What is "super star cluster"

n. a young, very massive, and very dense, star cluster

A super star cluster (SSC) is a very massive young open cluster 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 the Milky Way Galaxy'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.

The unique characteristics of SSCs are their large electron densities n = 10–10 cm and pressures P/ k = 10–10 K cm.