The yellow hypergiants are found in a stage between the massive Red Supergiants and the Wolf-Rayet stars. This review addresses current issues concerning the evolution of massive stars, concentrating on the transitional postRed Supergiant phase. Few yellow hypergiants are known and even fewer show direct evidence for having evolved off the Red Supergiant branch. Indeed, only two such rare objec...

The brightest blue supergiants represent the most massive evolved stars known. They are believed to be in the core helium-burning phase of evolution. To discuss the question of their stability, model interiors of core helium-burning stars with hydrogen-burning shells have been constructed and tested for pulsational instability (15-1000 M0). All models are found to be extremely stable against nu...

We present the serendipitous discovery of a young stellar cluster in the Galactic disk at l=12. Using Keck/NIRSPEC, we obtained high– and low–resolution spectroscopy of several stars in the cluster, and we identified one red supergiant and two blue supergiants. The radial velocity of the red supergiant provides a kinematic cluster distance of 4.7±0.4 kpc, implying luminosities of the stars cons...