Asymptotic Giant Branch
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A period of Stellar evolution undertaken by all low to intermediate mass stars (0.6-10 solar masses) late in their life. The Asymptotic Giant Branch is the name given to a region of the Hertzsprung-Russell Diagram populated by evolving low to medium-mass stars. When a star exhausts the supply of hydrogen in its core, the core contracts and its temperature increases, causing the outer layers of the star to expand and cool. The star's luminosity increases greatly, and it becomes a red giant, following a track leading into the upper-right hand corner of the HR diagram.
Eventually, once the temperature in the core has reached approximately 3x108K, helium burning begins. The onset of helium burning in the core halts the star's cooling and increase in luminosity, and the star instead moves back towards the left hand side of the HR diagram. This is the Horizontal Branch (for population II stars) or Red Clump (for population I stars). After the completion of helium burning in the core, the star again moves to the right and upwards on the diagram. Its path is almost aligned with its previous red giant track, hence the name of Asymptotic Giant Branch. Stars at this stage of stellar evolution are known as AGB stars.
The AGB phase is divided into two parts, the early AGB (E-AGB) and the thermally pulsing AGB (TP-AGB). During the E-AGB phase the main source of energy is helium fusion in a shell around a core consisting mostly of carbon and oxygen. During this phase the star swells up to giant proportions to become a red giant again. The star may become as large as one astronomical unit. After the helium shell runs out of fuel, the TP-AGB starts. Now the star derives its energy from fusion of hydrogen in a thin shell, inside of which lies the now inactive helium shell. However, on periods of 10,000 to 100,000 years the helium shell switches on again, and the hydrogen shells switches off, a process known as a helium shell flash. Due to these flashes, which only last a few thousand years, material from the core region is mixed into the outer layers, changing its composition, a process referred to as dredge-up. Because of this dredge-up AGB stars may show S-process elements in their spectra. Subsequent dredge-ups can lead to the formation of Carbon stars
AGB stars are typically long period variables, and suffer large mass loss in the form of a stellar wind. A star may lose 50 to 70% of its mass during the AGB phase. The stellar winds from AGB stars are sites of cosmic dust formation, and are believed to be the main production sites of dust in the universe. The stellar winds of AGB stars are often the site of maser emission. The masering molecules are SiO, H2O, and OH.
After these stars have lost nearly all of their envelope, and only the core region remains, they evolve further into short lived protoplanetary nebulae which are in turn followed by planetary nebulae.
