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Hyades (star cluster)
Encyclopedia
The Hyades is the nearest open cluster
to the Solar System
and one of the best-studied of all star cluster
s. The Hipparcos
satellite, the Hubble Space Telescope
, and infrared color-magnitude diagram fitting have been used to establish a distance to the cluster's center of ~153 ly (47 pc). The distance established via the three independent analyses agree, thereby making the Hyades an important rung on the cosmic distance ladder
. The cluster consists of a roughly spherical group of hundreds of stars sharing the same age, place of origin, chemical content, and motion through space. From the perspective of observers on Earth
, the Hyades Cluster appears in the constellation
Taurus
, where its brightest stars form a "V" shape along with the still brighter red giant
Aldebaran
. However, Aldebaran is completely unrelated to the Hyades, as it is located much closer to Earth (hence its apparent brightness) and merely happens to lie along the same line of sight.
The four brightest member stars of the Hyades are all red giants that began life as A-type stars
and have now evolved off the main sequence
. All are located within a few light years of each other. Their Bayer designation
s are Gamma
, Delta
, Epsilon
, and Theta Tauri
. They form an asterism
that was traditionally identified as the head of Taurus the Bull. Epsilon Tauri, also known as Ain (the "Bull's Eye"), harbors at least one gas giant
planet.
Data from the Hipparcos
satellite, the Hubble Space Telescope
, and the 2MASS
infrared photometric survey enabled astronomers to establish many fundamental parameters of the Hyades. The distance to its center is ~47 pc (153 light years) and its age is about 625 million years. The cluster core, where stars are most densely packed, has a radius of 2.7 parsecs (corresponding to a diameter of 17.6 light years), and the cluster's tidal radius is 10 parsecs (corresponding to a diameter of 65 light years). However, about one-third of confirmed member stars have been observed well outside this boundary, in the cluster's extended halo; these stars are probably in the process of escaping from its gravitational influence.
and other ordinary stars in the Solar neighborhood, with the overall cluster metallicity
measured at +0.14.
The Hyades Cluster is related to other stellar groups in the Sun's vicinity. Its age, metallicity, and proper motion
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 Galaxy
. Among the remaining members of the Hyades Stream, the exoplanet host star Iota Horologii
has recently been proposed as an escaped member of the primordial Hyades Cluster.
The Hyades are unrelated to two other nearby stellar groups, the Pleiades
and the Ursa Major Stream
, which are easily visible to the naked eye under clear dark skies.
In Greek mythology, the Hyades
were the five daughters of Atlas
and half-sisters to the Pleiades
. 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
to Ovid
. In Book 18 of the Iliad
the stars of the Hyades appear along with the Pleiades
, the Big Dipper
, and Orion
on the shield that the god Hephaistos
made for Achilles
.
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 Batista 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), 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.
, 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 1600 times the mass of our Sun, implying still larger numbers of individual stars.
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 2.3 Solar masses, meaning that all heavier stars have evolved into subgiants, giants, or white dwarf
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 3 Solar masses). 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 actually "retired A stars" of around 2.5 Solar masses. An additional "white giant" of type A7 III is the primary of Theta2 Tauri
, a binary
system that includes a less massive companion of spectral type A; this pair is visually associated with Theta1 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.
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, but few are later than M3, and only about 12 brown dwarfs are currently reported. 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.
. With the exception of its white dwarfs, the cluster's central 2 parsecs (6.5 light years) contain only star systems of at least 1 Solar mass. 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
), while the half-mass radius, within which half the cluster's mass is contained, is 5.7 parsecs (18.6 light years). The tidal radius of 10 parsecs (32.6 light years) 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.
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
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.
Open cluster
An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist...
to the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
and one of the best-studied of all star cluster
Star cluster
Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain less than...
s. The Hipparcos
Hipparcos
Hipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...
satellite, the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
, and infrared color-magnitude diagram fitting have been used to establish a distance to the cluster's center of ~153 ly (47 pc). The distance established via the three independent analyses agree, thereby making the Hyades an important rung on the cosmic distance ladder
Cosmic distance ladder
The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" to Earth...
. The cluster consists of a roughly spherical group of hundreds of stars sharing the same age, place of origin, chemical content, and motion through space. From the perspective of observers on Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, the Hyades Cluster appears in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
Taurus
Taurus (constellation)
Taurus is one of the constellations of the zodiac. Its name is a Latin word meaning 'bull', and its astrological symbol is a stylized bull's head:...
, where its brightest stars form a "V" shape along with the still brighter red giant
Red giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
Aldebaran
Aldebaran
Aldebaran is a red giant star located about 65 light years away in the zodiac constellation of Taurus. With an average apparent magnitude of 0.87 it is the brightest star in the constellation and is one of the brightest stars in the nighttime sky...
. However, Aldebaran is completely unrelated to the Hyades, as it is located much closer to Earth (hence its apparent brightness) and merely happens to lie along the same line of sight.
The four brightest member stars of the Hyades are all red giants that began life as A-type stars
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
and have now evolved off the main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
. All are located within a few light years of each other. Their Bayer designation
Bayer designation
A Bayer designation is a stellar designation in which a specific star is identified by a Greek letter, followed by the genitive form of its parent constellation's Latin name...
s are Gamma
Gamma Tauri
Gamma Tauri is a star in the constellation Taurus and a member of the Hyades star cluster. It has the traditional name Hyadum I, which is Latin for "First Hyad"....
, Delta
Delta Tauri
The Bayer designation Delta Tauri is shared by three star systems in the constellation Taurus. They are all members of the Hyades star cluster....
, Epsilon
Epsilon Tauri
Epsilon Tauri is an orange giant star, spectral type of K0 III, located approximately 147 light-years away from the Sun in the constellation of Taurus. It is a member of the Hyades open cluster...
, and Theta Tauri
Theta Tauri
Theta Tauri is a star system in the constellation Taurus and a member of the Hyades open cluster. It is approximately 155 light years from Earth. The system is dominated by two 3rd magnitude stars, θ¹ Tauri and θ² Tauri, which are separated by 5.62 arcminutes on the sky...
. They form an asterism
Asterism (astronomy)
In astronomy, an asterism is a pattern of stars recognized on Earth's night sky. It may form part of an official constellation, or be composed of stars from more than one. Like constellations, asterisms are in most cases composed of stars which, while they are visible in the same general direction,...
that was traditionally identified as the head of Taurus the Bull. Epsilon Tauri, also known as Ain (the "Bull's Eye"), harbors at least one gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
planet.
Data from the Hipparcos
Hipparcos
Hipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...
satellite, the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
, and the 2MASS
2MASS
Observations for the Two Micron All-Sky Survey began in 1997 and were completed in 2001 at two telescopes located one each in the northern and southern hemispheres to ensure coverage of the entire sky...
infrared photometric survey enabled astronomers to establish many fundamental parameters of the Hyades. The distance to its center is ~47 pc (153 light years) and its age is about 625 million years. The cluster core, where stars are most densely packed, has a radius of 2.7 parsecs (corresponding to a diameter of 17.6 light years), and the cluster's tidal radius is 10 parsecs (corresponding to a diameter of 65 light years). However, about one-third of confirmed member stars have been observed well outside this boundary, in the cluster's extended halo; these stars are probably in the process of escaping from its gravitational influence.
Proper motion
The stars of the Hyades are more enriched in heavy elements than our SunSun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
and other ordinary stars in the Solar neighborhood, with the overall cluster metallicity
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
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
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...
coincide with those of the larger and more distant Praesepe Cluster
Beehive Cluster
The Beehive Cluster, also known as Praesepe , M44, NGC 2632, or Cr 189, is an open cluster in the constellation Cancer. It is one of the nearest open clusters to the Solar System, and it contains a larger star population than most other nearby clusters...
, 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
Hyades Stream
The Hyades Stream is a large collection of scattered stars that also share a similar trajectory with the Hyades Cluster. In 1869, Richard A. Proctor observed that numerous stars at large distances from the Hyades share a similar motion through space...
, 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
Barred spiral galaxy
A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in approximately two-thirds of all spiral galaxies...
at the center of the Milky Way Galaxy
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
. Among the remaining members of the Hyades Stream, the exoplanet host star Iota Horologii
Iota Horologii
Iota Horologii is a solar twin, yellow dwarf star approximately 56 light-years away in the constellation of Horologium . The star is classified as a G0Vp yellow dwarf...
has recently been proposed as an escaped member of the primordial Hyades Cluster.
The Hyades are unrelated to two other nearby stellar groups, the Pleiades
Pleiades (star cluster)
In astronomy, the Pleiades, or Seven Sisters , is 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...
and the Ursa Major Stream
Ursa Major Moving Group
The Ursa Major Moving Group, also known as Collinder 285 or Ursa Major association, is a nearby stellar moving group, a set of stars with common velocities in space and thought to have a common origin. Its core is located roughly 80 light years away...
, which are easily visible to the naked eye under clear dark skies.
History
Hyades (mythology)
In Greek mythology, the Hyades , are a sisterhood of nymphs that bring rain.The Hyades were daughters of Atlas and sisters of Hyas in most tellings, although one version gives their parents as Hyas and Boeotia...
were the five daughters of Atlas
Atlas (mythology)
In Greek mythology, Atlas was the primordial Titan who supported the heavens. Although associated with various places, he became commonly identified with the Atlas Mountains in north-west Africa...
and half-sisters to the Pleiades
Pleiades (Greek mythology)
The Pleiades , companions of Artemis, were the seven daughters of the titan Atlas and the sea-nymph Pleione born on Mount Cyllene. They are the sisters of Calypso, Hyas, the Hyades, and the Hesperides...
. 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
In the Western classical tradition Homer , is the author of the Iliad and the Odyssey, and is revered as the greatest ancient Greek epic poet. These epics lie at the beginning of the Western canon of literature, and have had an enormous influence on the history of literature.When he lived is...
to Ovid
Ovid
Publius Ovidius Naso , known as Ovid in the English-speaking world, was a Roman poet who is best known as the author of the three major collections of erotic poetry: Heroides, Amores, and Ars Amatoria...
. In Book 18 of the Iliad
Iliad
The Iliad is an epic poem in dactylic hexameters, traditionally attributed to Homer. Set during the Trojan War, the ten-year siege of the city of Troy by a coalition of Greek states, it tells of the battles and events during the weeks of a quarrel between King Agamemnon and the warrior Achilles...
the stars of the Hyades appear along with the Pleiades
Pleiades (star cluster)
In astronomy, the Pleiades, or Seven Sisters , is 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 Big Dipper
Big Dipper
The Plough, also known as the Big Dipper or the Saptarishi , is an asterism of seven stars that has been recognized as a distinct grouping in many cultures from time immemorial...
, and Orion
Orion (constellation)
Orion, often referred to as The Hunter, is a prominent constellation located on the celestial equator and visible throughout the world. It is one of the most conspicuous, and most recognizable constellations in the night sky...
on the shield that the god Hephaistos
Hephaestus
Hephaestus was a Greek god whose Roman equivalent was Vulcan. He is the son of Zeus and Hera, the King and Queen of the Gods - or else, according to some accounts, of Hera alone. He was the god of technology, blacksmiths, craftsmen, artisans, sculptors, metals, metallurgy, fire and volcanoes...
made for Achilles
Achilles
In Greek mythology, Achilles was a Greek hero of the Trojan War, the central character and the greatest warrior of Homer's Iliad.Plato named Achilles the handsomest of the heroes assembled against Troy....
.
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
Green Grow the Rushes, O
Green Grow The Rushes, Ho , is a folk song popular across the English-speaking world. It is sometimes sung as a Christmas carol...
.
The cluster was probably first catalogued by Giovanni Batista Hodierna in 1654, and it subsequently appeared in many star atlases of the 17th and 18th centuries. However, Charles Messier
Charles Messier
Charles Messier was a French astronomer most notable for publishing an astronomical catalogue consisting of deep sky objects such as nebulae and star clusters that came to be known as the 110 "Messier objects"...
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 (star cluster)
In astronomy, the Pleiades, or Seven Sisters , is 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...
(Pleiades), and M67
Messier 67
Messier 67 is an open cluster in the constellation of Cancer. M67's Trumpler class is variously given as II 2 r, II 2 m, or II 3 r. It was discovered by Johann Gottfried Koehler in 1779. Age estimates for the cluster range between 3.2 and 5 billion years, with the most recent estimate implying...
.
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
Lewis Boss
-Life:He was born in Providence, Rhode Island and attended secondary school at the Lapham Institute in North Scituate and the New Hampton Institution in New Hampshire. In 1870 he graduated from Dartmouth College, then went to work as a clerk for the U.S. Government. He served as an assistant...
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." Only extremely massive clusters, orbiting far from the Galactic centerGalactic Center
The Galactic Center is the rotational center of the Milky Way galaxy. It is located at a distance of 8.33±0.35 kpc from the Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest...
, 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 1600 times the mass of our Sun, implying still larger numbers of individual stars.
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 dwarfBrown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...
s. 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 2.3 Solar masses, meaning that all heavier stars have evolved into subgiants, giants, or white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
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 3 Solar masses). 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 actually "retired A stars" of around 2.5 Solar masses. An additional "white giant" of type A7 III is the primary of Theta2 Tauri
Theta Tauri
Theta Tauri is a star system in the constellation Taurus and a member of the Hyades open cluster. It is approximately 155 light years from Earth. The system is dominated by two 3rd magnitude stars, θ¹ Tauri and θ² Tauri, which are separated by 5.62 arcminutes on the sky...
, a binary
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
system that includes a less massive companion of spectral type A; this pair is visually associated with Theta1 Tauri
Theta Tauri
Theta Tauri is a star system in the constellation Taurus and a member of the Hyades open cluster. It is approximately 155 light years from Earth. The system is dominated by two 3rd magnitude stars, θ¹ Tauri and θ² Tauri, which are separated by 5.62 arcminutes on the sky...
, 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.
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, but few are later than M3, and only about 12 brown dwarfs are currently reported. 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.
Mass segregation
The observed distribution of stellar types in the Hyades Cluster demonstrates a history of mass segregationMass segregation
In astronomy, dynamical mass segregation is the process by which heavier members of a gravitationally bound system, such as a star cluster or cluster of galaxies, tend to move toward the center, while lighter members tend to move farther away from the center. Primordial mass segregation is...
. With the exception of its white dwarfs, the cluster's central 2 parsecs (6.5 light years) contain only star systems of at least 1 Solar mass. 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
Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Seirios . The star has the Bayer designation Alpha Canis Majoris...
), while the half-mass radius, within which half the cluster's mass is contained, is 5.7 parsecs (18.6 light years). The tidal radius of 10 parsecs (32.6 light years) 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, binarityBinary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
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
Semi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...
are smaller than 50 astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
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.External links
- Simbad
- Information on the Hyades from SEDS
- Astronomy Picture of the Day (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)