Is There a Fundamental Upper Limit to the Mass of a Star Cluster?
  The discovery around the turn of the millenium of a population of very
massive (M$_\star$ > 2$\times$10$^6$ M$_\odot$) compact stellar systems (CSS)
with physical properties (radius, velocity dispersion, stellar mass etc.) that
are intermediate between those of the classical globular cluster (GC)
population and galaxies led to questions about their exact nature. Recently a
consensus has emerged that these objects, usually called ultra compact dwarfs
(UCDs), are a mass-dependent mixture of high mass star clusters and remnant
nuclei of tidally disrupted galaxies. The existence of genuine star clusters
with stellar masses >10$^7$ M$_\odot$ naturally leads to questions about the
upper mass limit of the star cluster formation process. In this work we compile
a comprehensive catalog of compact stellar systems, and reinforce the evidence
that the true ancient star cluster population has a maximum mass of M$_\star$ ~
5$\times$10$^7$ M$_\odot$, corresponding to a stellar mass at birth of close to
10$^8$ M$_\odot$. We then discuss several physical and statistical mechanisms
potentially responsible for creating this limiting mass.
