asymptotic giant branch (AGB)
šâxe-ye nâhamsâvi-ye qulân
Fr.: branche asymptotique des géantes
A region of the → Hertzsprung-Russell diagram populated by evolving → low-mass to → intermediate-mass stars. These stars have an electron → degenerate core of carbon and oxygen surrounded by two burning shells of helium and hydrogen. The H and He-burning shells are activated alternately in the deep layers of the star. An extended and tenuous convection envelope, having a radius of 104-105 times the size of the core, lies above these shells. The loosely bound envelope is gradually eroded by the strong → stellar wind, which forms a dusty → circumstellar envelope out to several hundreds of stellar radii. The convective envelope, stellar atmosphere, and circumstellar envelope have a rich and changing chemical composition provided by → nucleosynthesis processes in the burning shells in the deep interior.
blue horizontal branch star
setâre-ye âbi-ye šâxe-ye ofoqi
Fr.: étoile bleue de la branche horizontale
A member of a population of blue stars appearing on the → horizontal branch in the → Hertzsprung-Russell diagram of the Galactic → halo populations and → globular clusters. Belonging to → spectral types B3 to A0, they have evolved past the → red giant stage and are burning helium in their core.
→ blue; → horizontal; → branch, → star.
1) šâxé (#); 2) šâxé zadan (#)
Fr.: 1) branche; 2) se ramifier
1a) General: A shoot or arm-like limb of a tree; anything like a
limb of a tree; any offshoot from a main trunk.
M.E., from O.Fr. branche, from L.L. branca "a claw, paw."
1) Šâxé "branch," from Mid.Pers šâk, cf.
Mod.Pers. šâx, šax "branch; horn," Skt. sakha-
"a branch, a limb," Arm. cax, Lit. šaka,
O.S. soxa, PIE *kakhâ "branch."
The act of dividing into branches. → branching ratio.
Fr.: rapport de branchement
A quantity used to describe a → radionuclide that has more than one → decay mode. For a particular decay mode, the ratio of the number of atoms decaying by that decay mode to the number decaying in total: BRi = ki/(k1 + k2 + ...) = ki/k, where k is → decay constant.
extreme horizontal branch star (EHB)
setâre-ye šâxe-ye ofoqi-ye ostom
Fr.: étoile de la branche horizontale extrême
The hottest variety of stars on the → horizontal branch with temperatures ranging from 20,000 to 40,000 K. EHB stars are distinguished from normal horizontal branch stars by having extremely thin, inert hydrogen envelopes surrounding the helium-burning core. They are hot, dense stars with masses in a narrow range near 0.5 Msun. These stars have undergone such extreme mass loss during their first ascent up the giant branch that only a very thin hydrogen envelope survives. Stars identified as EHB stars are found in low metallicity globular clusters as an extension of the normal HB.
→ extreme; → horizontal; → branch; → star.
field horizontal branch star
setâre-ye šâxe-ye ofoqi-ye meydâni
Fr.: étoile de la branche horizontal du champ
A → horizontal branch star with high velocity.
→ field; → horizontal; → branch; → star.
šâxe-ye qulân, ~ qulpeykarân (#)
Fr.: branche des géantes
A conspicuous family of stars in the → Hertzsprung-Russell diagram composed of red, evolved stars with large sizes. → giant star; → red giant.
horizontal branch (HB)
šâxe-ye ofoqi (#)
Fr.: branche horizontale
A set of roughly horizontal points in the → Hertzsprung-Russell diagram of a typical → globular cluster. It displays a stage of stellar evolution which immediately follows the → red giant branch (RGB) in stars with an initial mass < 1.2 Msun. When the star's ascent of the RGB is terminated by the → helium flash, it moves down to the HB. The star's → effective temperature on the HB is higher than it was on the RGB, but the luminosity is considerably less than at the helium flash. Usually HB stars have two energy sources: in addition to the → helium burning in their cores, they experience → hydrogen fusion in a surrounding shell. The thickness of the shell determines the color of the HB stars. A thin shell, involving low → opacity, makes the star look blue. The HB domain encompasses a very large effective temperature range with several members: → extreme HB, → blue HB, → RR Lyrae, → red HB, and → red clump stars. The locations depend on many parameters, including stellar mass, metallicity, age, helium abundance, and rotation.
→ horizontal; → branch.
horizontal branch star
setâre-ye šâxe-ye ofoqi
Fr.: étoile de la branche horizontale
A star lying on the → horizontal branch.
→ horizontal; → branch; → star.
post-asymptotic giant branch star (post-AGB)
setâre-ye pasâ-šâxe-ye qulân-e nâhamsâvi
Fr.: étoile post-asymptotique
A star in a short-lived evolutionary stage evolving from the → asymptotic giant branch toward higher → effective temperatures. The majority of low and intermediate mass stars (1 to 8 → solar masses) are believed to pass through this stage on their way to becoming → planetary nebulae.
→ post-; → asymptotic giant branch.
red giant branch (RGB)
šâxe-ye qulân-e sorx
Fr.: branche des géantes rouges
The evolutionary path of a star that has exhausted its available hydrogen content in the core, between the → main-sequence turnoff and the → helium flash.
red horizontal branch star
setâre-ye sorx-e šâxe-ye ofoqi
Fr.: étoile rouge de la branche horizontale
A star found on the red part of the → horizontal branch. According to theoretical models, these stars result from the evolution of stars with a mass around 0.8 Msun, higher than that giving rise to → BHB stars. Upon helium burning in their cores, the remnant envelope of the red giant collapses.
→ red; → horizontal; → branch; → star.
supra-horizontal branch star
setâre-ye farâz-e šâxe-ye ofoqi
Fr.: étoile au-dessus de la branche horizontale
A member of a rare class of objects found in → globular clusters to lie about one magnitude above and to the blue part of the → horizontal branch. These stars are identified as post → EHB stars on their way from to the → asymptotic giant branch.
→ supra-; → horizontal; → branch; → star.
tip of the red giant branch method (TRGB)
raveš-e nok-e šâxe-ye qulhâ-ye sorx
Fr.: méthode du haut de la branche des géantes
A technique for deriving extragalactic distances which uses the → luminosity of the brightest → red giant branch stars in old → stellar populations as a → standard candle. For old (> 2-3 Gyr), → metal-poor ([Fe/H] < -0.7) stellar populations, this luminosity is relatively well determined, and the → absolute magnitude of these stars in the I band is roughly constant (MI = -4.1 ± 0.1).
zero age horizontal branch star (ZAHB)
setâre-ye šâxe-ye ofoqi bâ senn-e sefr
Fr.: étoile de la branche horizontale d'âge zéro
A star that has arrived on the → horizontal branch after leaving the → red giant branch. It begins → helium burning in its core.
→ zero; → age; → horizontal; → branch; → star.