It may seem like a boring star, but this star is very unique. This is Rho (ρ) Cassiopeia, a Yellow Hypergiant, one of the most luminous, and rare, stars. ρ Cassiopeia is about 8,150 light years away with a mass of 40 times of the sun.
Yellow Hypergiants are very rare as there are only 20 known to exist in the Milky Way. (ρ) Cassiopeia was the first star to be classified as a yellow hypergiant in 1991.
Yellow Hypergiants are very massive stars, that began their life as hot O class stars more than 15 times the mass of our sun. When massive stars start burning out their fuel, the star leaves the main branch of the H-R Diagram where the star begins to swell into a red supergiant. This is where the star starts to cool. During the transition to a red supergiant phase, it passes through what is called the “Yellow Evolutionary Void” where the star goes from being blue to red. Because these stars are unstable, they fluctuate in size, When the red supergiant begins to shrink again, it passes through this phases back to blue. The most luminous red supergiants may do this more than once, shedding their atmosphere.
This “Yellow Evolutionary Void” phase lasts for only a few thousand years and those that pass through the “blue loop” (shifting back and forth) may only be in this phase for a few decades.