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Pleiades (star cluster)
Encyclopedia
In astronomy
, the Pleiades, or Seven Sisters (Messier object 45
), is an open star cluster
containing middle-aged hot B-type stars located in the constellation of Taurus
. It is among the nearest star cluster
s to Earth and is the cluster most obvious to the naked eye
in the night sky
. Pleiades has several meanings in different cultures and traditions
.
The cluster is dominated by hot blue and extremely luminous stars that have formed within the last 100 million years. Dust that forms a faint reflection nebulosity
around the brightest stars was thought at first to be left over from the formation of the cluster (hence the alternate name Maia Nebula after the star Maia
), but is now known to be an unrelated dust cloud in the interstellar medium
that the stars are currently passing through. Astronomers estimate that the cluster will survive for about another 250 million years, after which it will disperse due to gravitational interactions with its galactic neighborhood.
The Pleiades are a prominent sight in winter in the Northern Hemisphere
and in summer in the Southern Hemisphere
, and have been known since antiquity to cultures all around the world, including the Māori
, Aboriginal Australians
, the Persians
, the Chinese
, the Japan
ese, the Maya
, the Aztec
, and the Sioux
and Cherokee
. In Tamil culture this star is attributed to Lord Muruga (supposed to have been raised by 6 women), in Sanskrit he is noted as Skanda.
The Babylonian star catalogues
name them MUL.MUL or "star of stars", and they head the list of stars along the ecliptic, reflecting the fact that they were close to the point of vernal equinox around the 23rd century BC. The earliest known depiction of the Pleiades is likely a bronze age
artifact known as the Nebra sky disk, dated to approximately 1600 BC. 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 and 38:31, as well as Amos
5:8). The Pleiades (Krittika
) are particularly revered in Hindu mythology
as the six mothers of the war god Skanda
, who developed six faces, one for each of them. Some scholars of Islam
suggested that the Pleiades (Ats-tsuraiya) are the Star in Najm
, which is mentioned in the Quran. The rising of the Pleiades on June 10 is mentioned in the Geoponica
, which can be dated to 0:00:00 Universal Time
June 10greg.
, 603 BC, viewed from the Kokino observatory
, which was 800,000 days (without year zero
) before the oldest biography of Nicolaus Copernicus
was completed on October 7, 1588, by Bernardino Baldi
. The Greeks oriented the Hecatompedon temple of 1150 BC and the Parthenon
of 438 BC to their rising.
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 motion
s, 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 that 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.
The distance to the Pleiades has elicited much controversy, namely since the cluster is an important first step in the cosmic distance ladder
, which lays the foundation for the astronomical distance scale. 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 parsec
s away from Earth. Data from Hipparcos yielded a surprising result, namely a distance of only 118 parsecs by measuring the parallax
of stars in the cluster—a technique that should yield the most direct and accurate results. Later work consistently argued that the Hipparcos distance measurement for the Pleiades was erroneous. In particular, distances derived to the cluster via the Hubble Space Telescope
and infrared color-magnitude diagram fitting favor a distance between 135–140 pc. However, the author of the 2009 catalog of revised Hipparcos
parallaxes reasserted that the distance to the Pleiades is ~120 pc, and reviewed the dissenting evidence. Some authors argue that the controversy over the distance to the Pleiades is a red herring
, since the cosmic distance ladder
can (presently) rely on a suite of other nearby clusters where consensus exists regarding the distances as established by Hipparcos and independent means (e.g., the Hyades
, Coma Ber, etc.).
The cluster core radius is about 8 light years and tidal radius
is about 43 light years. The cluster contains over 1,000 statistically confirmed members, although this figure excludes unresolved binary star
s. 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 mass
es.
The cluster contains many brown dwarf
s, which are objects with less than about 8% of the Sun
's mass, 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.
, 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, resulting 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 lithium's very low ignition temperature of 2.5 million kelvin
, the highest-mass brown dwarfs will burn it 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.
The cluster's relative motion will eventually lead it to be located, as seen from Earth many millennia in the future, passing below the feet of what is currently the constellation of Orion. Also, 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 cloud
s and the spiral arms of our galaxy also hastening its demise.
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.
