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Central massive object (CMO) refers to a mass concentration at the center of a galaxy. It can be either a supermassive black hole or a nuclear star cluster, i.e. a dense cluster of stars.
The most massive galaxies are thought to always contain a supermassive black hole (SBH); these galaxies do not contain nuclear star clusters, and the CMO is identified with the SBH. Fainter galaxies usually contain a nuclear star cluster (NSC). in most of these galaxies it is not known whether a supermassive black hole is present, and the CMO is identified with the NSC. A few galaxies, for instance the Milky Way and NGC 4395, are known to contain both a SBH and a NSC.
The mass associated with CMOs is roughly 0.1–0.3% times the total mass of the galactic bulge.