The Pleiades (/ˈplaɪ.ədiːz, ˈpliːə-/), also known as the Seven Sisters and Messier 45, are 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.
The cluster is dominated by hot blue and luminous stars that have formed within the last 100 million years. Reflection nebulae around the brightest stars were once thought to be left over material from the formation of the cluster, but are now considered likely to be an unrelated dust cloud in the interstellar medium through which the stars are currently passing.
Computer simulations have shown that the Pleiades were probably formed from a compact configuration that resembled the Orion Nebula. 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.
|Observation data (J2000 epoch)|
|Right ascension||03h 47m 24s|
|Declination||+24° 07′ 00″|
|Distance||444 ly on average (136.2±1.2 pc)|
|Apparent magnitude (V)||1.6|
|Apparent dimensions (V)||110' (arcmin)|
|Other designations||M45, Seven Sisters, Melotte 22|
The name of the Pleiades comes from Ancient Greek. It probably derives from plein ("to sail") because of the cluster's importance in delimiting the sailing season in the Mediterranean Sea: "the season of navigation began with their heliacal rising". However, in mythology the name was used for the Pleiades, seven divine sisters, the name supposedly deriving from that of their mother Pleione and effectively meaning "daughters of Pleione". In reality, the name of the star cluster almost certainly came first, and Pleione was invented to explain it.
The Pleiades are a prominent sight in winter in the Northern Hemisphere, and are easily visible out to mid-Southern latitudes. They have been known since antiquity to cultures all around the world, including the Celts, Hawaiians (who call them Makaliʻi), Māori (who call them Matariki), Aboriginal Australians (from several traditions), the Persians, the Arabs (who called them Thurayya), the Chinese (who called them 昴 mǎo), the Quechua, the Japanese, the Maya, the Aztec, the Sioux, the Kiowa, and the Cherokee. In Hinduism, the Pleiades are known as Krittika and are associated with the war-god Kartikeya. They are also mentioned three times in the Bible.
The earliest-known depiction of the Pleiades is likely a Northern German bronze age artifact known as the Nebra sky disk, dated to approximately 1600 BC. The Babylonian star catalogues name the Pleiades MULMUL (𒀯𒀯), meaning "stars" (literally "star star"), 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 Ancient Egyptians may have used the names "Followers" and "Ennead" in the prognosis texts of the Calendar of Lucky and Unlucky Days of papyrus Cairo 86637. Some Greek astronomers considered them to be a distinct constellation, and they are mentioned by Hesiod's Works and Days, Homer's Iliad and Odyssey, and the Geoponica. Some scholars of Islam suggested that the Pleiades (ath-thurayya) are the "star" mentioned in Sura An-Najm ("The Star") of the Quran.
In Japan, the constellation is mentioned under the name Mutsuraboshi ("six stars") in the 8th-century Kojiki. The constellation is now known in Japan as Subaru ("to unite"). It was chosen as the brand name of Subaru automobiles to reflect the origins of the firm as the joining of five companies, and is depicted in the firm's six-star logo.
Galileo Galilei was the first astronomer to view the Pleiades through a telescope. He thereby discovered that the cluster contains many stars too dim to be seen with the naked eye. He published his observations, including a sketch of the Pleiades showing 36 stars, in his treatise Sidereus Nuncius in March 1610.
The Pleiades have long been known to be a physically related group of stars rather than any chance alignment. 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 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 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 can be used as an important first step to calibrate the cosmic distance ladder. As the cluster is so close to the Earth, its distance is relatively easy to measure and has been estimated by many methods. 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. Yet some authors argue that the controversy over the distance to the Pleiades discussed below 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 the Hipparcos satellite and independent means (e.g., the Hyades, Coma Berenices cluster, etc.).
