It is known that the topological space $\omega_1$ is not compact. Further, $\omega_1$ has a least upper bound. Hence, by Theorem 27.1 of Munkres, every closed subset of $\omega_1$ is compact.
My question is: Is true that every compact subset of $\omega_1$ closed?
I do not know how to prove it and I also do not have a counterexample