The Pleiades (), also known as Seven Sisters and Messier 45 (M45), is an
asterism of an
open star cluster containing young
B-type stars in the northwest of the constellation
Taurus. At a distance of about 444
light-year
A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...
s, it is among the nearest
star clusters to
Earth
Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
and the nearest
Messier object
The Messier objects are a set of 110 astronomical objects catalogued by the French astronomer Charles Messier in his ' (''Catalogue of Nebulae and Star Clusters''). Because Messier was interested only in finding comets, he created a list of th ...
to Earth, being the most obvious star cluster to the
naked eye
Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnification, magnifying, Optical telescope#Light-gathering power, light-collecting optical instrument, such as a telescope or microsc ...
in the
night sky
The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon.
Natural light sources in a night sky include moonlig ...
. It is also observed to house the reflection nebula
NGC 1432, an
HII region. Around 2330 BC it marked the
vernal point. Due to the brightness of its stars, the Pleiades is viewable from most areas on Earth, even in locations with significant
light pollution
Light pollution is the presence of any unwanted, inappropriate, or excessive artificial Visible spectrum, lighting. In a descriptive sense, the term ''light pollution'' refers to the effects of any poorly implemented lighting sources, during the ...
.
The cluster is dominated by
hot blue luminous stars that have formed within the last 100 million years.
Reflection nebulae around the brightest stars were once thought to be leftover material from their formation, but are now considered likely to be an unrelated dust cloud in the
interstellar medium
The interstellar medium (ISM) is the matter and radiation that exists in the outer space, space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as cosmic dust, dust and cosmic rays. It f ...
through which the stars are currently passing.
This dust cloud is estimated to be moving at a speed of approximately 18 km/s relative to the stars in the cluster.
Computer simulations have shown that the Pleiades were probably formed from a compact configuration that once resembled the
Orion Nebula.
Astronomers estimate that the cluster will survive for approximately another 250 million years, after which the clustering will be lost due to gravitational interactions with the galactic neighborhood.
Together with the open star cluster of the
Hyades, the Pleiades form the
Golden Gate of the Ecliptic. The Pleiades have been said to "resemble a tiny dipper," and should not be confused with the "
Little Dipper," or Ursa Minor.
Origin of name
The name, Pleiades, comes from . It probably derives from ( 'to sail') because of the cluster's importance in delimiting the sailing season in the
Mediterranean Sea
The Mediterranean Sea ( ) is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean basin and almost completely enclosed by land: on the east by the Levant in West Asia, on the north by Anatolia in West Asia and Southern Eur ...
: "the season of navigation began with their
heliacal rising
The heliacal rising ( ) of a star or a planet occurs annually when it becomes visible above the eastern horizon at dawn just before sunrise (thus becoming "the Morning Star (disambiguation)#Astronomy, morning star"). A heliacal rising marks the ti ...
". In Classical Greek mythology the name was used for seven divine sisters called the
Pleiades
The Pleiades (), also known as Seven Sisters and Messier 45 (M45), is an Asterism (astronomy), asterism of an open cluster, open star cluster containing young Stellar classification#Class B, B-type stars in the northwest of the constellation Tau ...
. In time, the name was said to be derived from that of a mythical mother,
Pleione, effectively meaning "daughters of Pleione".
In reality, the ancient name of the star cluster related to sailing almost certainly came first in the culture, naming of a relationship to the sister deities followed, and eventually appearing in later myths, to interpret the group name, a mother, Pleione.
Astronomical role of M45 in antiquity
The M45 group played an important role in ancient times for the establishment of many calendars thanks to the combination of two remarkable elements. The first, which is still valid, is its unique and easily identifiable appearance on the celestial vault near the
ecliptic
The ecliptic or ecliptic plane is the orbital plane of Earth's orbit, Earth around the Sun. It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making.
Fr ...
. The second, essential for the ancients, is that in the middle of the third millennium BC, this asterism (a prominent pattern or group of stars that is smaller than a constellation) marked the
vernal point. (2330 BC with ecliptic latitude about +3.5° according to
Stellarium)

The importance of this asterism is also evident in northern Europe. The Pleiades cluster is displayed on the
Nebra sky disc that was found in Germany and is dated to around 1600 BC. On the disk the cluster is represented in a high position between the Sun and the Moon.
This asterism also marks the beginning of several ancient calendars:
* In ancient India, it constitutes, in the
Atharvaveda
The Atharvaveda or Atharva Veda (, , from ''wikt:अथर्वन्, अथर्वन्'', "priest" and ''wikt:वेद, वेद'', "knowledge") or is the "knowledge storehouse of ''wikt:अथर्वन्, atharvans'', the proced ...
