$U(n)$ and $U(1)\times SU(n)$ are not isomorphic Lie groups

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I am reading John Lee and on the chapter about group actions there is a problem that asks me to show that $U(n)$ and $U(1)\times SU(n)$ are not isomorphic Lie groups by showing that they don't have isomorphic centers. However, I can't see a clear way to compute their centers. Any hint would be appreciated.