A super star cluster (SSC) is a very large region that is thought to be the precursor of a globular cluster. They typically contain a very large number of young, massive stars that ionize a surrounding H II region, similar to our Milky Way's so-called "Ultra dense H II regions (UDHIIs)". An SSC's H II region is in turn surrounded by a cocoon of dust. In many cases, the stars and the H II regions will be invisible to optical observations due to high levels of extinction. As a result, the youngest SSCs are best observed in radio and infrared.
^Astrophysical Journal, "Star Formation in R136: A Cluster of O3 Stars Revealed by Hubble Space Telescope Spectroscopy", Massey, Philip; Hunter, Deidre A., 01/1998, v.493, p.180, doi:10.1086/305126 , Bibcode: 1998ApJ...493..180M