
in its spectrum
rather than absorption lines
, which is what stars usually show. 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
that emanates not just from the photosphere
, but from the disk as well. The geometry and kinematics
of this gaseous circumstellar environment are therefore 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 disks", which is material being actively ejected by the star as opposed to "accretion disks" which involves material falling toward the star.
Be Stars are fast rotators
(>200 km/s) with a large stellar wind
and high mass loss
rate, hence the causative factors behind these gaseous rings. Due to its apparent brightness, the star most recognized for its fast rotation is Achernar
, a phenomenon which causes it to be highly oblate
. Its rotational velocity, however, of 251 km/s is considerably slower than Pleione's 329 km/s. As a result, Pleione actually revolves on its axis once every 11.8 hours compared to Achernar's 48.4 hours. The Sun
by comparison takes 25.3 days to turn on its axis. Pleione is spinning so fast that it is close to the estimated breakup velocity for a B8V star of about 370–390 km/s. Another Be star whose rotational velocity is extremely fast is Alpha Arae at 470 km/s—a speed so extreme that it is on the verge of exploding.
What makes Pleione particularly unique is that it alternates between three different phases: 1) normal B star, 2) Be star
and 3) Be shell star
. The cause is likely the surrounding gaseous disk which in many Be stars will appear, then disappear, possibly reforming at a later time. Material in the disc is attracted back towards the star by the pull of gravity, but if it has enough energy it can escape into space, contributing to the stellar wind
. Sometimes, Be stars will form multiple gas rings or "decretion disks", each with its own evolution, creating 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. In fact, during the last 100 years, Pleione has demonstrated notable phase changes—as a Be phase until 1903, a B phase (1905–1936), a B-shell phase (1938–1954), and another Be phase (1955–1972). It then entered a Be-shell phase in 1972. Then, the star developed many 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 the minimum brightness in late 1973, the star gradually brightened. In 1989, Pleione entered a Be phase and stayed as a Be star until the summer of 2005.
The most recent disk responsible for these phase changes was formed in 1972. What's intriguing, however, is that Pleione's long-term polarimetric
observations show the intrinsic polarization angle
has changed, providing direct evidence for a spatial motion of the disk axis. Because Pleione has a stellar companion with a relatively 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.
Recent photometric and spectroscopic observations from 2005 to 2007 indicate that a new disk has formed around the equator—thus constituting a double disk phenomenon with disks at different angles. The inclination angle of the new disk is estimated at 60° whereas the previous disk was inclined at around 30°. Such a misaligned double-disk structure has never been observed before among Be stars. Thus, Pleione provides a rare opportunity to investigate the forming process of a new disk and the consequent interaction between the two.
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 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.
nymph
of Mount Kyllene in Arkadia (southern Greece), one of the three thousand daughters of the Titans
Oceanus
and Tethys
. The nymphs in Greek mythology
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
, the Titan
, and mother of the Hyas
, Hyades
and Pleiades
.
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 mythical 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
. This meaning also coincides with the biblical Kīmāh and the Arabic word for the Pleiades — Al Thurayya. 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
: , "dove
s"), a reference to the sisters' mythical transformation by Zeus
into a flock of doves following their pursuit by Orion
, the giant huntsman, across the heavens.
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
and orchids, the name Pleione is often associated with grace, speed and elegance. Some of the finest designs in racing yacht
s have the name Pleione, and the recent Shanghai Oriental Art Center draws its inspiration from an orchid. Fat Jon
in his new album
Hundred Eight Stars has a prismatic track
dedicated to 28 Tauri.
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
in the Pleiades
Pleiades (star cluster)
In astronomy, the Pleiades, or Seven Sisters , is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky...
star cluster
Open cluster
An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist...
(M45), located roughly 390 light years away in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
of Taurus
Taurus (constellation)
Taurus is one of the constellations of the zodiac. Its name is a Latin word meaning 'bull', and its astrological symbol is a stylized bull's head:...
