Hyades, Melotte 25

The Hyades are cluster of several hundred stars that makes up the head of Taurus, the Bull, formally catalogued as Melotte 25. The Hyades form a distinctive 'V" shaped group of stars, with bright red star Aldebaran marking the glaring "eye" of the angry Bull. Northwest of the Hyades is the beautiful Pleiades cluster, also in Taurus. Look for these clusters in evening sky during the months of November to January.

History and Mythology

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, the Big Dipper, and Orion on the shield that the god Hephaistos made for Achilles.

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. Their name (Greek: "the rainy ones") is derived from the ancient association of spring rain with the season of their heliacal (near dawn) rising. 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."

Properties and Stellar Populations

The Hyades consist of a roughly spherical group of 300 to 400 stars at a distance of 152 light years from Earth. They share the same age, place of origin, chemical content, and motion through space. As seen from Earth, Aldebaran appears to be a member of the cluster, but in fact it is much closer than the Hyades' distance of 152 light-years, and merely happens to lie along the same line of sight.

The stars in the Hyades are in a loose cluster, in which the stars move together through space like a flock of birds. The cluster core, where stars are most densely packed, has a diameter of about 17.6 light years, and the cluster's tidal radius is about 32 light years. However, around 1/3 of the confirmed member stars have been observed well outside this boundary; these stars are probably escaping from its gravitational influence.

Five members of the Hyades are visible to the unaided eye. Their Bayer designations are Gamma, Delta, Epsilon, and Theta1, and Theta2 Tauri. All are located within a few light years of each other. Four are type K0 III red giants that began life as type A stars of around 2.5 solar masses, and have now evolved off the main sequence. The primary of Theta2 Tauri is an additional "white giant" of type A7 III. 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 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 type A stars, 6 type F stars, and 21 type G stars.

The Hyades' lower-mass stars - of spectral types K and M - remains poorly understood. At least 48 type K dwarfs are confirmed members, along with about a dozen M dwarfs. 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 solar neighborhood stars.

The stars of the Hyades are more enriched in heavy elements than our Sun and other ordinary stars in the Solar neighborhood. Epsilon Tauri, also known as Ain, harbors at least one gas giant planet.

Evolution

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, M 44. The trajectories of both clusters can be traced back to the same region of space, indicating a common origin. With an age of 625 million years, the Hyades are not quite as old as the Beehive. The exoplanet host star Iota Horologii has recently been proposed as an escaped member of the primordial Hyades Cluster.

All stars form in clusters, but most clusters break up less than 50 million years after star formation concludes. Ninety percent of open clusters dissolve less than one billion years after formation, while only a tiny fraction survive to the present age of the Solar System (about 4.6 billion years). Only extremely massive clusters, orbiting far from the Galactic center, can survive 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 our Sun, implying still larger numbers of individual stars. Over the next few hundred million years, the Hyades will continue to lose members, as its brightest stars evolve off the main sequence, and its dimmest stars escape the cluster halo.