In the clear and unpolluted night skies of antiquity the Pleiades star cluster was an object of wonder and interest. It was the subject of myth and legend in almost every culture on the planet.
As the Pleiades cluster is close to the ecliptic (within 4°) in the constellation of Taurus it is a spring and autumnal 'seasonal' object in both the northern and southern hemispheres. Being close to the ecliptic, there are frequent occultations of the cluster with the Moon and planets. To our superstitious ancestors these were, no doubt, portentious events. Likewise, the apparent annual motion of the cluster would have been highly significant. The heliacal (near dawn) rising of the Pleiades in spring in the northern hemisphere has from ancient times augured the opening of the seafaring and farming season: while its dawn autumnal setting marked the season's end.
The Pleiades are among the first stars mentioned in literature, appearing in Chinese annals of about 2350 BC. The earliest European references are somewhat later, in a poem by Hesiod in about 1000 BC and in Homer's Odyssey and Iliad.
The Bible contains three direct references to the Pleiades in Job 9:9 and 38:31, and Amos 5:8, and a single indirect reference in the New Testament. This latter passage (Revelation 1:16) describes a vision of the coming of the Messiah – who holds, in his right hand, seven stars…
The etymological derivation of the name Pleiades (Πλειαδεσ) is uncertain. Robert Graves, the late English poet and writer, records in his 'The Greek Myths' (1955) that it may be derived from either the Greek 'plein' for 'to sail', or 'pleios' meaning 'many'. Another possible root is from Pindar, an early Greek poet, who named the cluster the Peleiades – 'a flock of Doves' – and this is, perhaps, the original form. A nearby cluster has retained its animalistic classical name of the Hyades, 'the Piglets'.
The 19th century poet Alfred Lord Tennyson probably did not realise how metaphorically close to the truth he was when he described, in his poem Locksley Hall, the rising Pleiades:
Many a night I saw the Pleiads, rising thro' the mellow shade,
Glitter like a swarm of fireflies tangled in a silver braid.
Poetic and apt – recent telescope observations have revealed that this most famous of open clusters is comprised of some four hundred stars wreathed in complex nebulæ of dust and gas. 
The Seven Sisters
In Greek mythology, the Pleiades were seven sisters: Maia, Electra, Alcyone, Taygete, Asterope, Celaeno and Merope. Their parents were Atlas, a Titan who held up the sky, and the oceanid Pleione, the protectress of sailing.
After a chance meeting with the hunter Orion, the Pleiades and their mother became the object of his pursuit. Enamoured with the young women he pursued them over the face of the Earth. In pity for their plight, Zeus changed them into a flock of doves, which he set in the heavens. Thus the olympian added the penalty of the absence of his wife and family to the Titan's original punishment of eternally supporting the heavens from the Earth.
Only six stars are distinctly visible to the naked eye. The ancient Greeks explained the sudden disappearance of the seventh star in various narratives. According to one, all the Pleiades were consorts to gods, with the exception of Merope. She deserted her sisters in shame, having taken a mortal husband, Sisyphus, the King of Corinth. Another explanation for the 'lost' star related to the myth of the Electra, an ancestress of the royal house of Troy. After the destruction of Troy, the grief stricken Electra abandoned her sisters and was transformed into a comet – everafter to be a sign of impending doom.
The Greek legends of the disappearing star are echoed in Jewish , Hindu and Mongolian folklore: their basis in an actual event seems to be corroborated by astronomical evidence that a clearly visible star in the cluster became extinct towards the end of the second millennium BC.
In an alternative myth, the Pleiades were the virgin companions of Artemis, to the ancient Greeks, the goddess of hunting and the Moon. Whilst stalking a hind, the great hunter Orion crept into a sunlit glade, disturbing the innocent play of the sisters. They fled in alarm. His immoderate passions enflamed by their beauty and grace, he pursued them relentlessly, as was fitting for the greatest mortal hunter. In frustration, Artemis pleaded with Zeus to for his intervention. With characteristic olympian sarcasm, he did. As the hunter closed in on his prey, Zeus transformed the sisters into a flock of doves. They flew into the heavens, beyond the reach of their pursuer, but also removed from earthly companionship with the goddess!