The nine brightest stars of the Pleiades are named for the Seven Sisters
of Greek mythology
: Sterope
, Merope
, Electra
, Maia
, Taygeta, Celaeno
, and Alcyone
, along with their parents Atlas
and Pleione
. As daughters of Atlas, the Hyades
were sisters of the Pleiades. The English name of the cluster itself is of Greek
origin, though of uncertain etymology. Suggested derivations include: from πλεîν pleîn, 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:
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, the Pleiades, or Seven Sisters (Messier object 45
Messier object
The Messier objects are a set of astronomical objects first listed by French astronomer Charles Messier in 1771. The original motivation of the catalogue was that Messier was a comet hunter, and was frustrated by objects which resembled but were not comets...
), is an open 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...
containing middle-aged hot B-type stars located in the constellation 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:...
. It is among the nearest star cluster
Star cluster
Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain less than...
s to Earth and is the cluster most obvious to the naked eye
Naked eye
The naked eye is a figure of speech referring to human visual perception unaided by a magnifying or light-collecting optical device, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked"...
in the night sky
Night sky
The term night sky refers to the sky as seen at night. The term is usually associated with astronomy, with reference to views of celestial bodies such as stars, the Moon, and planets that become visible on a clear night after the Sun has set. Natural light sources in a night sky include moonlight,...
. Pleiades has several meanings in different cultures and traditions
Pleiades in folklore and literature
The high visibility of the star cluster Pleiades in the night sky has guaranteed it a special place in many cultures, both ancient and modern.The astrological Pleiades was originally described in Three Books of Occult Philosophy by Heinrich Cornelius Agrippa .The heliacal rising of Pleiades often...
.
The cluster is dominated by hot blue and extremely luminous stars that have formed within the last 100 million years. Dust that forms a faint reflection nebulosity
Reflection nebula
In Astronomy, reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. The energy from the nearby star, or stars, is insufficient to ionize the gas of the nebula to create an emission nebula, but is enough to give sufficient scattering to make the dust...
around the brightest stars was thought at first to be left over from the formation of the cluster (hence the alternate name Maia Nebula after the star Maia
Maia (star)
Maia is a star in the constellation Taurus. It is the fourth brightest star in the Pleiades open star cluster , after Alcyone, Atlas and Electra, in that order. The name Maia originates with the and . She is one of the seven daughters of Atlas and Pleione in Greek mythology—stars which...
), but is now known to be an unrelated dust cloud in the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
that the stars are currently passing through. Astronomers estimate that the cluster will survive for about another 250 million years, after which it will disperse due to gravitational interactions with its galactic neighborhood.
Observational history
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...
and in summer in the Southern Hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...
, and have been known since antiquity to cultures all around the world, including the Māori
Maori culture
Māori culture is the culture of the Māori of New Zealand, an Eastern Polynesian people, and forms a distinctive part of New Zealand culture. Within the Māori community, and to a lesser extent throughout New Zealand as a whole, the word Māoritanga is often used as an approximate synonym for Māori...
, Aboriginal Australians
Indigenous Australians
Indigenous Australians are the original inhabitants of the Australian continent and nearby islands. The Aboriginal Indigenous Australians migrated from the Indian continent around 75,000 to 100,000 years ago....
, the Persians
Achaemenid Empire
The Achaemenid Empire , sometimes known as First Persian Empire and/or Persian Empire, was founded in the 6th century BCE by Cyrus the Great who overthrew the Median confederation...
, the Chinese
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
, the Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
ese, the Maya
Maya civilization
The Maya is a Mesoamerican civilization, noted for the only known fully developed written language of the pre-Columbian Americas, as well as for its art, architecture, and mathematical and astronomical systems. Initially established during the Pre-Classic period The Maya is a Mesoamerican...
, the Aztec
Aztec
The Aztec people were certain ethnic groups of central Mexico, particularly those groups who spoke the Nahuatl language and who dominated large parts of Mesoamerica in the 14th, 15th and 16th centuries, a period referred to as the late post-classic period in Mesoamerican chronology.Aztec is the...
, and the Sioux
Sioux
The Sioux are Native American and First Nations people in North America. The term can refer to any ethnic group within the Great Sioux Nation or any of the nation's many language dialects...
and Cherokee
Cherokee
The Cherokee are a Native American people historically settled in the Southeastern United States . Linguistically, they are part of the Iroquoian language family...