. Pleione was not given a Bayer designation
Bayer designation
A Bayer designation is a stellar designation in which a specific star is identified by a Greek letter, followed by the genitive form of its parent constellation's Latin name...
, but did receive a Flamsteed number—hence its designation 28 Tauri. Since the star is located close to Atlas
Atlas (star)
Atlas is a triple star system in the Pleiades open cluster . It is also known as 27 Tauri.The primary component, Atlas A, is a blue-white B-type giant with an apparent magnitude of +3.62. It is a spectroscopic binary whose components have magnitudes of +4.1 and +5.6. The binary makes one orbit...
, it's difficult for stargazers to distinguish with the naked eye, even though it's a hot type B star 190 times more luminous than the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. Pleione rotates even faster than Achernar
Achernar
Achernar , sometimes spelled Achenar, is the brightest star in the constellation Eridanus and the ninth-brightest star in the night sky. Of the top ten apparent brightest stars —Sirius, Canopus, Alpha Centauri, Arcturus, Vega, Capella, Rigel, Procyon, Achernar and Betelgeuse—Achernar is the hottest...
on its axis, close to its breakup velocity.
The brighter star of the binary pair, component A, is a classical Be star
Be star
A Be star is a B-type star with prominent emission lines of hydrogen in its spectrum. The designation is combined by the spectral class, B, and the lowercase e denoting emission in the spectral classification system. Line emission from other atomic ions might be present as well, but is typically...
with certain distinguishing traits: periodic phase changes and a complex circumstellar environment composed of two gaseous disks at different angles to each other. Although some research on the system has been performed, stellar characteristics of the orbiting B component are not well known. Pleione is the seventh brightest star in the Pleiades, after Taygeta
Taygeta (star)
Taygeta is a triple star system in the constellation Taurus and a member of the Pleiades star cluster. It is approximately 440 light years from Earth....
.
Visibility
With an apparent magnitudeApparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
of +5.05 in V
UBV photometric system
UBV photometric system, also called the Johnson system , is a wide band photometric system for classifying stars according to their colors. It is the first known standardized photoelectric photometric system. The letters U, B, and V stand for ultraviolet, blue, and visual magnitudes, which are...
, the star is rather difficult to make out with the naked eye, especially since its close neighbour Atlas
Atlas (star)
Atlas is a triple star system in the Pleiades open cluster . It is also known as 27 Tauri.The primary component, Atlas A, is a blue-white B-type giant with an apparent magnitude of +3.62. It is a spectroscopic binary whose components have magnitudes of +4.1 and +5.6. The binary makes one orbit...
is 3.7 times brighter and located less than 5 arcminutes away. 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
Dawn
Dawn is the time that marks the beginning of the twilight before sunrise. It is recognized by the presence of weak sunlight, while the sun itself is still below the horizon...
. 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
Cape Town
Cape Town is the second-most populous city in South Africa, and the provincial capital and primate city of the Western Cape. As the seat of the National Parliament, it is also the legislative capital of the country. It forms part of the City of Cape Town metropolitan municipality...
, South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
, at the tip of the Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
n continent
Continent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...
, the star rises almost 32° above the horizon
Horizon
The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting...
. Due to its declination of roughly +24°, Pleione is circumpolar
Circumpolar star
A circumpolar star is a star that, as viewed from a given latitude on Earth, never sets , due to its proximity to one of the celestial poles...
in the northern hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...
at latitudes greater than 66° North. Once late April arrives, the cluster can be spotted briefly in the deepening twilight
Twilight
Twilight is the time between dawn and sunrise or between sunset and dusk, during which sunlight scattering in the upper atmosphere illuminates the lower atmosphere, and the surface of the earth is neither completely lit nor completely dark. The sun itself is not directly visible because it is below...
of the western horizon, soon to disappear with the other setting stars.
