A hypergiant is an extremely luminous, extremely massive supergiant star that is generally identified by its extremely high luminosity and very high mass loss rates. Hypergiants don't necessarily have to be larger than supergiants in radius, though, that is generally the case. Hypergiants are the stage that extremely massive O-type stars or Wolf-Rayet stars take on when their cores start collapsing.
When a said star is about 15-20 times the mass of our sun and has already passed the red supergiant or hypergiant stages, the star can go through an incredibly rare stage called a yellow hypergiant before moving to becoming a blue supergiant or hypergiant. Yellow hypergiants are scarcely populated, with only 15 known in our galaxy. This is likely due to the fact that stars of the mass required to achieve the stage are very rare and the stage doesn't last long.
When a hypergiant's core collapses, it can go out in an extremely powerful supernova, sometimes dubbed a hypernova, and the remnant is often a black hole.
Most red hypergiants are larger than orange hypergiants; however, RW Cephei is the only known orange hypergiant.
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