Casimirs of Lie Algebras

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Are the eigenvalues of the Casimir of the lie algebra continuous if the corresponding lie group is noncompact and discrete is the lie group is compact ? One example I know is $SU(2)$ group, the Casimir of its corresponding lie algebra is discrete and are labelled by $j \in \mathbb{N}$ while the Casimir of the Lorentz algebra is continuous and is labelled by $m \in \mathbb{R}^{+}$. Is this a general fact ? If so, how do we prove it ?