Pleione is classified as a Gamma Cassiopeiae type variable star
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
, with brightness fluctuations that range between a 4.8 and 5.5 visual
UBV photometric system
UBV photometric system, also called the Johnson system , is a wide band photometric system for classifying stars according to their colors. It is the first known standardized photoelectric photometric system. The letters U, B, and V stand for ultraviolet, blue, and visual magnitudes, which are...
magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
. Its variable star designation
Variable star designation
Variable stars are named using a variation on the Bayer designation format of an identifying label combined with the Latin genitive of the name of the constellation in which the star lies...
is BU Tauri. The SIMBAD
SIMBAD
SIMBAD is an astronomical database of objects beyond the Solar System...
astronomical database lists its spectral class
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
as B8IVev, although the current classification recognized by many researchers is B8IVpe. The suffix "ev" stands for "Spectral emission that exhibits variability" whilst the suffix "pe" refers to "Emission lines with peculiarity". In the case of Pleione, the "peculiar" emissions come from gaseous circumstellar disk
Circumstellar disk
A circumstellar disk is a torus, pancake or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids or collision fragments in orbit around a star. Around the youngest stars, they are the reservoirs of material out of which planets may form...
s formed of material being ejected from the star.
There has been significant debate as to the star's actual distance from Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
. The debate revolves around the different methodologies to measure distance—parallax
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
being the most central, but photometric
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...
and spectroscopic observations yielding valuable insights as well. Before the Hipparcos
Hipparcos
Hipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...
mission, the estimated distance for the Pleiades star cluster was around 135 parsecs or 440 light years. However, when the Hipparcos Catalogue was published in 1997, the new parallax measurement indicated a much closer distance of about 119 ± 1.0 pc
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
(388 ± 3.2 ly), triggering substantial controversy among astronomers. 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 Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
's Fine Guidance Sensor
Fine Guidance Sensor
The Fine Guidance Sensor is an interferometric instrument on board the Hubble Space Telescope that provides high-precision pointing information as input to the observatory's attitude control systems....
s and corroborated by studies from Caltech and NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
's Jet Propulsion Laboratory
Jet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...
showed the original estimate of 135 pc
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
or 440 ly to be the correct figure. However a recent study published in 2009 has argued otherwise, publishing a new parallax measurement of 8.32 mas with a very tight error factor of 0.13 mas yielding a distance of 120.2 ± 1.9 pc
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
or 392.0 ± 6.0 ly. Which distance estimate future astrometric calculations will corroborate remains to be seen, although the upcoming Gaia mission with its expected launch in late 2012 could well prove to be the ultimate arbiter in this debate.
Properties
In 1942 Otto StruveOtto Struve
Otto Struve was a Russian astronomer. In Russian, his name is sometimes given as Otto Lyudvigovich Struve ; however, he spent most of his life and his entire scientific career in the United States...
, one of the early researchers of Be Stars, stated that Pleione is "the most interesting member of the Pleiades
Pleiades
Pleiades can refer to:Astronomy and science*Pleiades , an open cluster of stars in the constellation Taurus**Pleiades in folklore and literature, interpretations and traditional meanings of the star cluster among various human cultures...
cluster". Like many of the stars in the cluster
Open cluster
An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist...
, Pleione is a blue-white B-type
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
dwarf star
Dwarf star
The term dwarf star refers to a variety of distinct classes of stars.* Dwarf star alone generally refers to any main sequence star, a star of luminosity class V.** Red dwarfs are low-mass main sequence stars....
with a temperature of about 12,000 Kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
. It has a bolometric luminosity of 190 L☉ assuming a distance of roughly 120 pc
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
. With a radius of 3.2 R☉ and mass that is 3.4 M☉
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
, Pleione is considerably smaller than the brightest stars in the Pleiades. Alcyone
Alcyone (star)
Alcyone is a star system in the constellation Taurus. It is the brightest star in the Pleiades open cluster, which is a young cluster, aged at less than 50 million years. Alcyone is approximately 370 light years from Earth. It is named after the mythological figure Alcyone, one of the...
for instance has a radius that is 10 R☉ with a luminosity 2,400 L☉, making it roughly 30 times more voluminous than Pleione and about 13 times brighter.