, compiled around 1200-1000 BC, the first (Sanskrit name for lunar stations), which is called (), a revealing name since it literally means 'the Cuttings','' i.e. ''"Those that mark the break of the year". This is so before the classic list lowers this to third place, henceforth giving the first to the star couple
''β Arietis'' and
''γ Arietis'', which, notably in
Hipparchus
Hipparchus (; , ; BC) was a Ancient Greek astronomy, Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry, but is most famous for his incidental discovery of the precession of the equinoxes. Hippar ...
, at that time, marks the equinox.
* In Mesopotamia, the ''
MUL.APIN compendium'', the first known Mesopotamian astronomy treatise, discovered at Nineveh in the library of Assurbanipal and dating from no later than 627 BC, presents a list of deities
olders of starswho stand on "the path of the Moon", a list which begins with mul.MUL.
* In Greece, the () are a group whose name is probably functional before having a mythological meaning, as André Lebœuffle points out, who has his preference for the explanation by the
Indo-European
The Indo-European languages are a language family native to the northern Indian subcontinent, most of Europe, and the Iranian plateau with additional native branches found in regions such as Sri Lanka, the Maldives, parts of Central Asia (e. ...
root that expresses the idea of 'multiplicity, crowd, assembly'.
* Similarly, the Ancient Arabs begin their old
parapegma type calendar, that of the , with M45 under the name of (). And this before their classic calendar, that of the or 'lunar stations', also begins with the star couple
''β Arietis'' and
''γ Arietis'' whose name, (), is literally "the Two Marks
f entering the equinox
Although M45 is no longer at the vernal point, the asterism still remains important, both functionally and symbolically. In addition to the changes in the calendars based on the lunar stations among the Indians and the Arabs, consider the case of an ancient Yemeni calendar in which the months are designated according to an astronomical criterion that caused it to be named ''Calendar of the Pleiades'': the month of , literally 'five', is that during which the Sun and , i.e. the Pleiades, deviate from each other by five ''movements of the Moon'', i.e. five times the path that the Moon travels on average in one day and one night, to use the terminology of .
Nomenclature and mythology

The Pleiades are a prominent sight in winter in the
Northern Hemisphere
The Northern Hemisphere is the half of Earth that is north of the equator. For other planets in the Solar System, north is defined by humans as being in the same celestial sphere, celestial hemisphere relative to the invariable plane of the Solar ...
, and are easily visible from mid-southern latitudes. They have been known since antiquity to cultures all around the world, including the
Celts
The Celts ( , see Names of the Celts#Pronunciation, pronunciation for different usages) or Celtic peoples ( ) were a collection of Indo-European languages, Indo-European peoples. "The Celts, an ancient Indo-European people, reached the apoge ...
(, ); pre-colonial
Filipinos
Filipinos () are citizens or people identified with the country of the Philippines. Filipinos come from various Austronesian peoples, all typically speaking Filipino language, Filipino, Philippine English, English, or other Philippine language ...
(who called it , or , among other names), for whom it indicated the beginning of the year;
Hawaiians (who call them ),
Māori (who call them );
Indigenous Australians
Indigenous Australians are people with familial heritage from, or recognised membership of, the various ethnic groups living within the territory of contemporary Australia prior to History of Australia (1788–1850), British colonisation. The ...
(from
several traditions); the
Achaemenid Empire
The Achaemenid Empire or Achaemenian Empire, also known as the Persian Empire or First Persian Empire (; , , ), was an Iranian peoples, Iranian empire founded by Cyrus the Great of the Achaemenid dynasty in 550 BC. Based in modern-day Iran, i ...
, whence in
Persians
Persians ( ), or the Persian people (), are an Iranian ethnic group from West Asia that came from an earlier group called the Proto-Iranians, which likely split from the Indo-Iranians in 1800 BCE from either Afghanistan or Central Asia. They ...
(who called them or ); the
Arabs
Arabs (, , ; , , ) are an ethnic group mainly inhabiting the Arab world in West Asia and North Africa. A significant Arab diaspora is present in various parts of the world.
Arabs have been in the Fertile Crescent for thousands of yea ...
(who call them ;
); the
Chinese (who called them ; ); the
Quechua (who call them
Qullqa or the storehouse); the
Japan
Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...
ese (who call them ; , ); the
Maya; the
Aztec
The Aztecs ( ) were a Mesoamerican civilization that flourished in central Mexico in the Post-Classic stage, post-classic period from 1300 to 1521. The Aztec people included different Indigenous peoples of Mexico, ethnic groups of central ...
