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.