Nuclear star cluster
A nuclear star cluster (NSC) or compact stellar nucleus (sometimes called young stellar nucleus) is a star cluster with high density and high luminosity near the center of mass of most galaxies. NSCs are the central massive objects of fainter, low-mass galaxies where supermassive black holes (SMBHs) are not present or are of negligible mass. In the most massive galaxies, NSCs are entirely absent. Some galaxies, including the Milky Way, are known to contain both a NSC and a SMBH of comparable mass. Properties. Nuclear star clusters are found in most galaxies that can be resolved  sufficiently: NSCs are the densest known star clusters in the Universe. With apparent magnitudes between -14 and -10 mag in the infrared, they are on average 40 times brighter than globular clusters, although their effective radii are not larger than 2 to 5 parsecs. With a dynamic mass of 10 to 10 solar masses, they are at the upper end of the values reached by globular clusters. The majority of nuclear star clusters contain a mix of old (at least one billion years old) and young stellar populations and show signs of star formation within the last 100 million years . Formation. Although the mechanisms behind their formation are not entirely known, hypotheses provide four possibilities: Relationship with globular clusters. Because nuclear star clusters occur in most galaxy species, they should still be present in the halo of the resulting galaxy after the fusion of galaxies. This is a hypothesis for the formation of globular clusters. Thus, globular clusters could be the remains of nuclear star clusters excluded from gas incidence, in which no new star formation occurs. According to other hypotheses, however, the nuclear star clusters could be the result of a fusion of globular clusters captured by a supermassive  black hole in the center of the galaxy and dynamically destroyed.