; the
Sioux; the
Kiowa;
and the
Cherokee
The Cherokee (; , or ) people are one of the Indigenous peoples of the Southeastern Woodlands of the United States. Prior to the 18th century, they were concentrated in their homelands, in towns along river valleys of what is now southwestern ...
. In
Hinduism
Hinduism () is an Hypernymy and hyponymy, umbrella term for a range of Indian religions, Indian List of religions and spiritual traditions#Indian religions, religious and spiritual traditions (Sampradaya, ''sampradaya''s) that are unified ...
, the Pleiades are known as and are scripturally associated with the war deity and are also identified or associated with the (Seven Mothers). Hindus celebrate the first day (new moon) of the month of
Kartik (month) as
Diwali
Diwali (), also called Deepavali (IAST: ''Dīpāvalī'') or Deepawali (IAST: ''Dīpāwalī''), is the Hindu festival of lights, with variations celebrated in other Indian religions such as Jainism and Sikhism. It symbolises the spiritual v ...
, a festival of abundance and lamps. The Pleiades are also mentioned three times in the
Bible
The Bible is a collection of religious texts that are central to Christianity and Judaism, and esteemed in other Abrahamic religions such as Islam. The Bible is an anthology (a compilation of texts of a variety of forms) originally writt ...
.

The earliest known depiction of the Pleiades is likely a Northern German
Bronze Age
The Bronze Age () was a historical period characterised principally by the use of bronze tools and the development of complex urban societies, as well as the adoption of writing in some areas. The Bronze Age is the middle principal period of ...
artifact known as the
Nebra sky disk, dated to approximately 1600 BC.
The
Babylonian star catalogues name the Pleiades (), 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 the
vernal equinox around the twenty-third 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
A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object.
The first constellati ...
, and they are mentioned by
Hesiod's ''
Works and Days'',
Homer
Homer (; , ; possibly born ) was an Ancient Greece, Ancient Greek poet who is credited as the author of the ''Iliad'' and the ''Odyssey'', two epic poems that are foundational works of ancient Greek literature. Despite doubts about his autho ...
's ''
Iliad
The ''Iliad'' (; , ; ) is one of two major Ancient Greek epic poems attributed to Homer. It is one of the oldest extant works of literature still widely read by modern audiences. As with the ''Odyssey'', the poem is divided into 24 books and ...
'' and ''
Odyssey
The ''Odyssey'' (; ) is one of two major epics of ancient Greek literature attributed to Homer. It is one of the oldest surviving works of literature and remains popular with modern audiences. Like the ''Iliad'', the ''Odyssey'' is divi ...
'',
and the ''
Geoponica''. The Pleiades was the most well-known "star" among pre-Islamic Arabs and so often referred to simply as "the Star" (; ). Some scholars of
Islam
Islam is an Abrahamic religions, Abrahamic monotheistic religion based on the Quran, and the teachings of Muhammad. Adherents of Islam are called Muslims, who are estimated to number Islam by country, 2 billion worldwide and are the world ...
suggested that the Pleiades are the "star" mentioned in ('The Star') in the
Quran
The Quran, also Romanization, romanized Qur'an or Koran, is the central religious text of Islam, believed by Muslims to be a Waḥy, revelation directly from God in Islam, God (''Allah, Allāh''). It is organized in 114 chapters (, ) which ...
.
On numerous cylinder seals from the beginning of the first millennium BC, M45 is represented by seven points, while the ''Seven Gods'' appear, on low-reliefs of Neo-Assyrian royal palaces, wearing long open robes and large cylindrical headdresses surmounted by short feathers and adorned with three frontal rows of horns and a crown of feathers, while carrying both an ax and a knife, as well as a bow and a quiver.
As noted by scholar
Stith Thompson
Stith Thompson (March 7, 1885 – January 10, 1976) was an American folklore studies, folklorist: he has been described as "America's most important folklorist".
He is the "Thompson" of the Aarne–Thompson–Uther Index, which indexes Folklore, ...
, the constellation was "nearly always imagined" as a group of seven sisters, and their myths explain why there are only six. Some scientists suggest that these may come from observations as far back as 100,000 BC when
Pleione was farther from
Atlas
An atlas is a collection of maps; it is typically a bundle of world map, maps of Earth or of a continent or region of Earth. Advances in astronomy have also resulted in atlases of the celestial sphere or of other planets.
Atlases have traditio ...
and hence, more visible as a separate star to the unaided eye.
Subaru
In
Japan
Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...
, the cluster is mentioned under the name ("six stars") in the eighth-century ''
Kojiki''.
The cluster is now known in Japan as Subaru.
