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. The AGB phase is divided into two parts, the early AGB (E-AGB) and the thermally pulsing AGB (TP-AGB). Stars at this stage of stellar evolution are known as AGB stars. Its path is almost aligned with its previous red-giant track, hence the name asymptotic giant branch, although the star will become more luminous on the AGB than it did at the tip of the red-giant branch. Īfter the completion of helium burning in the core, the star again moves to the right and upwards on the diagram, cooling and expanding as its luminosity increases. This is the horizontal branch (for population II stars) or a blue loop for stars more massive than about 2.3 M ☉. The onset of helium burning in the core halts the star's cooling and increase in luminosity, and the star instead moves down and leftwards in the HR diagram. Eventually, once the temperature in the core has reached approximately 3 ×10 8 K, helium burning (fusion of helium nuclei) begins. The star becomes a red giant, following a track towards the upper-right hand corner of the HR diagram. When a star exhausts the supply of hydrogen by nuclear fusion processes in its core, the core contracts and its temperature increases, causing the outer layers of the star to expand and cool. Stellar evolution A sun-like star moves onto the AGB from the Horizontal Branch after core helium exhaustion A 5 M ☉ star moves onto the AGB after a blue loop when helium is exhausted in its core Its interior structure is characterized by a central and largely inert core of carbon and oxygen, a shell where helium is undergoing fusion to form carbon (known as helium burning), another shell where hydrogen is undergoing fusion forming helium (known as hydrogen burning), and a very large envelope of material of composition similar to main-sequence stars (except in the case of carbon stars). Observationally, an asymptotic-giant-branch star will appear as a bright red giant with a luminosity ranging up to thousands of times greater than the Sun. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) late in their lives. ![]() The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. End of main sequence, subgiant branch, and lower RGB
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