Highly ionized diffuse matter, particularly that associated with quasars, supernova remnants and X-ray sources - considered in "non-classical" nebular manner. Other aspects of X-ray sources and also very massive stars.
The nature of "Supernova Impostors": explosions that release as much radiation as a supernova, but leave the star almost intact. The most accessible example is Eta Carinae. SN impostors are far more mysterious than real supernovae, because their basic instabilities are largely unknown.
Expertise: Extremely massive stars, nebular processes, data analysis techniques, special problems. Has usually concentrated on a small number of special objects that give astrophysical information unobtainable anywhere else: Eta Carinae, Crab Nebula, etc.
