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Pleiades (star cluster)

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   The Pleiades are an open cluster dominated by hot blue stars surrounded
   by reflection nebulosity
   Enlarge
   The Pleiades are an open cluster dominated by hot blue stars surrounded
   by reflection nebulosity
   A shorter exposure shows less nebulosity.
   Enlarge
   A shorter exposure shows less nebulosity.

   The Pleiades (also known as M45 or the Seven Sisters) is the name of an
   open cluster in the constellation of Taurus. It is among the nearest to
   the Earth of all open clusters, probably the best known and certainly
   the most striking to the naked eye.

   The cluster is dominated by hot blue stars, which have formed within
   the last 100 million years. Dust that forms faint reflection nebulosity
   around the brightest stars was thought at first to be left over from
   the formation of the cluster but is now known to be an unrelated dust
   cloud that the stars are currently passing through. Astronomers
   estimate that the cluster will survive for about another 250 million
   years, after which time it will have dispersed due to gravitational
   interactions with the spiral arms of the galaxy and giant molecular
   clouds.

   Accurate knowledge of the distance to the cluster is very important in
   astronomy as it is a crucial first step on the cosmic distance ladder,
   the calibration of the distance scale of the whole universe. The
   cluster is now known to lie at a distance of about 135 parsecs (440
   light years).

History

   Comet Machholz passes near the Pleiades in early 2005
   Enlarge
   Comet Machholz passes near the Pleiades in early 2005

   The Pleiades are a prominent sight in the Northern Hemisphere in winter
   and in the Southern Hemisphere in summer, and have been known since
   antiquity to cultures all around the world, including the Māori and
   Australian Aborigines, the Japanese, the Aztec and the Sioux of North
   America. Some Greek astronomers considered them to be a distinct
   constellation, and they are mentioned by Hesiod, and in Homer's Iliad
   and Odyssey. They are also mentioned three times in the Bible ( Job
   9:9, 38:31; Amos 5:8). The Pleiades (Kartika) are particularly revered
   in Hindu mythology as the seven mothers of the war god Skanda.

   They have long been known to be a physically related group of stars
   rather than any chance alignment. The Reverend John Michell calculated
   in 1767 that the probability of a chance alignment of so many bright
   stars was only 1 in 500,000, and so correctly surmised that the
   Pleiades and many other clusters of stars must be physically related .
   When studies were first made of the stars' proper motions, it was found
   that they are all moving in the same direction across the sky, at the
   same rate, further demonstrating that they were related.

   Charles Messier measured the position of the cluster and included it as
   M45 in his catalogue of comet-like objects, published in 1771. Along
   with the Orion Nebula and the Praesepe cluster, Messier's inclusion of
   the Pleiades has been noted as curious, as most of Messier's objects
   were much fainter and more easily confused with comets—something which
   seems scarcely possible for the Pleiades. One possibility is that
   Messier simply wanted to have a larger catalogue than his scientific
   rival Lacaille, whose 1755 catalogue contained 42 objects, and so he
   added some bright, well-known objects to boost his list.

Distance

   The distance to the Pleiades is an important first step in the
   so-called cosmic distance ladder, a sequence of distance scales for the
   whole universe. The size of this first step calibrates the whole
   ladder, and the scale of this first step has been estimated by many
   methods. As the cluster is so close to the Earth, its distance is
   relatively easy to measure. Accurate knowledge of the distance allows
   astronomers to plot a Hertzsprung-Russell Diagram for the cluster
   which, when compared to those plotted for clusters whose distance is
   not known, allows their distances to be estimated. Other methods can
   then extend the distance scale from open clusters to galaxies and
   clusters of galaxies, and a cosmic distance ladder can be constructed.
   Ultimately astronomers' understanding of the age and future evolution
   of the universe is influenced by their knowledge of the distance to the
   Pleiades.

   Results prior to the launch of the Hipparcos satellite generally found
   that the Pleiades were about 135 parsecs away from Earth. Hipparcos
   caused consternation among astronomers by finding a distance of only
   118 parsecs by measuring the parallax of stars in the cluster—a
   technique which should yield the most direct and accurate results.
   Later work has consistently found that the Hipparcos distance
   measurement for the Pleiades was in error, but it is not yet known why
   the error occurred . The distance to the Pleiades is currently thought
   to be the higher value of about 135 parsecs ^, .

Composition

   X-ray images of the Pleiades reveal the stars with the hottest
   atmospheres. Green squares indicate the seven optically brightest
   stars.
   Enlarge
   X-ray images of the Pleiades reveal the stars with the hottest
   atmospheres. Green squares indicate the seven optically brightest
   stars.

