= Stellar core
{title2=$M_c=m(R_c)$}
A stellar core is the central region distinguished from the surrounding envelope by its composition, energy generation or thermodynamic state. A core boundary at radius $R_c$ encloses mass $M_c=m(R_c)=4\pi\int_0^{R_c}\rho(r)r^2dr$. In a hydrogen-shell-burning <red giant>, a helium-rich core can be inert while a narrow surrounding layer supplies the luminosity. Core mass and core radius are separate variables: the relation between them must come from a structural model, such as a <polytropic mass-radius relation>, rather than from the word core alone.
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