. In Tamil culture this star is attributed to Lord Muruga (supposed to have been raised by 6 women), in Sanskrit he is noted as Skanda.
The Babylonian star catalogues
Babylonian star catalogues
Babylonian astronomy collated earlier observations and divinations into sets of Babylonian star catalogues, during and after the Kassite rule over Babylonia. These star catalogues, written in cuneiform script, contained lists of constellations, individual stars, and planets...
name them MUL.MUL or "star of stars", and they head the list of stars along the ecliptic, reflecting the fact that they were close to the point of vernal equinox around the 23rd century BC. The earliest known depiction of the Pleiades is likely a bronze age
Bronze Age
The Bronze Age is a period characterized by the use of copper and its alloy bronze as the chief hard materials in the manufacture of some implements and weapons. Chronologically, it stands between the Stone Age and Iron Age...
artifact known as the Nebra sky disk, dated to approximately 1600 BC. Some Greek
Ancient Greece
Ancient Greece is a civilization belonging to a period of Greek history that lasted from the Archaic period of the 8th to 6th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Included in Ancient Greece is the...
astronomers considered them to be a distinct 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....
, and they are mentioned by Hesiod
Hesiod
Hesiod was a Greek oral poet generally thought by scholars to have been active between 750 and 650 BC, around the same time as Homer. His is the first European poetry in which the poet regards himself as a topic, an individual with a distinctive role to play. Ancient authors credited him and...
, and in Homer
Homer
In the Western classical tradition Homer , is the author of the Iliad and the Odyssey, and is revered as the greatest ancient Greek epic poet. These epics lie at the beginning of the Western canon of literature, and have had an enormous influence on the history of literature.When he lived is...
's Iliad
Iliad
The Iliad is an epic poem in dactylic hexameters, traditionally attributed to Homer. Set during the Trojan War, the ten-year siege of the city of Troy by a coalition of Greek states, it tells of the battles and events during the weeks of a quarrel between King Agamemnon and the warrior Achilles...
and Odyssey
Odyssey
The Odyssey is one of two major ancient Greek epic poems attributed to Homer. It is, in part, a sequel to the Iliad, the other work ascribed to Homer. The poem is fundamental to the modern Western canon, and is the second—the Iliad being the first—extant work of Western literature...
. They are also mentioned three times in the Bible
Bible
The Bible refers to any one of the collections of the primary religious texts of Judaism and Christianity. There is no common version of the Bible, as the individual books , their contents and their order vary among denominations...
(Job
Book of Job
The Book of Job , commonly referred to simply as Job, is one of the books of the Hebrew Bible. It relates the story of Job, his trials at the hands of Satan, his discussions with friends on the origins and nature of his suffering, his challenge to God, and finally a response from God. The book is a...
9:9 and 38:31, as well as Amos
Book of Amos
The Book of Amos is a prophetic book of the Hebrew Bible, one of the Twelve Minor Prophets. Amos, an older contemporary of Hosea and Isaiah, was active c. 750 BCE during the reign of Jeroboam II, making the Book of Amos the first biblical prophetic book written. Amos lived in the kingdom of Judah...
5:8). The Pleiades (Krittika
Krittika
The star cluster ', sometimes known as Kārtikā, corresponds to the open star cluster Pleiades in Indian astronomy and...
) are particularly revered in Hindu mythology
Hindu mythology
Hindu religious literature is the large body of traditional narratives related to Hinduism, notably as contained in Sanskrit literature, such as the Sanskrit epics and the Puranas. As such, it is a subset of Nepali and Indian culture...
as the six mothers of the war god Skanda
Murugan
Murugan also called Kartikeya, Skanda and Subrahmanya, is a popular Hindu deity especially among Tamil Hindus, worshipped primarily in areas with Tamil influences, especially South India, Singapore, Sri Lanka, Malaysia, Mauritius and Reunion Island. His six most important shrines in India are the...
, who developed six faces, one for each of them. Some scholars of Islam
Islam
Islam . The most common are and . : Arabic pronunciation varies regionally. The first vowel ranges from ~~. The second vowel ranges from ~~~...
suggested that the Pleiades (Ats-tsuraiya) are the Star in Najm
Najm
Najm is a given name in Middle East, Central Asia and South Asia. Najm is the male version of the name and Najma is female version of the name. Najm/Negm is also a common Arab family name....
