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The Hyades (;
Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...
Ὑάδες, also known as Caldwell 41, Collinder 50, or Melotte 25) is the nearest
open cluster An open cluster is a type of star cluster made of tens 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 ...
and one of the best-studied
star cluster A star cluster is a group of stars held together by self-gravitation. Two main types of star clusters can be distinguished: globular clusters, tight groups of ten thousand to millions of old stars which are gravitationally bound; and open cluster ...
s. Located about away from the
Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
, it consists of a roughly spherical group of hundreds of stars sharing the same age, place of origin, chemical characteristics, and motion through space. From the perspective of observers on
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 ...
, the Hyades Cluster appears in the
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 ...
Taurus, where its brightest stars form a "V" shape along with the still-brighter
Aldebaran Aldebaran () is a star in the zodiac constellation of Taurus. It has the Bayer designation α Tauri, which is Latinized to Alpha Tauri and abbreviated Alpha Tau or α Tau. Aldebaran varies in brightness from an apparent vis ...
. However, Aldebaran is unrelated to the Hyades, as it is located much closer to Earth (65 light-years) and merely happens to lie along the same line of sight. The five brightest member stars of the Hyades have consumed the hydrogen fuel at their cores and have
evolved Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...
into
giant star A giant star has a substantially larger radius and luminosity than a main-sequence (or ''dwarf'') star of the same surface temperature. They lie above the main sequence (luminosity class V in the Yerkes spectral classification) on the Hertzsp ...
s. Four of these stars, with
Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek alphabet, Greek or Latin letter followed by the genitive case, genitive form of its parent constellation's Latin name. The original list of Bayer design ...
s
Gamma Gamma (; uppercase , lowercase ; ) is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. In Ancient Greek, the letter gamma represented a voiced velar stop . In Modern Greek, this letter normally repr ...
, Delta1,
Epsilon Epsilon (, ; uppercase , lowercase or ; ) is the fifth letter of the Greek alphabet, corresponding phonetically to a mid front unrounded vowel or . In the system of Greek numerals it also has the value five. It was derived from the Phoenic ...
, and Theta2 Tauri, form an asterism that is traditionally identified as the head of Taurus the Bull. The fifth of these stars is Theta1 Tauri, a tight naked-eye companion to the brighter Theta2 Tauri. Epsilon Tauri, known as ''Ain'' (the "Bull's Eye"), has a
gas giant A gas giant is a giant planet composed mainly of hydrogen and helium. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" was originally synonymous with "giant planet". However, in the 1990s, it became known that Uranu ...
exoplanet candidate, the first planet to be found in any open cluster. The age of the Hyades is estimated to be about 625 million years. The core of the cluster, where stars are the most densely packed, has a radius of , and the cluster's tidal radius – where the stars become more strongly influenced by the gravity of the surrounding
Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...
galaxy – is . However, about one-third of confirmed member stars have been observed well outside the latter boundary, in the cluster's extended halo; these stars are probably in the process of escaping from its gravitational influence.


Location and motion

The cluster is sufficiently close to the Sun that its distance can be directly measured by observing the amount of parallax shift of the member stars as the Earth orbits the Sun. This measurement has been performed with great accuracy using 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 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 ...
. An alternative method of computing the distance is to fit the cluster members to a standardized infrared color–magnitude diagram for stars of their type, and use the resulting data to infer their intrinsic brightness. Comparing this data to the brightness of the stars as seen from Earth allows their distances to be estimated. Both methods have yielded a distance estimate of to the cluster center. The fact that these independent measurements agree makes the Hyades an important rung on 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 ...
method for estimating the distances of extragalactic objects. The stars of the Hyades are more enriched in heavier elements than the
Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
and other ordinary stars in the solar neighborhood, with the overall cluster
metallicity In astronomy, metallicity is the Abundance of the chemical elements, abundance of Chemical element, elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable (i.e. non-Dark matter, dark) matt ...
measured at +0.14. The Hyades Cluster is related to other stellar groups in the Sun's vicinity. Its age, metallicity, and
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 ...
coincide with those of the larger and more distant Praesepe Cluster, and the trajectories of both clusters can be traced back to the same region of space, indicating a common origin. Another associate is the Hyades Stream, a large collection of scattered stars that also share a similar trajectory with the Hyades Cluster. Recent results have found that at least 15% of stars in the Hyades Stream share the same chemical fingerprint as the Hyades cluster stars. However, about 85% of stars in the Hyades Stream have been shown to be completely unrelated to the original cluster on the grounds of dissimilar age and metallicity; their common motion is attributed to tidal effects of the massive rotating bar at the center of the
Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...
galaxy. Among the remaining members of the Hyades Stream, the
exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...
host star
Iota Horologii Iota Horologii, Latinisation of names, Latinized from ι Horologii, is a star in the Horologium (constellation), Horologium constellation. With an apparent magnitude of +5.40, it can be seen to the naked eye only from places not affecte ...
has recently been proposed as an escaped member of the primordial Hyades Cluster., announced in The Hyades are unrelated to two other nearby stellar groups, 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 ...
and the Ursa Major Stream, which are easily visible 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 ...
under clear dark skies. File:Hyades 40°N.png, Star chart of the Hyades cluster File:Galaxymap.com, map 100 parsecs (2022).png, Map of stars within 100 parsecs of the Sun, the Hyades is at 180° galactic longitude.


