Supergiant stars, including both red and blue supergiants, are rare making up less than 1% of stars. Yellow supergiants are an even rarer but important subclass that includes prominent stars such as Polaris and δ-Cephei.
Yellow Supergiants must meet two criteria: they have to be yellow with a spectral class of F or G, and they have to be bright with an absolute magnitude from about -5 to -8. It turns out that not many stars can satisfy both criteria for more than a short amount of time.
To understand them better, it is helpful to learn a bit about Hertzsprung-Russel Diagrams (H-R Diagrams) and stellar lifecycles. H-R diagrams help astronomers understand stellar evolution because stars fall into different positions and classes depending on where they are in their life cycle. Most stars, including our sun, spend most of their lifetime in the main sequence class where they produce energy by fusing hydrogen into helium. But as a star goes through its life stages, its luminosity and temperature change, hence its position on the H–R diagram also changes.
Our sun, for example, will spend about 10 billion years in the main sequence class and then expand and cool as it becomes a red giant. In doing so, its position on an H-R diagram will move up and to the right into the red giant class. The Sun will remain in the there for up to a billion years powered by the fusion of helium into carbon. After the helium is exhausted, the Sun will expel it’s outer layers as a planetary nebula then contract into a white dwarf. At this point, its position on the diagram moves into the white dwarf class where it remains for a long time. This lifecycle can be visualized as a path on an H-R diagram as shown below.
Yellow supergiants, on the other hand, start off in the main sequence class and remain there for just a few million years. They live in the “Instability Strip” as a sort of pit stop on their way to becoming red giants. Stars in the Instability Strip oscillates between contracting/heating up and expanding/cooling down. This results in periodic variations in the star’s luminosity making them variable stars. In fact, most yellow supergiants are Cepheid Variables – an important class for determining stellar distances. The prototypical Cepheid variable, the star δ-Cephei in Cepheus, is a yellow supergiant.
In some cases depending on chemical composition, a red giant can heat up to become a yellow supergiant. This transition is called the “blue loop” as labelled in the H-R diagram below.
Yellow supergiants only exist in the Instability Strip for a few thousand years. This short pit stop, coupled with the fact that 10+ solar-mass stars account for less than 1% of all stars explains the rarity of yellow supergiants. It is pretty cool that we can easily observe one with our naked eyes just by looking at Polaris, our prominent North Star.