Be star
Pleione is a classical Be starBe star
A Be star is a B-type star with prominent emission lines of hydrogen in its spectrum. The designation is combined by the spectral class, B, and the lowercase e denoting emission in the spectral classification system. Line emission from other atomic ions might be present as well, but is typically...
, often referred to as an "active hot star". Classical Be stars are B-type stars close to the main sequence with an "e" added on, signifying that Pleione exhibits emission lines
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
in its spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
rather than absorption lines
Spectrum analysis
Spectrum, also known as emission spectrochemical analysis, is the original scientific method of charting and analyzing the chemical properties of matter and gases by looking at the bands in their optical spectrum...
, which is what stars usually show. 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
Circumstellar disk
A circumstellar disk is a torus, pancake or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids or collision fragments in orbit around a star. Around the youngest stars, they are the reservoirs of material out of which planets may form...
, resulting in electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
that emanates not just from the photosphere
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...
, but from the disk as well. The geometry and kinematics
Kinematics
Kinematics is the branch of classical mechanics that describes the motion of bodies and systems without consideration of the forces that cause the motion....
of this gaseous circumstellar environment are therefore 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 disks", which is material being actively ejected by the star as opposed to "accretion disks" which involves material falling toward the star.
Be Stars are fast rotators
Stellar rotation
Stellar rotation is the angular motion of a star about its axis. The rate of rotation can be measured from the spectrum of the star, or by timing the movements of active features on the surface....
(>200 km/s) with a large stellar wind
Stellar wind
A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.Different types of stars have...
and high mass loss
Stellar mass loss
Stellar mass loss is a phenomenon observed in some massive stars. It occurs when a triggering event causes the ejection of a large portion of the star's mass. Stellar mass loss can also occur when a star gradually loses material to a binary companion or into interstellar space.-Causes:A number of...
rate, hence the causative factors behind these gaseous rings. Due to its apparent brightness, the star most recognized for its fast rotation is Achernar
Achernar
Achernar , sometimes spelled Achenar, is the brightest star in the constellation Eridanus and the ninth-brightest star in the night sky. Of the top ten apparent brightest stars —Sirius, Canopus, Alpha Centauri, Arcturus, Vega, Capella, Rigel, Procyon, Achernar and Betelgeuse—Achernar is the hottest...
, a phenomenon which causes it to be highly oblate
Flattening
The flattening, ellipticity, or oblateness of an oblate spheroid is a measure of the "squashing" of the spheroid's pole, towards its equator...
. Its rotational velocity, however, of 251 km/s is considerably slower than Pleione's 329 km/s. As a result, Pleione actually revolves on its axis once every 11.8 hours compared to Achernar's 48.4 hours. The Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
by comparison takes 25.3 days to turn on its axis. Pleione is spinning so fast that it is close to the estimated breakup velocity for a B8V star of about 370–390 km/s. Another Be star whose rotational velocity is extremely fast is Alpha Arae at 470 km/s—a speed so extreme that it is on the verge of exploding.
What makes Pleione particularly unique is that it alternates between three different phases: 1) normal B star, 2) Be star
Be star
A Be star is a B-type star with prominent emission lines of hydrogen in its spectrum. The designation is combined by the spectral class, B, and the lowercase e denoting emission in the spectral classification system. Line emission from other atomic ions might be present as well, but is typically...
and 3) Be shell star
Shell star
A shell star, also termed Gamma Cassiopeiae variable , is a star having a spectrum that exhibits features indicating a circumstellar disc of gas surrounding the star at the equator. They exhibit irregular variations in their luminosity due to the outflow of matter...
