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The Hyades (; Greek Ὑάδες, also known as Melotte 25 or Collinder 50) is the nearest open cluster to the Solar System and one of the best-studied of all star clusters. The Hipparcos satellite, the Hubble Space Telescope, and infrared color-magnitude diagram fitting have been used to establish a distance of ~153 ly (47 pc) to the cluster center.〔van Leeuwen, F. ("Parallaxes and proper motions for 20 open clusters as based on the new Hipparcos catalogue" ), ''A\&A'', 2009〕〔Majaess, D.; Turner, D.; Lane, D.; Krajci, T. ("Deep Infrared ZAMS Fits to Benchmark Open Clusters Hosting delta Scuti Stars" ), ''JAAVSO'', 2011〕〔McArthur, Barbara E.; Benedict, G. Fritz; Harrison, Thomas E.; van Altena, William ("Astrometry with the Hubble Space Telescope: Trigonometric Parallaxes of Selected Hyads" ), ''AJ'', 2011〕 The distances established by these three independent methods 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.〔〔Bouvier J, Kendall T, Meeus G, Testi L, Moraux E, Stauffer JR, James D, Cuillandre J-C, Irwin J, McCaughrean MJ, Baraffe I, Bertin E. (2008) Brown dwarfs and very low mass stars in the Hyades cluster: a dynamically evolved mass function. ''Astronomy & Astrophysics'', 481: 661-672. Abstract at http://adsabs.harvard.edu/abs/2008A%26A...481..661B.〕 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 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 five brightest member stars of the Hyades have all evolved away from the Main Sequence and now lie at the bottom of the giant branch. 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 other is Zeta 1 Tauri, which lies 2° further south. Epsilon Tauri, known as ''Ain'' (the "Bull's Eye"), has a gas giant exoplanet candidate,〔Sato B, Izumiura H, Toyota E, et al. (2007) A planetary companion to the Hyades giant Epsilon Tauri. ''Astrophysical Journal'', 661: 527-531. Abstract at http://adsabs.harvard.edu/abs/2007ApJ...661..527S.〕 the first planet to be found in any open cluster. The age of the Hyades is estimated to be 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 Sun 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.〔Famaey B, Pont F, Luri X, Udry S, Mayor M, Jorissen A. (2007) The Hyades stream: an evaporated cluster or an intrusion from the inner disk? ''Astronomy & Astrophysics'', 461: 957-962. Abstract at http://adsabs.harvard.edu/abs/2007A%26A...461..957F.〕 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.〔, announced in (【引用サイトリンク】 title=The Drifting Star )〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Hyades (star cluster)」の詳細全文を読む スポンサード リンク
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