Yellow Hypergiant - Characteristics

Characteristics

It is predicted that the majority of yellow hypergiants are post-red supergiants evolving blueward, while more stable and less luminous yellow supergiants are likely to be evolving to red supergiants for the first time. There exists strong chemical and surface gravity evidence that the brightest of the yellow supergiants, HD 33579, is a high mass star currently expanding from a blue supergiant to a red supergiant. Yellow hypergiants are dynamically unstable and show variation of their spectral type and temperature, at approximately constant luminosity, between an upper limit around 8,000K (the lower limit for LBV eruptions) and a lower limit around 4,000K. Examples such as Rho Cassiopeiae show slow irregular variations of small visual amplitude, but are observed to show occasional larger eruptions resulting in significant increase or decrease in brightness.

These stars are doubly rare because they are very massive, initially hot class O main sequence stars more than 15 times as massive as the sun, but also because they spend only a few thousand years in the unstable yellow void phase of their lives. Red supergiants may execute several "blue loops", shedding much of their atmosphere but without actually ever reaching the blue supergiant stage, each one taking only a few decades at most. Most of them are thought to explode as supernovae without ever becoming blue supergiants again, but some may eventually pass right through the yellow void and become low mass low luminosity Luminous Blue Variables, and possibly Wolf-Rayet stars after that. Recent discoveries of blue supergiant supernova progenitors have also raised the question of whether stars could explode directly from the yellow hypergiant stage.