Astrometry

A 2018 Gaia DR1 study of the Hyades Cluster determined a (U, V, W) group velocity of (−41.92 ± 0.16, −19.35 ± 0.13, −1.11 ± 0.11) km/sec, based on the space velocities of the 138 core stars. A 2019 Gaia DR2 study finds a (U, V, W) group velocity of (−42.24, −19.00, −1.48) km/sec, in very close agreement with the 2018 DR1 derivation. Another DR2 study from 2019 focused on mapping the 3D Topology & Velocities of the Hyades main body out to 30 parsecs, and included Sub-Stellar members as well. They identified 1764 member candidates, including 10 Brown Dwarfs and 17 White Dwarfs. The White Dwarfs included 9 single stars, and 4 binary systems. A 2022 Hyades study utilizing Gaia EDR3 derived a (U, V, W) group velocity of (-42.11±6.50, - 19.09±4.37, -1.32±0.44) km/sec, also with close agreement to DR1 and DR2 studies.


History

Together with the other eye-catching open star cluster of 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 ...
, the Hyades form the Golden Gate of the Ecliptic, which has been known for several thousand years. In Greek mythology, the Hyades were the five daughters of
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 half-sisters to 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 ...
. After the death of their brother, Hyas, the weeping sisters were transformed into a cluster of stars that was afterwards associated with rain. As a naked-eye object, the Hyades cluster has been known since prehistoric times. It is mentioned by numerous Classical authors from
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 ...
to
Ovid Publius Ovidius Naso (; 20 March 43 BC – AD 17/18), known in English as Ovid ( ), was a Augustan literature (ancient Rome), Roman poet who lived during the reign of Augustus. He was a younger contemporary of Virgil and Horace, with whom he i ...
.Information on the Hyades from SEDS
/ref> In Book 18 of the ''
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 ...
'' the stars of the Hyades appear along with 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 ...
,
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 Orion on the shield that the god
Hephaistos Hephaestus ( , ; eight spellings; ) is the Greek god of artisans, blacksmiths, carpenters, craftsmen, fire, metallurgy, metalworking, sculpture and volcanoes.Walter Burkert, ''Greek Religion'' 1985: III.2.ii; see coverage of Lemnos-based ...
made for
Achilles In Greek mythology, Achilles ( ) or Achilleus () was a hero of the Trojan War who was known as being the greatest of all the Greek warriors. The central character in Homer's ''Iliad'', he was the son of the Nereids, Nereid Thetis and Peleus, ...
.Homer. ''The Iliad.'' Translated by Richmond Lattimore. University of Chicago Press, 1951. In England the cluster was known as the "April Rainers" from an association with April showers, as recorded in the folk song " Green Grow the Rushes, O". The cluster was probably first catalogued by Giovanni Battista Hodierna in 1654, and it subsequently appeared in many star atlases of the 17th and 18th centuries. However, Charles Messier did not include the Hyades in his 1781 catalog of deep sky objects. It therefore lacks a Messier number, unlike many other, more distant open clusters – e.g., M44 (Praesepe), M45 (
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 ...
), and M67. In 1869, the astronomer R.A. Proctor observed that numerous stars at large distances from the Hyades share a similar motion through space. In 1908, Lewis Boss reported almost 25 years of observations to support this premise, arguing for the existence of a co-moving group of stars that he called the Taurus Stream (now generally known as the Hyades Stream or Hyades Supercluster). Boss published a chart that traced the scattered stars' movements back to a common point of convergence. By the 1920s, the notion that the Hyades shared a common origin with the Praesepe Cluster was widespread, with Rudolf Klein-Wassink noting in 1927 that the two clusters are "probably cosmically related". For much of the twentieth century, scientific study of the Hyades focused on determining its distance, modeling its evolution, confirming or rejecting candidate members, and characterizing individual stars.


