It is not understood whether long-lived star clusters possess a continuous range of sizes and masses (and hence densities) or if rather, they should be considered as distinct types with different origins. Utilizing the Hubble Space Telescope to measure sizes and long exposures on the Keck 10 m telescope to obtain distances, we have discovered the first confirmed star clusters that lie within a previously claimed size–luminosity gap dubbed the ‘avoidance zone’ by Hwang et al. The existence of these star clusters extends the range of sizes, masses and densities for star clusters, and argues against current formation models that predict well-defined size–mass relationships (such as stripped nuclei, giant globular clusters or merged star clusters). The red colour of these gap objects suggests that they are not a new class of object but are related to faint fuzzies observed in nearby lenticular galaxies. We also report a number of low-luminosity ultracompact dwarfs with sizes of up to 50 pc. Future, statistically complete, studies will be encouraged now that it is known that star clusters possess a continuous range of structural properties.
D. A. Forbes, V. Pota, C. Usher, Aaron J. Romanowsky, J. Strader, J. P. Brodie, J. A. Arnold, and L. R. Spitler. "Filling the gap: a new class of old star cluster?" Monthly Notices of the Royal Astronomical Society (2013): L6-L10. doi:10.1093/mnrasl/slt078