Artemis, enraged by these twofold masculine affronts, revenged herself on Orion. Apollo, her brother, having been affronted by the mortal hunter's prowess, was persuaded to set a monstrous scorpion to attack Orion. Not to be outdone in this, in another characteristic display of mordant wit, Zeus set the dead hunter in the heavens in a vain pursuit of the Pleiades through the night sky for eternity, with the constellation Scorpio ever chasing after Orion. Even so the Olympian had some compassion for his daughter: the path of the Moon in the heavens passes close to the Pleiades, and thus Artemis – as the goddess of the Moon – had the solace of their frequent reunions.
Coincidentally, a similar legend to that of the ancient Greeks is retold by the Kiowa tribe of North America. Seven maidens were transported in to the sky by the Great Spirit to save them from giant bears. The Spirit created the Mateo Tepe (the Devil's Tower National Monument, Wyoming) to place them beyond the bears. Yet the hunt continued, with the bears climbing the sheer cliffs – the vertical striations on the side of the rock formation were ascribed to be the bears' claw marks, gouged as they climbed after their prey. Seeing the bears close in on the maidens, the Spirit then placed them securely in the sky.
In Navajo legend, after the Earth was separated from the sky the Black Sky God had a cluster of stars on his ankle. These were The Flint Boys. In the Black God's first dance, with each stamp of his foot the Flint Boys jumped up his body, first to the knee, then the hip, shoulder and finally on to his forehead, where they remained as the sign that the Black God was Lord of the Sky.
The Western Mono Indians saw in the Pleiades a group of wives who were excessively fond of eating onions and were thrown out of their homes by their angry husbands. Repenting in their loneliness, the husbands sought after their wives, but in vain. They had wandered away into the sky, becoming the Pleiades.
To the Blackfoot tribe of south Alberta and north Montana the stars were known as the Orphan Boys. The fatherless boys were rejected by the tribe, but were befriended by a pack of wolves, who became their only companions. Saddened by their lives on earth they asked the Great Spirit to let them play together in the sky, and so he set them there as a group of small stars. As a reminder of their cruelty in contrast to the kindness of animals, every night the tribe were afflicted by the howling of the wolves, who pined after their lost friends.
The Inuit relate a legend that in early times a great bear threatened mankind. It was chased into the sky by a pack of dogs. As the Pleiades, they still pursue the bear through the heavens.
Many other civilizations have given names to the cluster:
Kartikeya (Skanda): is the Hindu god of masculinity and warfare - he is the leader of the armies of the gods. Born out of a magical spark created by Shiva, his name means "him of the Pleiades". (Indian)
Mao (昴), the hairy head of the white tiger of the West - alternatively, the Blossom Stars and Flower Stars. (Chinese)
Kimah: a cluster (כימה). (Hebrew)
Al-Thurayya: a cluster (الثريا). (Arabic)
Subaru: 'gathered together'. This was adopted as the trading name of a car manufacturer. (Japanese)
Hoki Boshi: 'dabs of paint on the sky', literally, the brush stars. (Japanese)
Ãlker: Mankind being afflicted with much evil and suffering, Tangri Ulgen (the creator god) met with the Sky Spirits of the West in the Pleiades (Ãlker). There they resolved to relieve these afflictions by sending an eagle as the first Shaman. To Turkic nomadic tribes the Pleiades was thus both a source of solace and their original point of contact with the realms of the gods. (Turkish)
Kungkarungkara: the ancestral women. (Australian aboriginal: Pitjantjatjara tribe)
Makara: the wives of the stars in the Orion constellation. (Australian aboriginal: Adnyamathanha tribe)
Matariki: literally the 'eyes of god' (mata ariki) or alternatively 'little eyes' (mata riki). One legend has it that when Ranginui (the sky father) and Papatūānuku (the earth mother) were separated by their offspring, Tāwhirimātea (the god of the winds) tore out his eyes in rage, flinging them far into the heavens—so forming the star cluster. Another has Matariki as a mother goddess surrounded by her six daughters, who rise up from their winter oceanic home to reinvigorate the Sun (Te Rā), weary from his annual journey. Matariki is thus associated with the antipodean winter solstice and the Māori New Year festival. (New Zealand: Māori)
Khuseti: the stars of rain, or rain bearers. (Southern Africa: Khoikhoi tribe)
Tianquiztli: the 'marketplace' or 'gathering place'. (Aztec)
The seed scatterer or sower. (Inca)
To the ancient Egyptians the Pleiades represented the goddess Net or Neith, the 'divine mother and lady of heaven'.
