Hypergiant stars
Cool hypergiants (yellow to red) and hot hypergiants (blue to white) are evolved massive stars with spectral types ranging from late A to M.
The star Rho Cassiopeiae is one of the rare example of yellow hypergiants in the Galaxy. The object is extremely unstable and in a very last stages of fast stellar evolution. The luminosity of Rho Cas is 550 000 time Sun value. The mass is 40 * Sun and the diameter is 450 * Sun.
From Dr Alex Lobel (Royal Observatory of Belgium) about Rho Cas:"In the summer of 2000 the star brightened to 4th visual magnitude, then abruptly dimmed to 5.3 mag., while displaying tremendous spectral changes during a new outburst event, first observed in 1946. During the variability phases before maximum brightness, photospheric absorption lines showed a strong overall collapse of the extended atmosphere. Over the following 100 days the atmosphere rapidly expanded, reaching a maximum velocity of 35 km/s in a massive eruption, while the optical spectrum changed from F- to M-type." - (see http://alobel.freeshell.org/phot.html).
HR 8752 and 6 Cas are other example yellow hypergiant stars.Mu Cep is an example of red supergiant star.
