In the summer and fall of 2000 the yellow hypergiant \rho Cassiopeiae dimmed by more than a visual magnitude , while its effective temperature decreased from \sim 7000 K to below 4000 K over \sim 200 d. We observed the highest mass-loss rate of \sim 5 % of the solar mass per year in a single stellar eruption so far ( Lobel et al .
2003 , ApJ , 583 , 923 ) .

It is the third outburst of \rho Cas on record in the last century .
During the outburst the enigmatic cool luminous hypergiant changed its spectral type form early F- to early M-type .
The outburst produced an outward propagating circumstellar shock wave , resulting in a tremendous cooling of the entire atmosphere .
The optical spectrum became comparable to that of the red supergiant Betelgeuse , and revealed strongly blue-shifted molecular absorption bands of titanium-oxide ( TiO ) .
We determine from the newly formed TiO bands a gas mass-loss rate of \dot { M } \simeq 5.4 \times 10 ^ { -2 } M _ { \odot } yr ^ { -1 } , which is of the same order of magnitude as has been proposed for the giant outbursts of the Luminous Blue Variable \eta Carinae .

During the pulsation cycles that followed the millennium outburst \rho Cas brightened up after mid 2002 , and started to dim in early March 2003 .
Over the past two years since the outburst event we observe a very prominent inverse P Cygni profile in Balmer H \alpha .
Strong H \alpha emission has not before been observed in the hypergiant over this long period of time , signaling an unusual strong collapse of the upper hydrogen atmosphere , which we also observed in the months before the 2000 outburst .
Since the brightness decrease of March 2003 we observe a remarkable transformation of the H \alpha profile into a P Cygni profile , signaling a strong expansion of Rho Cas ’ upper atmosphere , which could possibly result in a new outburst event .

Our radial velocity monitoring shows that the photospheric lines strongly shifted to longer wavelengths in the months before January 2003 .
High-resolution spectroscopic observations reveal that the lower photosphere rapidly expanded until May-June 2003 .
Very recent high-resolution observations of September 2003 show however that the photospheric absorption lines did not shift far blueward as observed in July 2000 before the strong brightness decrease of the outburst .
The Fe i \lambda 5572 is redshifted , signaling the collapse of the lower photosphere .
A new strong brightness decrease by more than a magnitude in V for the fall and winter of 2003 is therefore not expected .