The Bunch of Grapes / The Spring Virgins. (Classical Roman)
The Hen and Chicks. (Old English, Old German, Russian, Czech and Hungarian)
The Ancestors: an ancient Paraguayan tribe, the Abipones, even worshipped them as ancestors. A singularlypoor choice of origin, as it happens, since the youth of the stellar cluster indicates that it is extremely inhospitable to life, with other than rudimentary planetary formation [reference] anywhere in the cluster being highly improbable.
The Star Disk
A recently authenticated archaeological discovery in northern Germany may further enhance our appreciation of the significance of the Pleiades to our ancestors. Offering a glimpse into the human world of 3,600 years ago, the 'Nebra Star Disk' or 'Sangerhausen Star Disk' (Die Himmelsscheibe von Nebra) may transform our understanding of the astronomical knowledge of northern Europeans in the early Bronze Age.
1: The Ancient Greek astronomer Eudoxus of Knidos (c. 400-350 BC) accurately discerned them as a distinct and true con-stellation.
2: The 'lost' star(s) in Kimah: The Talmud Rosh Hashanah relates that God, angered by mankind's degeneracy, reformed the work of his creation by removing two stars from Kimah and caused the cluster to rise at daybreak, out of season. The biblical flood of Noah was the direct result.
The Pleiades in reality… the Pleiades star cluster
The Pleiades have inspired a wealth of mythology and legends: fascinating as these are the reality the star cluster is profoundly more wonderful. Historically, the Pleiades were seen as a group of 'seven' stars – its brightest stars: Alcyone, Atlas, Electra, Maia, Merope, Taygeta and Pleione are visible to the keen naked eye. However modern observations show that this most famous of open clusters is comprised of several hundred stars wreathed in intricately structured nebulosity.
At a distance of about 440 light years from the Earth, the Pleiades are one of the nearest galactic open clusters. The brightest stars in the cluster (Alcyone is magnitude +2.8, and Pleione +5.1) are distributed over about seven light years and although faint to naked sight these stars are from 40 to 1000 times brighter than our Sun. From the Earth the cluster's apparent size is 110 minutes of arc (almost 2°) in the plane of the ecliptic: in comparison, the diameter of a full Moon is about 0.5°.
The cluster has an apparent motion relative to the Earth of an angular rate of just over five seconds of arc per century towards the star lambda Tauri, that is, in a south-easterly direction. Thus the Pleiades takes some 60,000 years to traverse one degree.
The Pleiades exhibits one of the finest and nearest examples of a reflection nebula associated with a cluster of young stars. The nebulosity seen here is light reflected from the particles in an interstellar cloud of cold gas and dust into which the cluster has drifted. The apparent blue colour is due to the preferential scattering of blue light by these tiny interstellar particles and is 'streaky' in structure since the particles have been aligned by the magnetic fields between the stars.
This beautiful image of the Pleiades cluster was produced by David Malin of the Anglo-Australian Observatoryand is the copyright of the Anglo-Australian Observatory and the Royal Observatory, Edinburgh. The image is part of the AAO UK Schmidt series: the width is about 100 arc minutes, with the north east in the upper right. It is reproduced with the consent of the AAO. Resource: A high resolution image (113k).
Modern observing methods have revealed that over 400 mostly faint stars form the Pleiades star cluster. These are spread over a 110 minutes of arc or nearly 2 degree field at a low density in comparison to other nearby open clusters. The total mass contained in the cluster is estimated to be about 800 solar masses.