, which is mentioned in the Quran. The rising of the Pleiades on June 10 is mentioned in the Geoponica
Geoponica
The Geoponica is a twenty-book collection of agricultural lore, compiled during the 10th century in Constantinople for the Byzantine emperor Constantine VII Porphyrogenitus...
, which can be dated to 0:00:00 Universal Time
Universal Time
Universal Time is a time scale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time , i.e., the mean solar time on the Prime Meridian at Greenwich, and GMT is sometimes used loosely as a synonym for UTC...
June 10greg.
Gregorian calendar
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...
, 603 BC, viewed from the Kokino observatory
Kokino
Kokino is an important archaeological site and a megalithic observatory discovered in 2001 by archeologist Jovica Stankovski in the northeastern Republic of Macedonia, approximately 30 km from the town of Kumanovo, near the village of Staro Nagoričane Latitude: 42.260000N Longitude: 21.850000E....
, which was 800,000 days (without year zero
Year zero
"Year zero" does not exist in the widely used Gregorian calendar or in its predecessor, the Julian calendar. Under those systems, the year 1 BC is followed by AD 1...
) before the oldest biography of Nicolaus Copernicus
Nicolaus Copernicus
Nicolaus Copernicus was a Renaissance astronomer and the first person to formulate a comprehensive heliocentric cosmology which displaced the Earth from the center of the universe....
was completed on October 7, 1588, by Bernardino Baldi
Bernardino Baldi
Bernardino Baldi was an Italian mathematician and writer.Baldi descended from a noble family from Urbino, Marche, where he was born...
. The Greeks oriented the Hecatompedon temple of 1150 BC and the Parthenon
Parthenon
The Parthenon is a temple on the Athenian Acropolis, Greece, dedicated to the Greek goddess Athena, whom the people of Athens considered their virgin patron. Its construction began in 447 BC when the Athenian Empire was at the height of its power. It was completed in 438 BC, although...
of 438 BC to their rising.
John Michell
John Michell was an English natural philosopher and geologist whose work spanned a wide range of subjects from astronomy to geology, optics, and gravitation. He was both a theorist and an experimenter....
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 motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...
s, 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
Charles Messier
Charles Messier was a French astronomer most notable for publishing an astronomical catalogue consisting of deep sky objects such as nebulae and star clusters that came to be known as the 110 "Messier objects"...
measured the position of the cluster and included it as M45 in his catalogue
Messier object
The Messier objects are a set of astronomical objects first listed by French astronomer Charles Messier in 1771. The original motivation of the catalogue was that Messier was a comet hunter, and was frustrated by objects which resembled but were not comets...
of comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
-like objects, published in 1771. Along with the Orion Nebula
Orion Nebula
The Orion Nebula is a diffuse nebula situated south of Orion's Belt. It is one of the brightest nebulae, and is visible to the naked eye in the night sky. M42 is located at a distance of and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light...
and the Praesepe
Beehive Cluster
The Beehive Cluster, also known as Praesepe , M44, NGC 2632, or Cr 189, is an open cluster in the constellation Cancer. It is one of the nearest open clusters to the Solar System, and it contains a larger star population than most other nearby clusters...
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 that seems scarcely possible for the Pleiades. One possibility is that Messier simply wanted to have a larger catalogue than his scientific rival Lacaille
Nicolas Louis de Lacaille
Abbé Nicolas Louis de Lacaille was a French astronomer.He is noted for his catalogue of nearly 10,000 southern stars, including 42 nebulous objects. This catalogue, called Coelum Australe Stelliferum, was published posthumously in 1763. It introduced 14 new constellations which have since become...
, whose 1755 catalogue contained 42 objects, and so he added some bright, well-known objects to boost his list.
Distance
Cosmic distance ladder
The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" to Earth...
, which lays the foundation for the astronomical distance scale. 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
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...
satellite generally found that the Pleiades were about 135 parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
s away from Earth. Data from Hipparcos yielded a surprising result, namely a distance of only 118 parsecs by measuring the 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"...
of stars in the cluster—a technique that should yield the most direct and accurate results. Later work consistently argued that the Hipparcos distance measurement for the Pleiades was erroneous. In particular, distances derived to the cluster via the Hubble Space 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...
and infrared color-magnitude diagram fitting favor a distance between 135–140 pc. However, the author of the 2009 catalog of revised 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...
parallaxes reasserted that the distance to the Pleiades is ~120 pc, and reviewed the dissenting evidence. Some authors argue that the controversy over the distance to the Pleiades is a red herring
Red herring
A red herring is a deliberate attempt to divert attention.Red herring may refer to:* Red herring , the informal fallacy of presenting an argument that may in itself be valid, but does not address the issue in question....