. The cause is likely the surrounding gaseous disk which in many Be stars will appear, then disappear, possibly reforming at a later time. Material in the disc is attracted back towards the star by the pull of gravity, but if it has enough energy it can escape into space, contributing to the stellar wind
Stellar wind
A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.Different types of stars have...
. Sometimes, Be stars will form multiple gas rings or "decretion disks", each with its own evolution, creating complex circumstellar dynamics.
As a result of such dynamics, Pleione exhibits prominent long-term photometric
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...
and spectroscopic variations encompassing a period of about 35 years. In fact, during the last 100 years, Pleione has demonstrated notable phase changes—as a Be phase until 1903, a B phase (1905–1936), a B-shell phase (1938–1954), and another Be phase (1955–1972). It then entered a Be-shell phase in 1972. Then, the star developed many shell
Shell star
A shell star, also termed Gamma Cassiopeiae variable , is a star having a spectrum that exhibits features indicating a circumstellar disc of gas surrounding the star at the equator. They exhibit irregular variations in their luminosity due to the outflow of matter...
absorption lines
Absorption spectroscopy
Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field. The intensity of the absorption varies as a...
in its spectrum. At the same time, the star showed a decrease in brightness, beginning at the end of 1971. After reaching the minimum brightness in late 1973, the star gradually brightened. In 1989, Pleione entered a Be phase and stayed as a Be star until the summer of 2005.
The most recent disk responsible for these phase changes was formed in 1972. What's intriguing, however, is that Pleione's long-term polarimetric
Polarimetry
Polarimetry is the measurement and interpretation of the polarization of transverse waves, most notably electromagnetic waves, such as radio or light waves...
observations show the intrinsic polarization angle
Brewster's angle
Brewster's angle is an angle of incidence at which light with a particular polarization is perfectly transmitted through a transparent dielectric surface, with no reflection. When unpolarized light is incident at this angle, the light that is reflected from the surface is therefore perfectly...
has changed, providing direct evidence for a spatial motion of the disk axis. Because Pleione has a stellar companion with a relatively close orbit, the shift in the polarization angle has been attributed to the companion causing a precession
Precession
Precession is a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle is constant...
(wobble) of the disk, with a precession period of roughly 81 years.
Recent photometric and spectroscopic observations from 2005 to 2007 indicate that a new disk has formed around the equator—thus constituting a double disk phenomenon with disks at different angles. The inclination angle of the new disk is estimated at 60° whereas the previous disk was inclined at around 30°. Such a misaligned double-disk structure has never been observed before among Be stars. Thus, Pleione provides a rare opportunity to investigate the forming process of a new disk and the consequent interaction between the two.
Star system
Pleione is known to be a speckleSpeckle imaging
Speckle imaging describes a range of high-resolution astronomical imaging techniques based either on the shift-and-add method or on speckle interferometry methods...
binary
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
, 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
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...
of 218.0 days and a large eccentricity
Orbital elements
Orbital elements are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are generally considered in classical two-body systems, where a Kepler orbit is used...
of 0.6. 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 AU, assuming a distance of 120 parsecs.
Ethnological influences
Mythology
Pleione was an OceanidOceanid
In Greek mythology and, later, Roman mythology, the Oceanids were the three thousand daughters of the Titans Oceanus and Tethys. Each was the patroness of a particular spring, river, sea, lake, pond, pasture, flower or cloud...
nymph
Nymph
A nymph in Greek mythology is a female minor nature deity typically associated with a particular location or landform. Different from gods, nymphs are generally regarded as divine spirits who animate nature, and are usually depicted as beautiful, young nubile maidens who love to dance and sing;...
of Mount Kyllene in Arkadia (southern Greece), one of the three thousand daughters of the Titans
Titan (mythology)
In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, that ruled during the legendary Golden Age....
