Dredge up astronomy10/7/2023 ![]() ![]() ![]() As you stated, you begin to have a slight core contraction and then hydrogen in a shell around the core begins to burn, producing a helium shell. The star leaves the Zero-Age Main Sequence (ZAMS) once the core of the star has run out of hydrogen and fusion of hydrogen into helium ceases. This takes place "explosively" in a degenerate core if the star is below about $2M_$) Low Mass EvolutionĪbove is an H-R Diagram representing the path of a low mass star through the luminosity-temperature phase space. The tip of the red giant branch is where He is ignited. At the same time the envelope expands and becomes convective and the H-burning shell moves inwards and increases in temperature and luminosity. This occurs when the core grows significantly more massive, cannot support itself hydrostatically and begins to contract. The distinct upturn in luminosity marks the beginning of the red giant branch. The subgiant branch stars are those which have exhausted their hydrogen core and which are burning hydrogen in a shell but their He cores have not begun to contract significantly. The straightforward definition is in terms of where a star lies on its evolutionary track in the HR diagram (see below). ![]()
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