Гиады: Открытое звёздное скопление в Тельце

Open cluster in the constellation Taurus
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 fact that these independent measurements agree makes the Hyades an important rung on the cosmic distance ladder method for estimating the distances of extragalactic objects. The stars of the Hyades are more enriched in heavier elements than the Sun and other ordinary stars in the solar neighborhood, with the overall cluster metallicity measured at +0.14. 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.

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.

Together with the other eye catching open star cluster of the Pleiades, 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 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, Ursa Major, 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 Battista Hodierna in 1654, and it subsequently appeared in many star atlases of the 17th and 18th centuries. 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.

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 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 1,600 times the mass of the Sun , implying still larger numbers of individual stars.

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. 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 dwarfs, while less massive stars continue fusing hydrogen on the main sequence. corresponding to the final evolutionary stage of its original population of B type stars (each about ). 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). 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. 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. +Hyades brightest stars Designation HD Apparentmagnitude Stellarclassification Theta2 Tauri 28319 3.398 A7III Epsilon Tauri 28305 3.529 K0III Gamma Tauri 27371 3.642 G8III Delta1 Tauri 27697 3.753 G8III Theta1 Tauri 28307 3.836 G7III Kappa Tauri 27934 4.201 A7IV V 90 Tauri 29388 4.262 A6V Upsilon Tauri 28024 4.282 A8Vn Delta2 Tauri 27962 4.298 A2IV 71 Tauri 28052 4.480 F0V

Four stars in the Hyades have been found to host exoplanets. Epsilon Tauri has a superjovian planet, which was the first planet to be discovered in any open cluster. Astronomer Donald Olson questioned these findings, pointing out minor errors in the paper's astronomical data.