Yellow hypergiant stars are pretty amazing. They’re one of the rarest types of star in the known universe, with only a handful being known in the entire Milky Way. Classified in spectral class K up to late A, these celestial behemoths weight in at 20-50 solar masses. Internally, hypergiants consist of a convective core, composed partially or even entirely of degenerate matter under immense pressure. This core is surrounded by a radiative envelope which can shine with a luminosity of over half a million Suns.
The sheer force of the energy output from these huge stars (combined with a powerful magnetic field) mean that yellow hypergiants shed mass at a prodigous rate. Rho Cassiopeiae, the most well studied of it’s kind, sheds around 5% of a solar mass every year. This rapid mass loss is driven by the vast radiation pressure at the star’s surface. Essentially, there’s so much light being emitted that it accelerates stellar material to hundreds of kilometres per second. This leads them to have an extended “atmosphere” of circumstellar material. Some even have a disk of gas and dust, hinting at the possibility of planets.
According to theories, stars with between 10 – 60 solar masses start off as blue supergiants, before cooling, swelling and turning red. If a star has between 30-60 solar masses, it may then contract back and return to being a hotter (but smaller) blue supergiant. Thus, yellow hypergiants are rare, because they exist in what’s known as the “yellow evolutionary void”. Simply put, massive stars are only stable in either blue or red configurations. They only transit the “void” briefly while changing from one to another. Some theories ever suggest that a star cannot cross the void, destabilising too much once it turns yellow.
In fact, a yellow hypergiant will be unstable enough that it exhibits a lot of variability. For instance, Rho Cassiopeiae is extremely unstable. It’s been observed to drop in magnitude by 1.5 levels, and has been home to one of the largest stellar eruptions ever seen around 2001 when it ejected 3% of a solar mass in one sitting. Just prior to this, it was observed to have cooled from 7000K to 4000K over a few months. Seemingly such eruptions occur on Rho Cas every 50 years or so.
Using Rho Cas as a benchmark for comparison, it’s approximately 550,000 times solar luminosity – averaging -7.5 absolute magnitude. In other words, it’s one of the most luminous stars in the Galaxy, with a surface diameter 450 times that of the sun. Where planets are concerned, it’s habitable zone is estimated to be around 450AU (ten times Pluto’s orbit).
Being so massive, all hypergiant stars are extremely short lived, surviving only a few million years before exploding in a supernova (or possibly even a hypernova).