  We present an overview of the spectral variability of the peculiar F-type
hypergiant Rho Cas, obtained from our long-term monitoring campaigns over the
past 8.5 years with four spectrographs in the northern hemisphere. Between 2000
June and September an exceptional variability phase occurred when the
V-brightness dimmed by about a full magnitude. The star recovered from this
deep minimum by 2001 April. It is the third outburst of Rho Cas on record in
the last century. We observe TiO absorption bands in high-resolution near-IR
spectra obtained with the Utrecht Echelle Spectrograph during the summer of
2000. TiO formation in the outer atmosphere occurred before the deep brightness
minimum. Atmospheric models reveal that the effective temperature decreases by
at least 3000 K, and the TiO shell is driven supersonically with Mdot ~= 5.4
10^{-2} Msun/yr. Strong episodic mass loss and TiO have also been observed
during the outbursts of 1945-47 and 1985-86.
  A detailed analysis of the exceptional outburst spectra is provided, by
comparing with high-resolution optical spectra of the early M-type supergiants
Mu Cep (Ia) and Betelgeuse (Iab). The outburst spectra indicate the formation
of a low-temperature, optically thick circumstellar gas shell of 3 10^{-2} Msun
during ~200 d, caused by dynamic instability of the upper atmosphere of this
pulsating massive supergiant near the Eddington luminosity limit. We observe
that the mass-loss rate during the outburst is of the same order of magnitude
as has been proposed for the outbursts of Eta Carinae. We present calculations
that correctly predict the outburst time-scale, whereby the shell ejection is
driven by the release of hydrogen ionization-recombination energy.
