All stars form in clusters, but most clusters break up less than 50 million years after star formation concludes. The astronomical term for this process is "
evaporation." Only extremely massive clusters, orbiting far from the
Galactic Center, can avoid evaporation 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 1,600 times the
mass of the Sun (), implying still larger numbers of individual stars.
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. At an age of 625 million years, the cluster's main sequence turn-off is about , meaning that all heavier stars have evolved into subgiants, giants, or
white dwarfs, while less massive stars continue fusing hydrogen on the main sequence. corresponding to the final evolutionary stage of its original population of B-type stars (each about ). 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). 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 in the past. 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. About 35 L-type (7+1+6+4+3+3+3)
brown dwarfs are currently reported as Hyades members or candidate members. Meanwhile
Gaia DR2 allowed the identification of 710 cluster members within 30 parsec, including 23 candidates with estimated masses between 60 and 80 .
Mass segregation The observed distribution of stellar types in the Hyades Cluster demonstrates a history of
mass segregation. With the exception of its white dwarfs, the cluster's central contain only star systems of at least . Although the exact ratio of single to multiple systems in the cluster remains uncertain, this ratio has considerable implications for our understanding of its population. For example, Perryman and colleagues list about 200 high-probability Hyades members. If the binary fraction is 50%, the total cluster population would be at least 300 individual stars.
Future evolution Surveys indicate that 90% of open clusters dissolve less than 1 billion years after formation, while only a tiny fraction survive for the present age of the Solar System (about 4.6 billion years). Over the next few hundred million years, the Hyades will continue to lose both mass and membership as its brightest stars evolve off the main sequence and its dimmest stars evaporate out of the cluster halo. It may eventually be reduced to a remnant containing about a dozen star systems, most of them binary or multiple, which will remain vulnerable to ongoing dissipative forces. ==Brightest stars==