The Trumpler classification for the Pleiades as II,3,r (Trumpler, according to Kenneth Glyn Jones) or I,3,r,n (Götz and Sky Catalog 2000) – that is, the cluster appears detached and strong to moderately concentrated toward its centre, the stars exhibit a wide range of brightness, and it is rich (having more than 100 members). The central part of the cluster is spherical in form, with a core radius of slightly over 4.5 light years, however the outer part is markedly elliptical, with an ellipticity of 0.17. The tidal radius of the entire cluster is estimated at about 52 light years. [ Investigation of the Pleiades cluster IV. The radial structure. Authors: Raboud, D.; Mermilliod, J.-C. 1998: Astronomy & Astrophysics. ]
The brightest of these nebulae, around Merope, was discovered in 1859 by Ernst Wilhelm Leberecht (Wilhelm) Tempel at Venice with a 4-inch refractor. The nebula to Alcyone was discovered in 1875, and the nebulae around Electra, Celaeno and Taygeta in 1880. Their full complexity was revealed by the first astronomical cameras of the brothers Henry in Paris and Isaac Roberts in England, between 1885 and 1888.
The analysis of the spectra of the Pleiades nebulae by Vesto Slipher in 1912 showed them to be reflection nebulae, as their spectra correspond to the spectra of the associated stars.
Radio and infrared observations in the 1980s established that the nebulosity associated with the Pleiades results from a chance collision of the young stars with an interstellar cloud, rather than being the residual debris from the cluster's formation, as was previously believed.
A 3 car train wreck?
New data obtained at Kitt Peak National Observatory suggest that the Pleiades are actually encountering two clouds – a rare three-body collision in the vast emptiness of interstellar space.
Richard White, [note 1] of Smith College in Northampton, MA, interprets the new observations of sodium atoms in the Pleiades region in the context of other recent observations of the Pleiades region. These observations include significant new optical images of the Pleiades from the Burrell Schmidt telescope on Kitt Peak, published earlier this year in the Astrophysical Journal by Steven Gibson of the University of Calgary and Kenneth Nordsieck of the University of Wisconsin, [note 2] and applied Dr. Gibson's radio maps of neutral hydrogen in the cluster.
Sodium atoms in gas found in interstellar space absorb two specific wavelengths of yellow starlight. Because of the Doppler effect, the motion of the gas along our line of sight produces subtle shifts in the observed wavelengths.
The orientation of features in the optical and radio imagery provides clues to gas and dust motions across the sky, which can be combined with the spectroscopically measured velocities from Kitt Peak to allow astronomers to reconstruct the three-dimensional configuration of the interstellar matter near the Pleiades.
The sodium absorption lines reveal that there always is one feature between Earth and the Pleiades stars. This is moving toward the cluster with a line of sight velocity of about 10 kilometers per second. White associates this feature with the Taurus-Auriga interstellar cloud complex, the bulk of which lies about 40 light-years to the east.
However, toward some stars there are two or more absorption features. White argues that a shock-wave from the collision between the Pleiades and gas associated with the Taurus-Auriga complex can account for splitting of one feature into three in some areas, primarily on the south and east sides of the Pleiades. Even so, White concludes that the presence of an additional feature in the data, primarily on the west side and moving into the cluster at about 12 kilometers per second, defies understanding unless a second cloud also is converging The Pleiades.
The wake of the Pleiades
A large emission 'cavity' has been recently discovered whose bright rims extend about 5 deg eastward from the Pleiades and is pressurized by the soft-UV radiation of the cluster. This structure delineates the wake of the Pleiades as it moves through the Interstellar Medium [note 3]. The trajectory of the principal gas cloud forming the Pleiades nebulae can be traced back to an origin in Gould's Belt some 15 Million years ago, in a blowout of gas from an exploding star (PSR 1919+21) into the Galactic halo.
Alcyone (eta Tauri) is a significant example of a nearby multiple star. It is a giant star of more than 10 Solar masses and is almost a thousand times more luminous than the Sun. It is orbited by 3 faint companion stars. The entire cluster contains numerous double stars and a few triples.