The name was chosen for that of the
Subaru Telescope, the flagship telescope of the
National Astronomical Observatory of Japan, located at the
Mauna Kea Observatory on the island of
Hawaii
Hawaii ( ; ) is an island U.S. state, state of the United States, in the Pacific Ocean about southwest of the U.S. mainland. One of the two Non-contiguous United States, non-contiguous U.S. states (along with Alaska), it is the only sta ...
. It had the largest
monolithic primary mirror
A primary mirror (or primary) is the principal light-gathering surface (the objective) of a reflecting telescope.
Description
The primary mirror of a reflecting telescope is a spherical, parabolic, or hyperbolic shaped disks of polished ...
in the world from its commissioning in 1998 until 2005.
It also 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.
Tolkien's Legendarium
In
J. R. R. Tolkien
John Ronald Reuel Tolkien (, 3 January 1892 – 2 September 1973) was an English writer and philologist. He was the author of the high fantasy works ''The Hobbit'' and ''The Lord of the Rings''.
From 1925 to 1945, Tolkien was the Rawlinson ...
's legendarium, where ''
The Lord of the Rings
''The Lord of the Rings'' is an Epic (genre), epic high fantasy novel written by English author and scholar J. R. R. Tolkien. Set in Middle-earth, the story began as a sequel to Tolkien's 1937 children's book ''The Hobbit'' but eventually d ...
'' is set, Pleiades is referred to as Remmirath, the netted star, as are several other celestial bodies, such as the constellation Orion as Menelvagor, swordsman of the Sky.
Observational history
Galileo Galilei
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642), commonly referred to as Galileo Galilei ( , , ) or mononymously as Galileo, was an Italian astronomer, physicist and engineer, sometimes described as a poly ...
was the first
astronomer
An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...
to view the Pleiades through a
telescope
A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...
. 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
John Michell (; 25 December 1724 – 21 April 1793) was an English natural philosopher and clergyman who provided pioneering insights into a wide range of scientific fields including astronomy, geology, optics, and gravitation. Considered "on ...
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 must consist of physically related stars. When studies were first made of the
proper motion
Proper motion is the astrometric measure of changes in the apparent places of stars or other celestial objects as they move relative to the center of mass of the Solar System. It is measured relative to the distant stars or a stable referenc ...
s of the stars, 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
A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or Coma (cometary), coma surrounding ...
-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 the number on his list.
Edme-Sébastien Jeaurat then drew in 1782 a map of 64 stars of the Pleiades from his observations in 1779, which he published in 1786.
Distance
The distance to the Pleiades can be used as a key first step to calibrate the
cosmic distance ladder
The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A ''direct'' distance measurement of an astronomical object is possible ...
. As the cluster is relatively close to the Earth, the distance should be 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 with those plotted for clusters whose distance is not known, allows their distances to be estimated. Other methods may then extend the distance scale from open clusters to galaxies and clusters of galaxies, and a cosmic distance ladder may 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
''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions and distances of ...
'' satellite and independent means (e.g., the
Hyades, the
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 approximately 135
parsec
The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (AU), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...
s (pc) away from Earth. Data from ''Hipparcos'' yielded a surprising result, namely a distance of only 118 pc, by measuring the
parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...
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
The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...
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 inner pair of stars within ]Atlas
An atlas is a collection of maps; it is typically a bundle of world map, maps of Earth or of a continent or region of Earth. Advances in astronomy have also resulted in atlases of the celestial sphere or of other planets.
Atlases have traditio ...
(a bright triple star in the Pleiades) 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.] In 2012, Francis and Anderson proposed that a systematic effect on ''Hipparcos'' parallax errors for stars in clusters would bias calculation using the weighted mean; they gave a ''Hipparcos'' parallax distance of 126 pc and photometric distance of 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 may 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,[
See also commentary by
] 134 ± 6 pc and 136.2 ± 5.0 pc, respectively. The Gaia Data Release 1 team were cautious about their result, and the VLBI authors assert "that the ''Hipparcos''-measured distance to the Pleiades cluster is in error".
The most recent distance estimate of the distance to the Pleiades based on the Gaia Data Release 3 is .
Composition
The cluster core radius is approximately 8 light-year
A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...
s and tidal radius is approximately 43 light-years. The cluster contains more than 1,000 statistically confirmed members, not counting the number that would be added if all binary stars could be resolved.[
] Its light is dominated by young, hot blue stars, up to 14 of which may be seen with the naked eye, depending on local observing conditions and visual acuity of the observer. The brightest stars form a shape somewhat similar to that of Ursa Major
Ursa Major, also known as the Great Bear, is a constellation in the Northern Sky, whose associated mythology likely dates back into prehistory. Its Latin name means "greater (or larger) bear", referring to and contrasting it with nearby Ursa M ...
and Ursa Minor. The total mass contained in the cluster is estimated to be approximately 800 solar mass
The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxie ...
es and is dominated by fainter and redder stars. An estimate of the frequency of binary stars in the Pleiades is approximately 57%.