, since the cosmic distance ladder
Cosmic distance ladder
The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" to Earth...
can (presently) rely on a suite of other nearby clusters where consensus exists regarding the distances as established by Hipparcos and independent means (e.g., the Hyades
Hyades
Hyades may refer to:*Hyades *Hyades , an open star cluster in the constellation Taurus...
, Coma Ber, etc.).
Composition
Roche limit
The Roche limit , sometimes referred to as the Roche radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction...
is about 43 light years. The cluster contains over 1,000 statistically confirmed members, although this figure excludes unresolved binary star
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...
s. It is dominated by young, hot blue stars
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...
, 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
Ursa Major
Ursa Major , also known as the Great Bear, is a constellation visible throughout the year in most of the northern hemisphere. It can best be seen in April...
and Ursa Minor
Ursa Minor
Ursa Minor , also known as the Little Bear, is a constellation in the northern sky. Like the Great Bear, the tail of the Little Bear may also be seen as the handle of a ladle, whence the name Little Dipper...
. The total mass contained in the cluster is estimated to be about 800 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es.
The cluster contains many brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...
s, which are objects with less than about 8% of 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...
's mass, not heavy enough for nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
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.
Age and future evolution
Ages for star clusters can be estimated by comparing the Hertzsprung-Russell diagram for the cluster with theoretical models of stellar evolutionStellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...
, 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
Convective overshoot
Convective overshoot is a phenomenon of convection carrying material beyond an unstable region of the atmosphere into a stratified, stable region...
, in which a convective
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
zone within a star penetrates an otherwise non-convective zone, resulting 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
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...
stars, lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
is rapidly destroyed in nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
reactions, but brown dwarfs can retain their lithium. Due to lithium's very low ignition temperature of 2.5 million 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...
, the highest-mass brown dwarfs will burn it 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.
The cluster's relative motion will eventually lead it to be located, as seen from Earth many millennia in the future, passing below the feet of what is currently the constellation of Orion. Also, 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 cloud
Molecular cloud
A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....
s and the spiral arms of our galaxy also hastening its demise.
Reflection nebulosity
Reflection nebula
In Astronomy, reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. The energy from the nearby star, or stars, is insufficient to ionize the gas of the nebula to create an emission nebula, but is enough to give sufficient scattering to make the dust...
, 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
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...
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
Radiation pressure
Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
. Instead, it seems that the cluster is simply passing through a particularly dusty region of the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
.
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
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
pressure as the dust has moved towards the stars.
Brightest stars in 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...
of 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...
: Sterope
Sterope (Pleiad)
In Greek mythology, Sterope , also called Asterope , was one of the seven Pleiades and the wife of Oenomaus ....
, Merope
Merope
Merope was originally the name of several, probably unrelated, characters in Greek mythology. The name may refer to:-Greek mythology:* Merope , one of the Heliades, daughter of Helios and Clymene...
, Electra
Electra (Pleiad)
The Pleiad Electra of Greek mythology was one of the seven daughters of Atlas and Pleione. Electra was the wife of Corythus. She was raped by Zeus and gave birth to Dardanus, who became the founder of Troy, ancestor of Priam and his house. According to one legend, she was the lost Pleiad,...
, Maia
Maia (mythology)
In Greek mythology, Maia is one of the Pleiades and the mother of Hermes. The goddess known as Maia among the Romans may have originated independently, but attracted the myths of Greek Maia because the two figures shared the same name.-Birth:...
, Taygeta, Celaeno
Celaeno
In Greek mythology, Celaeno referred to several different figures.*Celaeno, one of the Harpies, whom Aeneas encountered at Strophades. She gave him prophecies of his coming journeys.*Celaeno, one of the Pleiades...
, and 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...