Oceanus
Oceanus
Oceanus ; , Ōkeanós) was a pseudo-geographical feature in classical antiquity, believed by the ancient Greeks and Romans to be the world-ocean, an enormous river encircling the world....
and Tethys
Tethys (mythology)
In Greek mythology, Tethys , daughter of Uranus and Gaia was an archaic Titaness and aquatic sea goddess, invoked in classical Greek poetry but not venerated in cult. Tethys was both sister and wife of Oceanus...
. The nymphs in Greek mythology
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...
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
Atlas (mythology)
In Greek mythology, Atlas was the primordial Titan who supported the heavens. Although associated with various places, he became commonly identified with the Atlas Mountains in north-west Africa...
, the Titan
Titan (mythology)
In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, that ruled during the legendary Golden Age....
, and mother of the Hyas
Hyas
Hyas, in Greek mythology, was a son of the Titan Atlas by Aethra . He was a notable archer who was killed by his intended prey. Some stories have him dying after attempting to rob a lion of its cubs. Some have him killed by a serpent, but most commonly he is said to have been gored by a wild boar...
, Hyades
Hyades (mythology)
In Greek mythology, the Hyades , are a sisterhood of nymphs that bring rain.The Hyades were daughters of Atlas and sisters of Hyas in most tellings, although one version gives their parents as Hyas and Boeotia...
and Pleiades
Pleiades (Greek mythology)
The Pleiades , companions of Artemis, were the seven daughters of the titan Atlas and the sea-nymph Pleione born on Mount Cyllene. They are the sisters of Calypso, Hyas, the Hyades, and the Hesperides...
.
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 mythologicalMythology
The term mythology can refer either to the study of myths, or to a body or collection of myths. As examples, comparative mythology is the study of connections between myths from different cultures, whereas Greek mythology is the body of myths from ancient Greece...
Pleiades (the 'Seven Sisters'). The term "Pleiades" was used by Valerius Flaccus
Gaius Valerius Flaccus
Gaius Valerius Flaccus was a Roman poet who flourished in the "Silver Age" under the emperors Vespasian and Titus and wrote a Latin Argonautica that owes a great deal to Apollonius of Rhodes' more famous epic....
to apply to the cluster as a whole, and Riccioli
Giovanni Battista Riccioli
Giovanni Battista Riccioli was an Italian astronomer and a Catholic priest in the Jesuit order...
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 mythical 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
Lexeme
A lexeme is an abstract unit of morphological analysis in linguistics, that roughly corresponds to a set of forms taken by a single word. For example, in the English language, run, runs, ran and running are forms of the same lexeme, conventionally written as RUN...
with many English derivatives like pleiotropy
Pleiotropy
Pleiotropy occurs when one gene influences multiple phenotypic traits. Consequently, a mutation in a pleiotropic gene may have an effect on some or all traits simultaneously...
, pleomorphism
Pleomorphism
Pleomorphism is a concept that two or more structural forms can occur during a life cycle, especially of certain plants, including many marine plants such as Mastocarpus stellatus...
, pleonasm
Pleonasm
Pleonasm is the use of more words or word-parts than is necessary for clear expression: examples are black darkness, or burning fire...
, pleonexia
Pleonexia
Pleonexia, sometimes called pleonexy, originates from the Greek language πλεονεξια and is a philosophical concept employed both in the New Testament and in writings by Plato and Aristotle...
, plethora and Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...
. This meaning also coincides with the biblical Kīmāh and the Arabic word for the Pleiades — Al Thurayya. 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
Peleiades
Peleiades were the sacred women of Zeus and the Mother Goddess, Dione, at the Oracle at Dodona. Pindar made a reference to the Pleiades as the "peleiades" a flock of doves, but the connection seems witty and poetical, rather than mythic. The chariot of Aphrodite was drawn by a flock of doves,...
(Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...
