= Mass-radius relation
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A <mass-radius relation> connects the sizes of a class of self-gravitating objects with their <masses>. It encodes both material properties and the mechanism supporting the object against <gravity>. For an illustrative sequence of uniform, fixed <mass density> $\rho$, the identity $M=4\pi\rho R^3/3$ gives $R\propto M^{1/3}$. Real self-compression changes that scaling.
A <planetary mass-radius relation> depends strongly on composition and any gas envelope. In compact objects, <electron degeneracy pressure> can make a larger <mass> correspond to a smaller <radius>. A relation therefore applies to a stated family under stated assumptions, rather than every object obeying one universal size law. Deviations can test an <equation of state>, but can also reflect differences in composition or age.
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