Spectral Synthesis of Massive Stars in Clusters
  Stellar clusters are thought to be the simplest stellar systems and the closest observational counterparts to theoretical models for single stellar populations. Progress in our understanding of the atmospheres and evolution of massive stars has led to generally reliable synthesis models. The future release of new evolution models with rotation, however, will require non-trivial updates to previously published synthesis models, in particular for all Wolf-Rayet and red supergiant related quantities. Cluster synthesis work is currently progressing from a purely stellar approach to a more comprehensive stellar+cluster perspective. The photometric evolution of stars and the dynamical evolution of clusters are delicately interwoven. Recent work attempts to combine these seemingly related fields. 
Claus Leitherer