   The cluster is about 12 light years in diameter and contains
   approximately 500 stars in total. It is dominated by young, hot blue
   stars, up to 14 of which can be seen with the naked eye depending on
   local observing conditions. The arrangement of the brightest stars is
   somewhat similar to Ursa Major and Ursa Minor. The total mass contained
   in the cluster is estimated to be about 800 solar masses.

   The cluster contains many brown dwarfs — objects with less than about
   8% of the Sun's mass, which are not heavy enough for nuclear fusion
   reactions to start in their cores and become proper stars. They may
   constitute up to 25% of the total population of the cluster, although
   they contribute less than 2% of the total mass . Astronomers have made
   great efforts to find and analyse brown dwarfs in the Pleiades and
   other young clusters, because they are still relatively bright and
   observable, while brown dwarfs in older clusters have faded and are
   much more difficult to study.

   Also present in the cluster are several white dwarfs. Given the young
   age of the cluster normal stars are not expected to have had time to
   evolve into white dwarfs, a process which normally takes several
   billion years. It is believed that, rather than being individual low-
   to intermediate-mass stars, the progenitors of the white dwarfs must
   have been high-mass stars in binary systems. Transfer of mass from the
   higher-mass star to its companion during its rapid evolution would
   result in a much quicker route to the formation of a white dwarf.

Age and future evolution

   Ages for star clusters can be estimated by comparing the H-R diagram
   for the cluster with theoretical models of stellar evolution, and using
   this technique, ages for the Pleiades of between 75 and 150 million
   years have been estimated. The spread in estimated ages is a result of
   uncertainties in stellar evolution models. In particular, models
   including a phenomenon known as convective overshoot, in which a
   convective zone within a star penetrates an otherwise non-convective
   zone, result in higher apparent ages.

   Another way of estimating the age of the cluster is by looking at the
   lowest-mass objects. In normal main sequence stars, lithium is rapidly
   destroyed in nuclear fusion reactions, but brown dwarfs can retain
   their lithium. Due to its very low ignition temperature of 2.5 million
   kelvins, the highest-mass brown dwarfs will burn lithium eventually,
   and so determining the highest mass of brown dwarfs still containing
   lithium in the cluster can give an idea of its age. Applying this
   technique to the Pleiades gives an age of about 115 million years.

   Like most open clusters, the Pleiades will not stay gravitationally
   bound forever, as some component stars will be ejected after close
   encounters and others will be stripped by tidal gravitational fields.
   Calculations suggest that the cluster will take about 250 million years
   to disperse, with gravitational interactions with giant molecular
   clouds and the spiral arms of the galaxy also hastening its demise.

Reflection nebulosity

   Hubble Space Telescope image of reflection nebulosity near Merope
   Enlarge
   Hubble Space Telescope image of reflection nebulosity near Merope

   Under ideal observing conditions, some hint of nebulosity may be seen
   around the cluster, and this shows up in long-exposure photographs. It
   is a reflection nebula, caused by dust reflecting the blue light of the
   hot, young stars.

   It was formerly thought that the dust was left over from the formation
   of the cluster, but at the age of about 100 million years generally
   accepted for the cluster, almost all the dust originally present would
   have been dispersed by radiation pressure. Instead, it seems that the
   cluster is simply passing through a particularly dusty region of the
   interstellar medium.

   Studies show that the dust responsible for the nebulosity is not
   uniformly distributed, but is concentrated mainly in two layers along
   the line of sight to the cluster. These layers may have been formed by
   deceleration due to radiation pressure as the dust has moved towards
   the stars.

Names and technical information

   A map of the Pleiades
   Enlarge
   A map of the Pleiades

   The nine brightest stars of the Pleiades are named for the Seven
   Sisters of Greek mythology: Sterope, Merope, Electra, Maia, Taygete,
   Celaeno and Alcyone, along with their parents Atlas and Pleione. As
   daughters of Atlas, the Hyades were sisters of the Pleiades. The name
   of the cluster itself is of Greek origin, though of uncertain
   etymology. Suggested derivations include: from πλεîν plein, to sail,
   making the Pleiades the "sailing ones"; from pleos, full or many; or
   from peleiades, flock of doves. The following table gives details of
   the brightest stars in the cluster:

   CAPTION: Pleiades Bright Stars

   Name Pronunciation ( IPA & respelling) Designation Apparent magnitude
   Stellar classification
   Alcyone /æl'saɪəni/, al-sye'-ə-nee Eta (25) Tauri 2.86 B7IIIe
   Atlas /'ætləs/, at'-ləs 27 Tauri 3.62 B8III
   Electra /i'lɛktrə/, ee-lek'-trə 17 Tauri 3.70 B6IIIe
   Maia /'meɪə, 'maɪə/; may'-ə, mye'-ə 20 Tauri 3.86 B7III
   Merope /'mɛrəpi/, mair'-ə-pee 23 Tauri 4.17 B6IVev
   Taygeta /tei'ɪʤəti/, tay-ij'-ə-tee 19 Tauri 4.29 B6V
   Pleione /'plaɪəni/, plye'-ə-nee 28 (BU) Tauri 5.09 (var.) B8IVep
   Celaeno /sə'lino/, sə-lee'-no 16 Tauri 5.44 B7IV
   Asterope /ə'stɛrəpi/, ə-stair'-ə-pee 21 and 22 Tauri 5.64;6.41 B8Ve/B9V
   — — 18 Tauri 5.65 B8V

The Pleiades in folklore

   The Pleiades' high visibility in the night sky has guaranteed it a
   special place in many cultures, both ancient and modern. In Greek
   mythology, they represented the Seven Sisters, while to the Vikings,
   they were Freyja's hens, and their name in many old European languages
   compares them to a hen with chicks.

   To the Bronze Age people of Europe, such as the Celts (and probably
   considerably earlier), the Pleiades were associated with mourning and
   with funerals, since at that time in history, on the cross-quarter day
   between the autumn equinox and the winter solstice (see Samhain, also
   Halloween or All Souls Day), which was a festival devoted to the
   remembrance of the dead, the cluster rose in the eastern sky as the
   sun's light faded in the evening. It was from this acronychal rising
   that the Pleiades became associated with tears and mourning. As a
   result of precession over the centuries, the Pleiades no longer marked
   the festival, but the association has nevertheless persisted, and
   accounts for the significance of the Pleiades astrologically.
   A bronze disk, 1600 BC, from Nebra, Germany, is the one of the oldest
   known representations of the cosmos. The Pleiades are top right. See
   Nebra sky disk
   Enlarge
   A bronze disk, 1600 BC, from Nebra, Germany, is the one of the oldest
   known representations of the cosmos. The Pleiades are top right. See
   Nebra sky disk

   The ancient Aztecs of Mexico and Central America based their calendar
   upon the Pleiades. Their calendric year began when priests first
   remarked the asterism rising heliacally in the east, immediately before
   the sun's dawn light obliterated the view of the stars.

   Heliacal risings very often mark important calendar points for ancient
   peoples. The heliacal rising of the Pleiades (around June) also begins
   the new year for the Māori of New Zealand, who call the Pleiades
   Matariki.

Indigenous Australians

   Depending on the tribe or clan, some Indigenous Australian peoples
   believed the Pleiades were a woman who had been nearly raped by Kidili,
   the man in the moon.

   Another version, often painted by Gabriella Possum Nungurayyi as this
   is her dreaming (or creation story), daughter of the late Clifford
   Possum Tjapaltjarri from the Central desert art movement of Papunya,
   depicts the story of seven Napaltjarri sisters being chased by a man
   named Jilbi Tjakamarra. He would practice love magic to seduce the
   sisters but they had no intention of being with him and ran away. They
   sat down at Uluru to search for honey ants but when they saw Jilbi,
   they went to Kurlunyalimpa and with the spirits of Uluru, transformed
   into stars. Jilbi transforms himself into what is commonly known as the
   Morning Star in Orion's belt, thus continuing to chase the seven
   sisters across the sky. (Source: Aboriginal Fine Art Gallery)

Native Americans

   The Sioux of North America had a legend that linked the origin of the
   Pleiades to Devils Tower. It was common among the indigenous peoples of
   the Americas to measure keenness of vision by the number of stars the
   viewer could see in the Pleiades, a practice which was also used in
   historical Europe, especially in Greece.

   In Japan, the Pleiades are known as Subaru, and have given their name
   to the car manufacturer. In Chinese constellations, they are 昴 mao, the
   hairy head of the white tiger of the West, while the name of the Hindu
   God Kartikeya means him of the Pleiades.

   In Western astrology they represent coping with sorrow and were
   considered a single one of the medieval fixed stars. As such, they are
   associated with quartz and fennel. In Indian astrology the Pleiades
   were known as the asterism ( nakshatra) Krittika (which in Sanskrit is
   translated as "the cutters.") The Pleiades are called the star of fire,
   and their ruling deity is the Vedic god Agni, the god of the sacred
   fire. It is one of the most prominent of the nakshatras, and is
   associated with anger and stubbornness.

   The word has acquired a meaning of "multitude", inspiring the name of
   the French literary movement La Pléiade and an earlier group of
   Alexandrian poets, the Alexandrian Pleiad.
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