Yellow hypergiant
In 1991, Rho Cassiopeiae was the first to be described as a yellow hypergiant,[5] likely becoming grouped as a new class of luminous stars during discussions at the Solar physics and astrophysics at interferometric resolution workshop in 1992.
The area is bounded on the high-temperature side by the Yellow Evolutionary Void where stars of this luminosity become extremely unstable and experience severe mass loss.
In fact, it is difficult to explain even the small number of observed yellow hypergiants, relative to red supergiants of comparable luminosity, from simple models of stellar evolution.
[14] Recent discoveries of blue supergiant supernova progenitors have also raised the question of whether stars could explode directly from the yellow hypergiant stage.
[20] A handful of possible yellow supergiant supernova progenitors have been discovered, but they all appear to be of relatively low mass and luminosity, not hypergiants.
[24] Specifically, more massive stars and those with higher mass loss rates due to rotation or high metallicity will evolve beyond the yellow hypergiant stage to hotter temperatures before reaching core collapse.
[26] Because of their extreme luminosity and internal structure,[27] yellow hypergiants suffer high rates of mass loss[28] and are generally surrounded by envelopes of expelled material.
An example of the nebulae that can result is IRAS 17163-3907, known as the Fried Egg, which has expelled several solar masses of material in just a few hundred years.
[29] The yellow hypergiant is an expected phase of evolution as the most luminous red supergiants evolve bluewards, but they may also represent a different sort of star.
LBVs during eruption have such dense winds that they form a pseudo-photosphere which appears as a larger cooler star despite the underlying blue supergiant being largely unchanged.
At the bistability jump which occurs around 21,000 K (20,700 °C; 37,300 °F) blue supergiant winds become several times denser and could be result in an even cooler pseudo-photosphere.