Measurements of the distance have elicited much controversy. Results prior to the launch of the Hipparcos satellite generally found that the Pleiades were about 135 parsecs (pc) away from Earth. Data from Hipparcos yielded a surprising result, namely a distance of only 118 pc 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 (so-called "spectroscopic parallax") favor a distance between 135 and 140 pc; a dynamical distance from optical interferometric observations of the Pleiad double Atlas favors a distance of 133 to 137 pc. However, the author of the 2007–2009 catalog of revised Hipparcos parallaxes reasserted that the distance to the Pleiades is ~120 pc and challenged the dissenting evidence. Recently, Francis and Anderson proposed that a systematic effect on Hipparcos parallax errors for stars in clusters biases calculation using the weighted mean and gave a Hipparcos parallax distance of 126 pc and photometric distance 132 pc based on stars in the AB Doradus, Tucana-Horologium, and Beta Pictoris moving groups, which are all similar in age and composition to the Pleiades. Those authors note that the difference between these results can be attributed to random error. More recent results using very-long-baseline interferometry (VLBI) (August 2014) and preliminary solutions using Gaia Data Release 1 (September 2016) and Gaia Data Release 2 (August 2018), determine distances of 136.2 ± 1.2 pc, 134 ± 6 pc and 136.2 ± 5.0 pc, respectively. Although the Gaia Data Release 1 team was cautious about their result, the VLBI authors assert "that the Hipparcos-measured distance to the Pleiades cluster is in error".
|2004||134.6 ± 3.1||Hubble Fine Guidance Sensor|
|2009||120.2 ± 1.9||Revised Hipparcos|
|2014||136.2 ± 1.2||Very-long-baseline interferometry|
|2016||134 ± 6||Gaia Data Release 1|
|2018||136.2 ± 5||Gaia Data Release 2|
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 stars. Its light 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 and is dominated by fainter and redder stars.
The cluster contains many brown dwarfs, 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.
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 (Πλειάδες), though of uncertain etymology. Suggested derivations include: from πλεῖν plein, "to sail", making the Pleiades the "sailing ones"; from πλέος pleos, "full, many"; or from πελειάδες peleiades, "flock of doves". The following table gives details of the brightest stars in the cluster:
|Name||Pronunciation (IPA & respelling)||Designation||Apparent magnitude||Stellar classification|
|Alcyone||/ælˈsaɪ.əni/ al-SY-ə-nee||Eta (25) Tauri||2.86||B7IIIe|
|Atlas||/ˈætləs/ AT-ləs||27 Tauri||3.62||B8III|
|Electra||/ɪˈlɛktrə/ i-LEK-trə||17 Tauri||3.70||B6IIIe|
|Maia||/ˈmeɪ.ə, ˈmaɪ.ə/ M(A)Y-ə||20 Tauri||3.86||B7III|
|Merope||/ˈmɛrəpi/ MERR-ə-pee||23 Tauri||4.17||B6IVev|
|Taygeta||/teɪˈɪdʒɪtə/ tay-IJ-i-tə||19 Tauri||4.29||B6V|
|Pleione||/ˈplaɪ.əni/ PLY-ə-nee||28 (BU) Tauri||5.09 (var.)||B8IVpe|
|Celaeno||/sɪˈliːnoʊ/ si-LEE-noh||16 Tauri||5.44||B7IV|
|Sterope, Asterope||/(ə)ˈstɛrəpi/ (ə)-STERR-ə-pee||21 and 22 Tauri||5.64;6.41||B8Ve/B9V|
Ages for star clusters can be estimated by comparing the Hertzsprung–Russell diagram for the cluster with theoretical models of stellar evolution. Using this technique, ages for the Pleiades of between 75 and 150 million years have been estimated. The wide spread in estimated ages is a result of uncertainties in stellar evolution models, which include factors such 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. Brown dwarfs can retain their lithium, however. Due to lithium's very low ignition temperature of 2.5 × 106 K, 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 is slowly moving in the direction of the feet of what is currently the constellation of Orion. Like most open clusters, the Pleiades will not stay gravitationally bound forever. Some component stars will be ejected after close encounters with other stars; 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 our galaxy also hastening its demise.