, along with their parents 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...
and Pleione
Pleione (mythology)
Pleione was an Oceanid nymph. She lived in a southern region of Greece called Arcadia, on a mountain named Mount Kyllini. She married Atlas and gave birth to the Hyades, Hyas and the Pleiades.-The Pleiades:...
. As daughters of Atlas, the 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...
were sisters of the Pleiades. The English name of the cluster itself is of Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
origin, though of uncertain etymology. Suggested derivations include: from πλεîν pleîn, to sail, making the Pleiades the "sailing ones"; from pleos, full or many; or 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,...
, flock of doves. The following table gives details of the brightest stars in the cluster:
| Name | Pronunciation (IPA & respelling) | Designation Star designation Designations of stars are done by the International Astronomical Union . Many of the star names in use today were inherited from the time before the IAU existed. Other names, mainly for variable stars , are being added all the time.Approximately 10,000 stars are visible to the naked eye... |
Apparent 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... |
Stellar classification 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... |
|---|---|---|---|---|
| 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... |
/ælˈsaɪ.əniː/ | Eta (25) Tauri | 2.86 | B7IIIe |
| 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... |
/ˈætləs/ | 27 Tauri | 3.62 | B8III |
| Electra Electra (star) Electra, also cataloged as 17 Tauri, is a blue-white giant star in the constellation of Taurus. The star is one of the nine brightest stars in the Pleiades open cluster. The most visible stars in this group are named for the Seven Sisters of Greek mythology.The star has an apparent brightness of... |
/ |
17 Tauri | 3.70 | B6IIIe |
| Maia Maia (star) Maia is a star in the constellation Taurus. It is the fourth brightest star in the Pleiades open star cluster , after Alcyone, Atlas and Electra, in that order. The name Maia originates with the and . She is one of the seven daughters of Atlas and Pleione in Greek mythology—stars which... |
/ˈmeɪə, ˈmaɪə/ , | 20 Tauri | 3.86 | B7III |
| Merope Merope (star) Merope is a star in the constellation Taurus and a member of the Pleiades star cluster. It is approximately 360 light years from Earth.Merope is a blue-white B-type subgiant with a mean apparent magnitude of +4.14... |
/ˈmɛrəpiː/ | 23 Tauri | 4.17 | B6IVev |
| 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.... |
/teɪˈɪdʒ |
19 Tauri | 4.29 | B6V |
| Pleione Pleione (star) Pleione is a binary star in the Pleiades star cluster , 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... |
/ˈplaɪ.əniː/ | 28 (BU) Tauri | 5.09 (var.) 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... |
B8IVpe |
| Celaeno Celaeno (star) Celaeno is a star in the constellation Taurus and a member of the Pleaides open cluster of stars. Celaeno is sometimes called the Lost Pleiad, a name alluded to in a poem by Alfred Austin... |
/s |
16 Tauri | 5.44 | B7IV |
| Sterope, Asterope | /ˈstɛrɵpiː, əˈstɛrɵpiː/ | 21 and 22 Tauri | 5.64;6.41 | B8Ve/B9V |
| — | — | 18 Tauri | 5.65 | B8V |
See also
- List of Messier objects
- List of open clusters
- New General CatalogueNew General CatalogueThe New General Catalogue of Nebulae and Clusters of Stars is a well-known catalogue of deep sky objects in astronomy. It contains 7,840 objects, known as the NGC objects...
- Pleiades in folklore and literaturePleiades in folklore and literatureThe high visibility of the star cluster Pleiades in the night sky has guaranteed it a special place in many cultures, both ancient and modern.The astrological Pleiades was originally described in Three Books of Occult Philosophy by Heinrich Cornelius Agrippa .The heliacal rising of Pleiades often...
External links
- The Pleiades (M45) At the astro-photography site of Mr. T. Yoshida.
- Photos and information on the Pleiades from the University of Calgary
- Information on the Pleiades from SEDS
- Information and images from the Anglo-Australian Observatory
- WEBDA open cluster database webpage on Pleiades cluster – E. Pauzen (Univ. Vienna)
- NightSkyInfo.com: The Pleiades
- Maya Astronomy
- Doppler Imaging: Results first Doppler image of a Pleiades solar-type G dwarf – HII314, Strassmeier & Rice 2001, A&A 377, 264
- Dark Atmospheres Photography (deep nebulosity exposure)
- Pleiades in Hungarian and Middle Asian culture