Yellow hypergiant stars

An evolved, very massive star of spectral type F or G with a very high luminosity (~10^5 times solar) lying near the empirical upper luminosity boundary in the H-R diagram (Humphreys-Davidson limit). Yellow hypergiants have high mass loss rates (10^-5 - 10^-3 solar masses per year) and are in a short, transitional evolutionary stage. Their evolutionary state is thought to correspond to post-red supergiants rapidly evolving in blueward loops in the H-R diagram. In their post-RSG blueward evolution these stars enter a temperature range (6000-9000 K), called yellow void, with increased dynamical instability. Their link to other advanced evolutionary phases of massive stars such as Luminous Blue Variables and Wolf-Rayet stars is still an open issue in stellar evolution theory. The most famous yellow hypergiant is Rho Cassiopeiae.