The cluster contains many brown dwarf
Brown dwarfs are substellar objects that have more mass than the biggest gas giant planets, but less than the least massive main sequence, main-sequence stars. Their mass is approximately 13 to 80 Jupiter mass, times that of Jupiter ()not big en ...
s, such as Teide 1. These are objects with less than approximately 8% of the Sun's mass, insufficient for nuclear fusion
Nuclear fusion is a nuclear reaction, reaction in which two or more atomic nuclei combine to form a larger nuclei, nuclei/neutrons, neutron by-products. The difference in mass between the reactants and products is manifested as either the rele ...
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 analyze 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.
Brightest stars
The brightest stars of the cluster are named the Seven Sisters in early Greek mythology
Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of ancient Greek folklore, today absorbed alongside Roman mythology into the broader designation of classical mythology. These stories conc ...
: Sterope, Merope, Electra
Electra, also spelt Elektra (; ; ), is one of the most popular Greek mythology, mythological characters in tragedies.Evans (1970), p. 79 She is the main character in two Greek tragedies, ''Electra (Sophocles play), Electra'' by Sophocles and ''Ele ...
, Maia, Taygeta, Celaeno, and Alcyone. Later, they were assigned parents, Pleione and Atlas
An atlas is a collection of maps; it is typically a bundle of world map, maps of Earth or of a continent or region of Earth. Advances in astronomy have also resulted in atlases of the celestial sphere or of other planets.
Atlases have traditio ...
. As daughters of Atlas, the Hyades were sisters of the Pleiades.
The following table gives details of the brightest stars in the cluster:
Age and future evolution
Ages for star clusters may be estimated by comparing the Hertzsprung–Russell diagram for the cluster with theoretical models of stellar evolution
Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is consi ...
. 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
Lithium (from , , ) is a chemical element; it has chemical symbol, symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard temperature and pressure, standard conditions, it is the least dense metal and the ...
is rapidly destroyed in nuclear fusion
Nuclear fusion is a nuclear reaction, reaction in which two or more atomic nuclei combine to form a larger nuclei, nuclei/neutrons, neutron by-products. The difference in mass between the reactants and products is manifested as either the rele ...
reactions. Brown dwarf
Brown dwarfs are substellar objects that have more mass than the biggest gas giant planets, but less than the least massive main sequence, main-sequence stars. Their mass is approximately 13 to 80 Jupiter mass, times that of Jupiter ()not big en ...
s can retain their lithium, however. Due to lithium's very low ignition temperature of 2.5 million 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 may 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 approximately 250 million years to disperse, because of gravitational interactions with giant molecular clouds and the spiral arms of our galaxy hastening its demise.
Reflection nebulosity
With larger amateur telescopes, the nebulosity around some of the stars may be easily seen, especially when long-exposure photographs are taken. Under ideal observing conditions, some hint of nebulosity around the cluster may be seen even 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 approximately 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
The interstellar medium (ISM) is the matter and radiation that exists in the outer space, space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as cosmic dust, dust and cosmic rays. It f ...
.
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
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes:
* ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...
pressure as the dust has moved toward the stars.
Possible planets
Analyzing deep-infrared images obtained by the Spitzer Space Telescope and Gemini North telescope, astronomers discovered that one of the stars in the cluster, 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.[
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Videos
Gallery
File:Pleiades.png, A star chart of the Pleiades and their nebulae
File:M45 - The Pleiades.jpg, A widefield view of the Pleiades showing the surrounding dust, image taken with 7 hours of total exposure time
File:Pleiades Deep dive.jpg, A widefield view of the Pleiades showing the surrounding dust, image taken with 56 hours of total exposure time
File:Full Color Image of the Pleiades (Messier 45).jpg, Wide-field, deep color image of the Pleiades and their surroundings. Processed by Adam Block (astrophotographer).
File:M45-Star-Cluster.tif, M45 captured with an 8-inch telescope reveals dense dust all around the stars
See also
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* Stozhary
* Matrikas
* The Seven Sages
References
External links
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Information on the Pleiades from SEDS
{{DEFAULTSORT:Pleiades (Star Cluster)
Astronomical objects known since antiquity
Messier objects
NGC objects
Orion–Cygnus Arm
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Taurus (constellation)