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The asymptotic giant branch is the region of the Hertzsprung–Russell diagram populated by evolving low- to medium-mass stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (0.6–10 solar masses) late in their lives. Observationally, an asymptotic-giant-branch (AGB) star will appear as a bright red giant with a luminosity thousands of times the Sun. Its interior structure is characterized by a central and 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.〔 〕 ==Stellar evolution== 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. 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 , helium burning (fusion of helium nuclei) begins. 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. 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 ''asymptotic giant branch''. Stars at this stage of stellar evolution are known as AGB stars.〔 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Asymptotic giant branch」の詳細全文を読む スポンサード リンク
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