The yellow hypergiant Rho Cassiopeiae (F-G Ia0) has recently become very active with a tremendous outburst event in the fall of 2000. During the event the pulsating supergiant dimmed by more than a visual magnitude, while its effective temperature decreased from 7000 K to below 4000 K over about 200 d, and we directly observed the largest mass-loss rate of about 5% of the solar mass in a single stellar outburst so far. Over the past three years since the eruption we observed a very prominent inverse P Cygni profile in Balmer H alpha, signaling a strong collapse of the upper atmosphere, also observed before the 2000 event. Continuous spectroscopic monitoring reveals that the H alpha line profile has transformed into a P Cygni profile since June 2003, presently (Sept 2004) signaling supersonic expansion velocities up to ~120 km/s in the extended upper atmosphere. Based on the very recent unique spectral evolution we observed the far-UV spectrum with the FUSE satellite in July 2004. The FUSE spectrum reveals that high-temperature plasma emission lines of O VI and C III are absent in the hypergiant, also observed for the red supergiant Alpha Ori (M2 Iab). On the other hand, we observe prominent transition region emission lines in the smaller (less luminous) classical Cepheid variable Beta Dor (F-G Iab-Ia), indicating that the mean atmospheric extension and surface gravity acceleration (as compared to effective temperature and atmospheric pulsation) play a major role for the formation of high-temperature stellar atmospheric plasmas. We present an overview of the recent spectral variability phases of Rho Cas with enhanced mass-loss from this enigmatic cool star.
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