Some of the Pleiades stars are rapidly rotating, at velocities of 150 to 300 km/sec at their surfaces, which is common among main sequence stars of a certain spectral type (A-B). Due to this rotation, they must be oblate spheroids rather than regular spherical bodies. The rotation can be detected because it leads to broadened and diffuse spectral absorption lines, as parts of the stellar surface approach us on the one side, while those on the opposite side recede from us, relative to the star's mean radial velocity. The most prominent example of a rapidly rotating star in the cluster is the varible star Pleione (variablity mag 4.77 to 5.50). Spectroscopic observations have shown that between 1938 and 1952 the rapid rotation of Pleione caused the ejection a gas shell.
Cecilia Payne-Gaposhkin has observed that the Pleiades contain several White Dwarf stars. These stars give rise to a specific problem of stellar evolution: How can white dwarfs exist in such a young star cluster? As they are numerous, it is highly probable that the stars are original cluster members and not field stars which have been captured. From the theory of stellar evolution, it follows that white dwarfs cannot have masses above a limit of about 1.4 solar masses (the Chandrasekhar limit), as if more massive they would collapse due to their own gravitation field. But stars with such a low mass evolve so slowly that it takes billions of years to attain that final state, not the mere 100 million year age of the Pleiades cluster.
A possible explanation may be that these white dwarf stars were formerly very massive and therefore evolved rapidly. Under intense effects, such as strong stellar winds, mass loss to close neighbours, or rapid rotation, these stars may have lost a high proportion part of their mass in planetary nebulae. Thus the residual stars – previously the stars' cores – have come below the Chandrasekhar limit, forming the observed stable white dwarfs.
An exotic type of star has been revealed in observations of the Pleiades since 1995. These Brown Dwarfswere, until recently, hypothetical objects thought to have a mass intermediate between that of giant planets (like Jupiter) and small stars. According to the theory of stellar structure, bodies that produce energy by fusion sometime in their lifetime, must have at least about 6 to 7 per cent of a solar mass, that is, some 60 to 70 Jupiter masses. Following this theory, brown dwarfs should have 10 to about 60 times the mass of Jupiter. They are assumed to be visible in the infrared spectrum, with diameters less than or similar to that of Jupiter (143 000 km), and densities of 10 to 100 times that of Jupiter, since they have stronger gravitation fields.
Many of stars in the cluster are X-ray sources at levels that are up to a thousand times higher than that of the sun. Systematic X-ray surveys of the Pleiades by ROSAT [note 4] have now identified some 170 stars in the Pleiades as X-ray emitters.
The relatively tight grouping is a sign of the youthfulness of the Pleiades cluster, although it is drifting apart and will gradually disperse. Its youth is also indicated by the absence of red giant stars in the group. None of the stars has yet had time to reach that stage of maturity, although the brightest member stars are hot B-type blue-white giants.
According to a recent calculation [G. Meynet, J.-C. Mermilliod, and A. Maeder in Astron. Astrophys. Suppl. Ser. 98, 477-504, 1993], the age of the Pleiades cluster is about 100 million years, whereas the "canonical" age is considered to be 60 to 80 million years (for instance, the Sky Catalog 2000 gives 78 million). The Pleiades may have an anticipated lifetime as a cluster of only some 250 million years – after then they will have been dispersed as single (or multiple) isolated stars along their orbital path.
The distance of the Pleiades from the Earth
There is an unresolved dispute as to the distance of the Pleiades cluster from the Earth.
The distance of the Pleiades cluster from the Earth was determined in 1997 using direct parallax measurements from the European Space Agency Hipparcos astrometric satellite. On this basis the Pleiades are about 118 parsecs (380 light years) distant from the Solar System. The previously accepted estimated distance was some 408 light years. This lower value requires an explanation for the comparatively faint apparent magnitudes of the stars in the cluster.
The implication of this reduced estimate was that either current stellar models were flawed or Hipparcos was giving incorrect distances. However recent research [note 5] has derived a distance of 133-137 parsecs (434-446 light years), thus reaffirming the fidelity of current stellar models.
