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A yellow hypergiant is a huge hypergiant star. It starts off a mass of about 20–60 solar masses, but it soon loses about half that mass (because they are very unstable). It has a spectral class from A to K.
They are sometimes referred to as 'cool hypergiants' in comparison to O- and B-type stars, and sometimes as 'warm hypergiants' in comparison to red supergiants. Yellow hypergiants have evolved off the main sequence and have little hydrogen left in their centres.