With larger telescopes, the nebulosity around some of the stars can be easily seen; especially when long-exposure photographs are taken. Under ideal observing conditions, some hint of nebulosity around the cluster may even be seen with small telescopes or average binoculars. 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.
Analyzing deep-infrared images obtained by the Spitzer Space Telescope and Gemini North telescope, astronomers discovered that one of the cluster's stars—HD 23514, which has a mass and luminosity a bit greater than that of the Sun, is surrounded by an extraordinary number of hot dust particles. This could be evidence for planet formation around HD 23514.
Alcyone (; Ancient Greek Ἁλκυόνη Αlkuónē, derived from alkyon αλκυων "kingfisher"), in Greek mythology, was the name of one of the Pleiades, daughters of Atlas and Pleione or, more rarely, Aethra. She attracted the attention of the god Poseidon and bore him several children, variously named in the sources: Hyrieus, Hyperenor, and Aethusa; Hyperes and Anthas; and Epopeus. By a mortal, Anthedon, Alcyone became the mother of the fisherman Glaucus, who was later transformed into a marine god. There are various etymological interpretations of her name's origin.Alcyone (star)
Alcyone , designated η Tauri (Eta Tauri, abbreviated Eta Tau, η Tau), is a multiple star system in the constellation of Taurus. Approximately 440 light years from the Sun, it is the brightest star in the Pleiades open cluster, which is a young cluster, around 100 million years old. There are a number of fainter stars very close to Alcyone, all most probably members of the same cluster.Billy Meier
Eduard Albert Meier (born February 3, 1937) is a Swiss citizen who is the source of many photographs of alleged unidentified flying objects (UFOs), which he presents in support of his claim that he is in contact with extraterrestrial beings. He also presented other material during the 1970s such as metal samples, sound recordings and film footage. Meier reports regular contacts with extraterrestrials he calls the Plejaren. Meier claims to be the seventh reincarnation after six prophets common to Judaism, Christianity, and Islam: Enoch, Elijah, Isaiah, Jeremiah, Jmmanuel (Jesus), and Mohammed.Meier has been widely characterized as a fraud by skeptics and ufologists, who suggest that he used models to hoax photos claimed to show alien spacecraft.Celaeno
In Greek mythology, Celaeno (; Ancient Greek: Κελαινώ Kelaino, lit. 'the dark one', also Celeno or Kelaino, sometimes [misspelled] Calaeno) 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. She was said to be mother of Lycus and Nycteus by Poseidon; of Eurypylus (or Eurytus), King of Cyrene, and Lycaon, also by Poseidon; and of Lycus and Chimaereus by Prometheus.
Celaeno, one of the Danaïdes, the daughters of Danaus. Her mother was Crino. She married and killed Hyperbius, son of Aegyptus and Hephaestine. She was also believed to have had a son Celaenus by Poseidon.
Celaeno, an Amazon. She was killed by Heracles whilst he was undertaking the ninth labour.
Celaeno, daughter of Hyamus and granddaughter of Lycorus. She was the mother of Delphus by Apollo.
Celaeno, daughter of Ergea by Poseidon.Celaeno (star)
Celaeno , designated 16 Tauri, is a star in the constellation of Taurus and a member of the Pleiades open star cluster (M45) of stars.Electra (Pleiad)
The Pleiad Electra (; Greek: Ἠλέκτρα, Ēlektra "amber") of Greek mythology was one of the seven daughters of Atlas and Pleione. Electra was the wife of Corythus, to whom she bore Iasion. She was seduced by Zeus and gave birth to Dardanus. According to one legend, she was the lost Pleiad, disappearing in grief after the destruction of Troy. She was called Atlantis by Ovid, personifying the family of Pleiades. Electra means "amber," "shining," and "bright."Electra (star)
Electra , designated 17 Tauri, is a blue-white giant star in the constellation of Taurus. It is the third-brightest star in the Pleiades open star cluster (M45); the most visible stars in this group are named for the Seven Sisters of Greek mythology.IC 2602
IC 2602, generally known as the Southern Pleiades or Theta Carinae Cluster, is an open cluster in the constellation Carina that was discovered by Abbe Lacaille in 1751 from South Africa. Easily seen with the naked eye, this cluster is one of the closest to us, whose distance is about 167.7 parsecs (547 light-years) away from Earth.