A brief history of observation
The Pleiades are among those objects which are known since the earliest times. At least 6 member stars are visible to the naked eye, while under moderate conditions this number increases to 9, and under clear dark skies jumps up to more than a dozen (Vehrenberg, in his 'Atlas of Deep Sky Splendors', mentions that in 1579, well before the invention of the telescope, the astronomer Moestlin had correctly drawn 11 Pleiades stars, while Kepler quotes observations of up to 14).
The cluster was first examined telescopically by Galileo, who recorded more than 40 member stars. It was an early subject for astronomical photography, being first photographed by Paul and Prosper Henry in 1885.
The cluster was the final entry, as M45, in an early modern astronomical catalogue by Charles Messier, Tables des Nebuleuses, ainsi que des amas d'Etoiles, que l'on decouvre parmi les Etoiles fixes sur l'horizon de Paris; observes a l'Observatoire de la Marine; Memoires de l'Academie des Sciences for 1771, Paris (Table of nebulae and star clusters, which have been discovered between the fixed stars over the horizon of Paris; observed at the Marine Observatory).
In 1846 Johann von Maedler of the Estonian Dorpat Observatory, an eminent lunar cartographer, had been measuring the motions of the various Pleiades stars. Finding that they showed no relative motion within the cluster, he that they were at the centre of the Galaxy, and Alcyone was the star at the centre of the known universe. For a brief period, until this erroneous argument was exposed, the Pleiades were the subject of intense public debate and much baseless speculation.
Recent advances in telescopy are revealing some of the fine structure of our galactic neighbours – such as the Pleiades cluster. Eerie though it appears, the picture below is not the product of a fertile imagination. This is a photograph of an interstellar cloud in the process of disintegration by intense radiation from an adjacent hot star. The cloud – IC 349 or Barnard's Merope Nebula – is illuminated by Merope in the Pleiades star cluster.
The cloud, which is part of Tempel's Nebula or NGC 1432, is drifting through the Pleiades star cluster. Since the cluster is itself dispersing and moving through space, the combined velocity of Merope and the nebula is some 11 kilometres per second.
As a result of its close proximity of the cloud and the star – in astronomical terms – of about 0.06 light-years (say 550 billion kilometres) the cloud has been extensively deformed. The is an effect of the phenomenon of 'radiation pressure' due to the intense stellar radiation emitted by the nearby star, which acts differentially on the dust particles composing the cloud. It selectively decelerates the particles: less massive dust particles are subject to greater deceleration than larger particles. The radiation pressure thus acts as a sieve, sifting the particles by size.
The clearly formed linear structures directed toward the star are streams of larger particles, whereas the smaller and thus more decelerated particles are, for the moment, retained within the main body of the cloud. If the nebula is not entirely dispersed or absorbed by the star during its close passage, it will pass Merope by and continue on into interstellar space.
Gibson, Steven J., Nordsieck, Kenneth H. (2003), The Pleiades Reflection Nebula. II. Simple Model Constraints on Dust Properties and Scattering Geometry, The Astrophysical Journal, v.589, p. 362 Return to [note 2]
White, Richard E., Bally, John. Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 409, no. 1, p. 234-247, 1993. Return to [note 3]
The ROSAT Mission (1990-1999). ROSAT, the Röntgen Satellite, was an X-ray observatory developed through a cooperative programme between Germany, the United States, and the United Kingdom. The satellite was proposed by the Max-Planck-Institut für extraterrestrische Physik (MPE) and designed, built and operated in Germany and launched by the United States. Return to [note 4]
Letters: Nature 427, 326 - 328 (22 January 2004). A distance of 133-137 parsecs to the Pleiades star cluster. Xiaopei Pan1, M. Shao1 & S.R. Kulkarni2 1 Jet Propulsion Laboratory, California Institute of Technology. 2 Caltech Optical Observatories, California Institute of Technology.
The researchers applied the orbital parameters of the bright double star Atlas in the Pleiades, using long-baseline optical/infrared interferometry. From the data they derived a firm lower bound of 127 pc, with the most likely range being 133 – 137 pc. Return to [note 5]
The apparent rays of light focused on the star are not present in reality, they are an optical phenomenon produced in the apparatus. Return to [note 6]