Hyades (star Cluster)

Hyades (star Cluster)

The Hyades ( /ˈhaɪ.ədiːz/; 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. 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. 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. 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 designations 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.

The age of the Hyades 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.