Morphology and evolution

All stars form in clusters, but most clusters break up less than 50 million years after star formation concludes. The astronomical term for this process is "
evaporation Evaporation is a type of vaporization that occurs on the Interface (chemistry), surface of a liquid as it changes into the gas phase. A high concentration of the evaporating substance in the surrounding gas significantly slows down evapora ...
." Only extremely massive clusters, orbiting far from the
Galactic Center The Galactic Center is the barycenter of the Milky Way and a corresponding point on the rotational axis of the galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a ...
, can avoid evaporation over extended timescales. As one such survivor, the Hyades Cluster probably contained a much larger star population in its infancy. Estimates of its original mass range from 800 to 1,600 times the mass of the Sun (), implying still larger numbers of individual stars.Weideman V, Jordan S, Iben I, Casertano S. (1992) White dwarfs in the halo of the Hyades Cluster – The case of the missing white dwarfs. ''Astronomical Journal,'' 104: 1876-1891. 1992AJ....104.1876W.


Star populations

Theory predicts that a young cluster of this size should give birth to stars and substellar objects of all spectral types, from huge, hot O stars down to dim
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. However, studies of the Hyades show that it is deficient in stars at both extremes of mass. At an age of 625 million years, the cluster's main sequence turn-off is about , meaning that all heavier stars have evolved into subgiants, giants, or
white dwarf A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...
s, while less massive stars continue fusing hydrogen on the main sequence. Extensive surveys have revealed a total of 8 white dwarfs in the cluster core, corresponding to the final evolutionary stage of its original population of B-type stars (each about ). The preceding evolutionary stage is currently represented by the cluster's four red clump giants. Their present spectral type is K0 III, but all are "retired A stars" of around . An additional "white giant" of type A7 III is the primary of θ2 Tauri, a binary system that includes a less massive companion of spectral type A; this pair is visually associated with θ1 Tauri, one of the four red giants, which also has an A-type binary companion. The remaining population of confirmed cluster members includes numerous bright stars of spectral types A (at least 21), F (about 60), and G (about 50). All these star types are concentrated much more densely within the tidal radius of the Hyades than within an equivalent 10-parsec radius of the Earth. By comparison, our local 10-parsec sphere contains only 4 A stars, 6 F stars, and 21 G stars.Research Consortium on Nearby Stars (RECONS). Ten-parsec census at http://joy.chara.gsu.edu/RECONS/census.posted.htm. The Hyades' cohort of lower-mass stars – spectral types K and M – remains poorly understood, despite proximity and long observation. At least 48 K dwarfs are confirmed members, along with about a dozen M dwarfs of spectral types M0-M2. Additional M dwarfs have been proposed in the past. This deficiency at the bottom of the mass range contrasts strongly with the distribution of stars within 10 parsecs of the Solar System, where at least 239 M dwarfs are known, comprising about 76% of all neighborhood stars. In more recent studies more low-mass members were discovered. This is due targeted searches and an improvement in
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 ...
searches. About 35 L-type (7+1+8+6+3+4+3+3) and 15 T-type (2+1+3+1+4+4)
brown dwarfs Brown dwarfs are substellar objects that have more mass than the biggest gas giant planets, but less than the least massive main-sequence stars. Their mass is approximately 13 to 80 times that of Jupiter ()not big enough to sustain nuclear fu ...
are currently reported as Hyades members or candidate members. Meanwhile
Gaia In Greek mythology, Gaia (; , a poetic form of ('), meaning 'land' or 'earth'),, , . also spelled Gaea (), is the personification of Earth. Gaia is the ancestral mother—sometimes parthenogenic—of all life. She is the mother of Uranus (S ...
DR2 allowed the identification of 710 cluster members within 30 parsec, including 23 candidates with estimated masses between 60 and 80 .


Mass segregation

The observed distribution of stellar types in the Hyades Cluster demonstrates a history of mass segregation. With the exception of its white dwarfs, the cluster's central contain only star systems of at least . This tight concentration of heavy stars gives the Hyades its overall structure, with a core defined by bright, closely packed systems and a halo consisting of more widely separated stars in which later spectral types are common. The core radius is 2.7 parsecs (8.8 light-years, a little more than the distance between the Sun and
Sirius Sirius is the brightest star in the night sky. Its name is derived from the Greek word (Latin script: ), meaning 'glowing' or 'scorching'. The star is designated  Canis Majoris, Latinized to Alpha Canis Majoris, and abbr ...
), while the half-mass radius, within which half the cluster's mass is contained, is . The tidal radius of represents the Hyades' average outer limit, beyond which a star is unlikely to remain gravitationally bound to the cluster core. Stellar evaporation occurs in the cluster halo as smaller stars are scattered outward by more massive insiders. From the halo they may then be lost to tides exerted by the Galactic core or to shocks generated by collisions with drifting hydrogen clouds. In this way the Hyades probably lost much of its original population of M dwarfs, along with substantial numbers of brighter stars.


Stellar multiplicity

Another result of mass segregation is the concentration of binary systems in the cluster core. More than half of the known F and G stars are binaries, and these are preferentially located within this central region. As in the immediate Solar neighborhood, binarity increases with increasing stellar mass. The fraction of binary systems in the Hyades increases from 26% among K-type stars to 87% among A-type stars. Hyades binaries tend to have small separations, with most binary pairs in shared orbits whose semimajor axes are smaller than 50
astronomical unit The astronomical unit (symbol: au or AU) is a unit of length defined to be exactly equal to . Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its m ...
s. Although the exact ratio of single to multiple systems in the cluster remains uncertain, this ratio has considerable implications for our understanding of its population. For example, Perryman and colleagues list about 200 high-probability Hyades members. If the binary fraction is 50%, the total cluster population would be at least 300 individual stars.


Future evolution

Surveys indicate that 90% of open clusters dissolve less than 1 billion years after formation, while only a tiny fraction survive for the present age of the Solar System (about 4.6 billion years). Over the next few hundred million years, the Hyades will continue to lose both mass and membership as its brightest stars evolve off the main sequence and its dimmest stars evaporate out of the cluster halo. It may eventually be reduced to a remnant containing about a dozen star systems, most of them binary or multiple, which will remain vulnerable to ongoing dissipative forces.


Brightest stars

This is a list of Hyades cluster member stars that are fourth magnitude or brighter.


Planets

Five stars in the Hyades have been found to host
exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...
s. Epsilon Tauri has a superjovian planet, which was the first planet to be discovered in any open cluster. HD 285507 has a
hot Jupiter Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter (i.e. Jupiter analogue, Jupiter analogues) but that have very short orbital periods (). The close proximity to t ...
, K2-25 has a Neptune-sized planet, K2-136 has a system of three planets, and TOI-4364 has a
mini-Neptune A Mini-Neptune (sometimes known as a gas dwarf or transitional planet) is a planet less massive than Neptune but resembling Neptune in that it has a thick hydrogen-helium atmosphere, probably with deep layers of ice, rock or liquid oceans (made o ...
. Another star, HD 283869, may also host a planet, but this has not been confirmed as only one transit has been detected.


In culture

In the works of Robert W. Chambers,
H. P. Lovecraft Howard Phillips Lovecraft (, ; August 20, 1890 – March 15, 1937) was an American writer of Weird fiction, weird, Science fiction, science, fantasy, and horror fiction. He is best known for his creation of the Cthulhu Mythos. Born in Provi ...
, and others, the fictional city of Carcosa is located on a planet in the Hyades. A 2018 archaeoastronomical paper suggested that the Hyades may have inspired the Norse myth of
Ragnarök In Norse mythology, (also Ragnarok; or ; ) is a foretold series of impending events, including a great battle in which numerous great Norse mythological figures will perish (including the Æsir, gods Odin, Thor, Týr, Freyr, Heimdall, a ...
. Astronomer Donald Olson questioned these findings, pointing out minor errors in the paper's astronomical data.


See also

* List of open clusters


References


External links

*
Information on the Hyades from SEDS


(2000-09-29)
WEBDA open cluster database website for Hyades cluster
– E. Paunzen (Univ. Vienna)
Distance to the Hyades undergraduate lab
– J. Lucey (University of Durham)
Warburg Institute Iconographic Database (medieval and early modern images of the Hyades)
*
Hyades at Constellation Guide
{{DEFAULTSORT:Hyades (Star Cluster) Open clusters Taurus (constellation) 041b Hyades (mythology)