Here's why:
* Supergiants are massive stars that have exhausted their hydrogen fuel and have expanded significantly. They are very luminous and have short lifespans.
* Yellow giants are stars that have evolved from main sequence stars like our Sun. They are larger and cooler than main sequence stars, but still much smaller than supergiants.
Polaris is a yellow-white star, not yellow, and it is a cepheid variable, meaning its brightness changes over time. It is larger and brighter than our Sun, but it is not a supergiant.
