 The SSC, however, does not have to physically be larger than other clusters of lower mass and luminosity.They typically contain a very large number of young, massive stars that ionize a surrounding HII region or a so-called "Ultra dense HII regions (UDHIIs)" in the Milky Way Galaxy as well as in other galaxies (however, SSCs do not always have to be inside an HII region).An SSC's HII region is in turn surrounded by a cocoon of dust.