The Southern Pleiades (IC 2602) has a total apparent magnitude of 1.9, which is 70% fainter than the Taurean Pleiades, and contains about seventy-four stars. It is the third-brightest open cluster in the sky, following the Hyades. Like its northern counterpart, the Southern Pleiades spans a sizeable area of sky, approximately 50 arcminutes, so it is best viewed with large binoculars or telescope with a wide-angle eyepiece.
θ Carinae is the brightest star within the open cluster, with the apparent visual magnitude of +2.74. p Carinae (PP Carinae) is another third-magnitude star known to be a member of IC 2602, although it lies well outside the main visible grouping of stars. All the other members the cluster are of the fifth magnitude and fainter, but several are naked-eye objects, including HR 4196 (V518 Car), HR 4204, HR 4205, HR 4219, HR 4220, HR 4222, HD 92536, HD 93738, and V364 Carinae.
IC 2602 is likely about the same age as the open cluster IC 2391, which has a lithium depletion boundary age of 50 million years old, though the accepted age from its HR-Diagram is about 13.7 million years. IC 2602 stars are also all likely a part of the Lower Scorpius-Centaurus Association.Maia
Maia ( or ; Greek: Μαῖα; Latin: Maia), in ancient Greek religion, is one of the Pleiades and the mother of Hermes.
Maia is the daughter of Atlas and Pleione the Oceanid, and is the oldest of the seven Pleiades. They were born on Mount Cyllene in Arcadia, and are sometimes called mountain nymphs, oreads; Simonides of Ceos sang of "mountain Maia" (Maiados oureias) "of the lovely black eyes." Because they were daughters of Atlas, they were also called the Atlantides.Maia (star)
Maia , designated 20 Tauri (abbreviated 20 Tau), is a star in the constellation of Taurus. It is the fourth-brightest star in the Pleiades open star cluster (M45), after Alcyone, Atlas and Electra, in that order. Maia is a blue giant of spectral type B8 III, and a mercury-manganese star.
Maia's visual magnitude is 3.871, requiring darker skies to be seen. Its total bolometric luminosity is 660 times solar, mostly in the ultraviolet, thus suggesting a radius that is 5.5 times that of the Sun and a mass that is slightly more than 4 times solar. It was thought to be a variable star by astronomer Otto Struve. A class of stars known as Maia variables was proposed, which included Gamma Ursae Minoris, but Maia and some others in the class have since been found to be stable.Maia is surrounded by the Maia Nebula (also known as NGC 1432), one of the brightest patches of nebulosity within the Pleiades star cluster.Merope (Pleiades)
In Greek mythology, Merope (; Greek: Μερόπη) is one of the seven Pleiades, daughters of Atlas and Pleione. Pleione, their mother, is the daughter of Oceanus and Tethys and is the protector of sailors. Their transformation into the star cluster known as the Pleiades is the subject of various myths.NGC 1435
The Merope Nebula (also known as Tempel's Nebula and NGC 1435) is a diffuse reflection nebula in the Pleiades star cluster, surrounding the 4th magnitude star Merope. It was discovered on October 19, 1859 by the German astronomer Wilhelm Tempel. John Herschel, in his New General Catalogue (NGC), described it as a very faint nebula about the size of the full moon.
The Merope Nebula has an apparent magnitude starting at 13 and quickly dimming by a factor of about 15, making most of the nebula dimmer than magnitude 16. It is illuminated entirely by the star Merope, which is embedded in the nebula. It contains a bright knot, IC 349, about half an arcminute wide near Merope. It appears blue in photographs because of the fine carbon dust spread throughout the cloud. Though it was once thought the Pleiades formed from this and surrounding nebulae, it is now known that the Pleiades nebulosity is caused by a chance encounter with the cloud.
