Hyades (star cluster)

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The Hyades (Greek Ὑάδες, also known as Melotte 25 or Collinder 50 or Caldwell 41) is the nearest open cluster to the Solar System and one of the best-studied of all star clusters. At a distance of 151 light years, it consists of a roughly spherical group of 300 to 400 stars that share the same age, place of origin, chemical content, and motion through space.[1][2] 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 massive A-type stars and have now evolved off the main sequence.[3] All are located within a few light years of each other. Their Bayer designations are Gamma, Delta, Epsilon, and Theta Tauri. They form an asterism that was traditionally identified as the head of Taurus the Bull.[3] Epsilon Tauri, also known as Ain (the "Bull's Eye"),[4] harbors at least one gas giant planet.[5]

Data from the Hipparcos satellite in the 1990s enabled astronomers to establish many fundamental parameters of the Hyades. The distance to its center is 46.34 parsecs (151 light years) and its age is about 625 million years.[1] 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).[1] 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.[1]

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