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The S-Star Cluster is the stellar nucleus at the center of the Milky Way that surrounds its central supermassive black hole (SMBH), Sagittarius A*. The cluster doesn't have a consensus name; another is the Sagittarius A* Cluster. The name S-Star Cluster stems from the term S star used for stars extremely close to the black hole (I speculate "S" is for Sagittarius), whose orbits were studied to verify the black hole's existence and assist in estimating its mass. One if its stars termed S2 has an orbital period of 16 years, which for a few years was the shortest known orbital period around the black hole, but stars with shorter periods have now been identified.
The cluster has a stellar number density competitive with the most dense globular clusters of the galaxy: I've seen the figure 100,000 stars per cubic parsec. The center of the galaxy is hidden by dust, and viewing the stars requires microwave or infrared viewing (much of this not available through atmospheric windows), and research revealing the above information about the cluster is fairly recent.