: , "dove
Dove
Pigeons and doves constitute the bird family Columbidae within the order Columbiformes, which include some 300 species of near passerines. In general terms "dove" and "pigeon" are used somewhat interchangeably...
s"), a reference to the sisters' mythical transformation by Zeus
Zeus
In the ancient Greek religion, Zeus was the "Father of Gods and men" who ruled the Olympians of Mount Olympus as a father ruled the family. He was the god of sky and thunder in Greek mythology. His Roman counterpart is Jupiter and his Etruscan counterpart is Tinia.Zeus was the child of Cronus...
into a flock of doves following their pursuit by Orion
Orion (mythology)
Orion was a giant huntsman in Greek mythology whom Zeus placed among the stars as the constellation of Orion....
, the giant huntsman, across the heavens.
Modern legacy
In the best-selling 1955 natureNature
Nature, in the broadest sense, is equivalent to the natural world, physical world, or material world. "Nature" refers to the phenomena of the physical world, and also to life in general...
book published by Time-Life
Time-Life
Time–Life is a creator and direct marketer of books, music, video/DVD, and multimedia products. Its products are sold throughout North America, Europe, Australia, and Asia through television, print, retail, the Internet, telemarketing, and direct sales....
called The World We Live In
The World We Live In
The World We Live In appeared in the pages of LIFE magazine from December 8, 1952, to December 20, 1954. A science series, it comprised 13 chapters published on an average of every eight months....
, there is an artist's impression of Pleione entitled Purple Pleione. The illustration is from the famed space artist Chesley Bonestell
Chesley Bonestell
Chesley Bonestell was an American painter, designer and illustrator. His paintings were a major influence on science fiction art and illustration, and he helped inspire the American space program...
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
Sailing is the propulsion of a vehicle and the control of its movement with large foils called sails. By changing the rigging, rudder, and sometimes the keel or centre board, a sailor manages the force of the wind on the sails in order to move the boat relative to its surrounding medium and...
and orchids, the name Pleione is often associated with grace, speed and elegance. Some of the finest designs in racing yacht
Yacht
A yacht is a recreational boat or ship. The term originated from the Dutch Jacht meaning "hunt". It was originally defined as a light fast sailing vessel used by the Dutch navy to pursue pirates and other transgressors around and into the shallow waters of the Low Countries...
s have the name Pleione, and the recent Shanghai Oriental Art Center draws its inspiration from an orchid. Fat Jon
Fat Jon
Fat Jon the Ample Soul Physician is an American hip hop producer from Cincinnati, Ohio and one of the four members of Five Deez, alongside Pace Rock, Sonic, and Kyle David...
in his new album
Album
An album is a collection of recordings, released as a single package on gramophone record, cassette, compact disc, or via digital distribution. The word derives from the Latin word for list .Vinyl LP records have two sides, each comprising one half of the album...
Hundred Eight Stars has a prismatic track
Track (CD)
On an optical disc, a track or title is a subdivision of its content. Specifically, it is a consecutive set of sectors on the disc containing a block of data. One session may contain one or more tracks of the same or different types...
dedicated to 28 Tauri.
See also
- Lists of stars in the constellation Taurus
- Class B Stars
- Be stars
- Shell starShell starA shell star, also termed Gamma Cassiopeiae variable , is a star having a spectrum that exhibits features indicating a circumstellar disc of gas surrounding the star at the equator. They exhibit irregular variations in their luminosity due to the outflow of matter...
- Circumstellar diskCircumstellar diskA circumstellar disk is a torus, pancake or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids or collision fragments in orbit around a star. Around the youngest stars, they are the reservoirs of material out of which planets may form...
External links
- Jim Kaler's Stars, University of Illinois:PLEIONE (28 Tauri)
- Philippe Stee's in-depth information on: Hot and Active Stars Research
- Olivier Thizy's in-depth information on: Be Stars
- High-resolution LRGB image based on 4 hrs total exposure: M45 - Pleiades Open Cluster
- APODAstronomy Picture of the DayAstronomy Picture of the Day is a website provided by NASA and Michigan Technological University . According to the website, "Each day a different image or photograph of our universe is featured, along with a brief explanation written by a professional astronomer."The photograph is not necessarily...
Pictures: