|Distance: 153 LY|
|Mass: 400 Solar Masses|
|Radius: 10 LY (Core Radius)|
|Age: 625 Million Years|
|Other Names: Melotte 25|
The Hyades is the nearest open cluster and one of the best-studied star clusters. This fact made it the prime candidate for the Exodus Event of 2371. Located about 153 lightyears away from Sol, 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 in the Sol system, the Hyades Cluster appears in the constellation Taurus, where its brightest stars form a "V" shape along with the still brighter Aldebaran. Although Aldebaran visually appears with the Hyades Cluster, Aldebaran is unrelated to the Hyades. Aldebaran is located much closer to Earth (approximately 75 light years) and merely happens to lie along the same line of sight. This fact made Aldebaran an important navigational beacon during the voyage to Hyades.
The five brightest member stars of the Hyades have consumed the hydrogen fuel at their cores and are now evolving into giant stars. Four of these stars, with Bayer designations Gamma, Delta 1, Epsilon, and Theta 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") and to the colonists who live in the system as Coronis, was the first star to have a 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 8.8 Light Years. This was another important reason in selecting Hyades as the destination for the Exodus Event - at that distance the colonies would all be relatively close together and could easily travel between the systems making a strong network of trade and commerce. The cluster's tidal radius − where the stars become more strongly influenced by the gravity of the surrounding Milky Way galaxy − is 33 Light Years. 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 Sol that while the Exodus voyage to the cluster would be challenging and perilous, it was not deemed impossible. The distance of Hyades was measured in the 21st Century by the Hipparcos satellite and the Hubble Space Telescope. An alternative method of computing the distance is to fit the cluster members to a standardized infrared Hertzsprung–Russell 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 yielded a distance estimate of 153 LY to the cluster center. The fact that three independent measurements agreed made the Hyades an important rung on the cosmic distance ladder method for estimated the distances of extragalactic objects.
The stars of the Hyades are more enriched in heavier elements than Sol and other ordinary stars in the Solar neighborhood. 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. 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 Galaxy. Among the remaining members of the Hyades Stream, the exoplanet host star Iota Horologii has 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 from Sol 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 from Sol, 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, Ursa Major, and Orion on the shield that the god Hephaestus 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.
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 "Photoevaporation." Only extremely massive clusters, orbiting far from the Galactic center, 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 Sol.
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 dwarfs. 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 dwarfs, 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 Solar mass 2.5. (The Star Coronis is of this type.)
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.
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.
This is a map of Hyades cluster member stars.
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