Also known as Collinder 50 and Melotte 25, the Hyades cluster is the closest open cluster to Earth, and one of the most intensely studied clusters in the entire sky. All told, the Hyades contains several hundred stars that all originated at about the same time, have similar chemical compositions, and share a common proper motion across the sky. One of the brightest stars in the cluster, Epsilon Tauri, also known as Ain, is suspected of hosting a giant gas planet- if this is confirmed Epsilon Tauri will be the first star known to have a planet in an open cluster.
• Constellation: Taurus
• Coordinates: RA 4h 27m |Dec. +15° 52″
• Distance: 153 light years
• Object type: Open star cluster
• Apparent diameter: 330 minutes of arc
• True diameter: 33 light years
• Apparent magnitude: 0.5
• Age: 625 million years
• Other designations: Melotte 25, Collinder 50, Caldwell 41
The Hyades cluster is a naked-eye object whenever the constellation Taurus is visible, which is during the autumn and winter time from the northern hemisphere, although best seen at about 9 PM Local Time during the month of January, when it is highest in the sky. The core of the cluster is shown below, but note that the bright red star in the left of the circle is Aldebaran, which is not a member of the Hyades, but merely a foreground star that lies along the same line of sight as the main cluster. The little cluster of blue stars at the top right is the Pleiades (M45).
Despite the Hyades being the closest open cluster to Earth, much about this deep-sky object remains uncertain at best, and unknown at worst. For instance, while theory predicts that a young open cluster of the size and mass of the Hyades should produce, or at least contain stars and sub-stellar objects across the entire range of possible masses, and spectral types, observations have shown the opposite.
By rights, the star cluster should contain a range of stellar objects from massive O-type stars to small, dim brown dwarfs, but observations have shown that the cluster is largely devoid of stars at both ends of the mass spectrum. Extensive studies of the cluster have shown that at an age of 625 million years, the main sequence turn-off point in the cluster occurs at about 2.3 solar masses, which means that stars that are more massive than this have by now all evolved into sub-giants, giants, and white dwarfs.
This is borne out by the presence of at least 8 white dwarfs in the cluster’s core, which represents the final evolutionary stages of the cluster’s original population of B-type stars, which all had about 3 solar masses. The preceding evolutionary stage is represented by the clusters’ four red giants; however, while these four stars are classified as K-type stars, they are in fact “retired” or very old A-type stars that have about 2.5 solar masses.
The bulk of the remaining stellar population is concentrated within the central 10-parsecs of the cluster, and consists of at least 21 A-type stars, about 60 or so F-type stars, and about 50 or so Sun-like G-type stars. By comparison, the 10-parsec space around Earth contains only 4 A-type stars, 6 F-type stars, and 21 G-type stars. The reasons for the deficiency of lower-mass K- and M-type stars in the cluster remain somewhat of a mystery, despite the cluster’s close proximity, and investigators’ best efforts to resolve the matter. To date, only 48 K-type dwarfs, about 12 M-type dwarfs, and about 12 brown dwarfs have been found in the inner-most 10 parsecs of the cluster, while at least 239 M-type dwarfs are known to inhabit the 10 parsec-wide region surrounding the solar system, which number represents about 76% of the stellar population in the Sun’s immediate neighborhood.
One other notable aspect of the Hyades cluster is the fact that it has far outlived the time it takes typical open clusters to disperse. While most open clusters typically disperse within 50 million years or so after the formation of the cluster has ended, the Hyades’ distance from the galactic core has largely prevented the process known as “evaporation”, in which stars are tidally stripped from the cluster. However, a large percentage (about 30%) of the cluster’s members are now at, or close to the tidal radius, and it is expected that the Milky Way will completely disrupt the cluster over the next few hundred million years.
Perhaps the most notable feature of the Hyades cluster is the fact that its structure derives mainly from a process of mass segregation. In this case, and with the exception of several white dwarfs, the innermost 2 parsecs of the cluster contains only stars of at least one solar mass, mainly in binary and multiple star systems.
This gives the Hyades its noticeable bright, dense core which is surrounded by an extended halo that largely consists of widely separated stars of later spectral types. Moreover, the core of the cluster spans only 8.8 light years, while 50% of the cluster’s mass is concentrated in a half-mass radius of only 18.6 light years. The Hyades’s halo extends to a distance of about 32.6 light years from the core, beyond which a star in not likely to remain gravitationally bound to the cluster for extended periods.