What composition characterizes the inert core during the initial red giant expansion phase?
Answer
Dense, hot, non-fusing helium.
Immediately following the cessation of core hydrogen burning, the star's center is left with what is often termed 'helium ash.' This material is extremely dense and hot due to gravitational compression, but it is not yet hot enough to initiate the next stage of fusion, which is helium burning. Therefore, during this initial expansion phase of the red giant, the core is composed almost entirely of this non-fusing, degenerate helium waiting for the temperature and density to climb further until the necessary threshold for helium ignition is met.
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