Star Populations
Theory predicts that a young cluster of this size should give birth to stars and substellar objects of all spectral types, from huge, hot O stars down to dim brown dwarfs. However, studies of the Hyades show that it is deficient in stars at both extremes of mass. At an age of 625 million years, the cluster's main sequence turn-off is about 2.3 Solar masses, meaning that all heavier stars have evolved into subgiants, giants, or white dwarfs, while less massive stars continue fusing hydrogen on the main sequence. 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 preceding evolutionary stage is currently represented by the cluster's four red clump giants. Their present spectral type is K0 III, but all are actually "retired A stars" of around 2.5 Solar masses. An additional "white giant" of type A7 III is the primary of θ² Tauri, a binary system that includes a less massive companion of spectral type A; this pair is visually associated with θ¹ Tauri, one of the four red giants, which also has an A-type binary companion.
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 A stars, 6 F stars, and 21 G stars.
The Hyades' cohort of lower-mass stars – spectral types K and M – remains poorly understood, despite proximity and long observation. At least 48 K dwarfs are confirmed members, along with about a dozen M dwarfs of spectral types M0-M2. Additional M dwarfs have been proposed, but few are later than M3, and 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 neighborhood stars.
Read more about this topic: Hyades (star Cluster), Morphology and Evolution
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