ستارهی قلاب آبی
setare-ye qollab-e abi
Fr.: étoile du crochet bleu
A rare class of → horizontal branch (HB)
stars that so far have been
found in only very few Galactic → globular clusters.
These stars are such called because they
form a blue hook at the hot end of the HB in
→ far ultraviolet
(FUV) → color-magnitude diagrams.
The physical mechanism that produces blue hook populations
is still uncertain. At least two scenarios have been proposed.
In the first scenario these stars are explained as a consequence of extreme
→ mass loss during the
→ red giant branch phase
and late helium flashing while descending the
→ white dwarf cooling track.
Due to the thin residual hydrogen envelope, helium is mixed into
the envelope and hydrogen is
mixed into the core during the late → helium flash.
As a result, the
stars are hotter and UV-fainter than canonical
→ extreme horizontal branch stars (EHB).
By contrast, in the He self-enrichment scenario the
EHB and blue hook stars are produced via the normal evolution
of He-enriched sub-populations in globular clusters.
These sub-populations might have formed from the ejecta
of intermediate-mass → asymptotic giant branch (AGB)
stars of the first generation of stars. For the same age and
→ metallicity, He-enriched HB stars have
smaller masses than
normal HB stars, resulting in bluer
→ zero age horizontal branch star (ZAHB)
locations. They are also brighter in the FUV, but this
effect is reversed for very hot He-enriched HB stars with
→ effective temperatures
larger than 19000 K.
See Dieball A., et al., 2013, arXiv:0901.1309v1, and for blue hook stars
in ω Cen cluster, M. Tailo et al., 2015, Nature 523, 318.
→ blue; → hook;
1) A curved or bent piece of metal or other hard material for catching,
holding, or hanging something.
2) Something curved or bent like a hook.
M.E. hoke, O.E. hoc "hook, angle;" cf.
M.Du. hoek, Du. haak, Ger. Haken "hook."
Qollab "a hook, a hooked device," probably ultimately from Proto-Ir.
gart- "to turn;" cf. Pers. gard-, gardidan, gaštan
"to turn, to wind;" cognate with dialectal qellidan "to roll."
قانون ِ هوک
qânun-e Hooke (#)
Fr.: loi de Hooke
The law stating that if a body is deformed the → strain
produced is directly proportional to the applied → stress.
If the elastic limit is not exceeded, the material returns to its original shape and
size on the removal of the stress. Hooke's law forms the basis of the theory of
More specifically, within certain limits, the force required to stretch an elastic
object such as a metal spring is directly proportional to the extension of
the spring. It is commonly written: F = -kx,
where F is the force, x is the length of extension/compression and
k is a constant of proportionality known as the spring constant.
Named after Robert Hooke (1635-1703), British scientist who described the
relationship in 1676; → law.