A small and unique nebula which is close to Merope was discovered by Edward Emerson Barnard in November 1890. It is naturally very bright but is almost hidden in the radiance of Merope.Pleiades (Greek mythology)
The Pleiades (; Greek: Πλειάδες, Ancient Greek: [pleːádes]), companions of Artemis, were the seven daughters of the titan Atlas and the sea-nymph Pleione born on Mount Cyllene. They were the sisters of Calypso, Hyas, the Hyades, and the Hesperides. The Pleiades were nymphs in the train of Artemis, and together with the seven Hyades were called the Atlantides, Dodonides, or Nysiades, nursemaids and teachers to the infant Dionysus. They were thought to have been translated to the night sky as a cluster of stars, the Pleiades, and were associated with rain.Pleiades (satellite)
The Pléiades constellation is composed of two very-high-resolution optical Earth-imaging satellites. Pléiades-HR 1A and Pléiades-HR 1B provide the coverage of Earth's surface with a repeat cycle of 26 days. Designed as a dual civil/military system, Pléiades will meet the space imagery requirements of European defence as well as civil and commercial needs.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 heliacal rising of Pleiades often marks important calendar points for ancient peoples.Pleione (star)
Pleione (pronounced or ) is a binary star and the seventh-brightest star in the Pleiades star cluster (Messier 45). It has the variable star designation BU Tauri (BU Tau) and the Flamsteed designation 28 Tauri (28 Tau). The star is located approximately 120 parsecs (390 light-years) from the Sun, appearing in the constellation of Taurus. Pleione is located close on the sky to the brighter star Atlas, so is difficult for stargazers to distinguish with the naked eye despite being a fifth magnitude star.
The brighter star of the Pleione binary pair, component A, is a hot type B star 190 times more luminous than the Sun. It is classified as Be star with certain distinguishing traits: periodic phase changes and a complex circumstellar environment composed of two gaseous disks at different angles to each other. The primary star rotates rapidly, close to its breakup velocity, even faster than Achernar. Although some research on the companion star has been performed, stellar characteristics of the orbiting B component are not well known.Sterope (Pleiad)
In Greek mythology, Sterope (; Ancient Greek: Στερόπη, [sterópɛː], from στεροπή, steropē, lightning), also called Asterope (Ἀστερόπη), was one of the seven Pleiades, the daughters of Atlas and Pleione, born to them at Mount Cyllene in Arcadia. She was the wife of King Oenomaus of Pisa, or according to some accounts, his mother by Ares. Sterope was also credited to be the mother of Evenus (father of Marpessa) by the said Olympian god.Taurus (constellation)
Taurus (Latin for "the Bull") is one of the constellations of the zodiac, which means it is crossed by the plane of the ecliptic. Taurus is a large and prominent constellation in the northern hemisphere's winter sky. It is one of the oldest constellations, dating back to at least the Early Bronze Age when it marked the location of the Sun during the spring equinox. Its importance to the agricultural calendar influenced various bull figures in the mythologies of Ancient Sumer, Akkad, Assyria, Babylon, Egypt, Greece, and Rome.
A number of features exist that are of interest to astronomers. Taurus hosts two of the nearest open clusters to Earth, the Pleiades and the Hyades, both of which are visible to the naked eye. At first magnitude, the red giant Aldebaran is the brightest star in the constellation. In the northwest part of Taurus is the supernova remnant Messier 1, more commonly known as the Crab Nebula. One of the closest regions of active star formation, the Taurus-Auriga complex, crosses into the northern part of the constellation. The variable star T Tauri is the prototype of a class of pre-main-sequence stars.Wish Upon the Pleiades
Wish Upon the Pleiades (放課後のプレアデス, Hōkago no Pureadesu, lit. "After-School Pleiades") is an original net animation series produced by Gainax in collaboration with Japanese automaker Subaru. The series was released on YouTube in four parts on February 1, 2011 with the English-subtitled version appearing six days later. An anime television series aired in Japan between April and June 2015.