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.
šâxe-ye qulân, ~ qulpeykarân (#)
Fr.: branche des géantes
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.
red giant branch (RGB)
šâxe-ye qulân-e sorx
Fr.: branche des géantes rouges
tip of the red giant branch method
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).